Surgical Management of Large (≥3 cm) Trigeminal Schwannomas: Functional Outcomes and Approach Selection in Multicompartmental Schwannomas

Abstract

Introduction Trigeminal schwannoma surgery has shown a remarkable improvement in functional recovery and tumor resection. In the era of radiosurgery, these outcomes need to be characterized for tumors which are outside the realm of being treated with radiosurgery. We present a series of trigeminal schwannomas larger than 3 cm, surgical approaches used, and outcomes with an emphasis on functional recovery in a high-volume center with radiosurgery facilities.

Method All consecutive cases of trigeminal schwannoma from January 2012 to May 2021 which were more than 3 cm in size and underwent microsurgery were included in this series. The surgical approach, neurological outcomes, and extent of resection were defined objectively with pre/postoperative magnetic resonance imaging.

Results A total of 83 such cases (>3 cm) were found, with cranial nerve symptoms (5th most common) being the commonest. Twenty three percent cases had blindness due to secondary optic atrophy and eighteen percent had long tract motor symptoms signifying the tumor burden in our series. Radiological gross total excision was achieved in 75.9% cases.

Conclusion Large-volume schwannomas present with cranial nerve involvement and may need extensive skull base approaches. Functional outcomes need to be prioritized and can be achieved albeit with lesser gross resection rates. Hearing and facial preservation in addition to relief of trigeminal symptoms should be the goal of resection with minimal additional morbidity.

Keywords

trigeminal - schwanomma - microsurgery - radiosurgery

Introduction

Schwannomas were first described in 1910 by Verocay[1] and originate from Schwann cells, predominantly affecting the sensory cranial nerves. Vestibular schwannomas are by far the most common but among the nonvestibular ones, trigeminal schwannomas represent the majority accounting for 0.8 to 8%.[2] These lesions are benign and can present at any age but have a peak incidence in the third and fourth decade.[3] [4] [5] [6] [7]

Relevant Surgical Anatomy

The trigeminal nerve fibers join the brainstem around the midpoint of the ventral pons and are composed of a large sensory root and a small medial motor root. Fibers pass upward toward the petrous apex, traversing the cerebellopontine cistern, and leave the posterior fossa through the porus trigeminus. Once through the porous trigeminus, the fibers converge to form the trigeminal ganglion (Gasserian ganglion), all except the motor component. The trigeminal ganglion sits within Meckel's cave, which is formed by dura and arachnoid in a recess shaped like a “trident.” Each finger of the glove corresponds to branches of the trigeminal nerve. The ophthalmic branch (V1) passes along the lateral wall of the cavernous sinus and into the orbit. The maxillary nerve (V2) passes beneath the dura, below the point where the medial and lateral walls of the cavernous sinus dura fuse, and exits the skull through the foramen rotundum. The mandibular root (V3) passes extradurally through the foramen ovale, forming the mandibular nerve which has a sensory component and motor component that supply the muscles of mastication. The petrous and lacerum part of the internal carotid artery lie beneath Meckel's cave within the petrous bone.

Classification of Trigeminal Schwannomas

For an in-depth understanding of the classification, it is important to understand the surgical anatomy of the trigeminal nerve and its surrounding structures. The classification systems[3] [4] [5] [6] [7] mirror the anatomy ([Table 1]), but they can be broadly understood as middle fossa predominantly, middle and posterior fossa, and posterior fossa predominant tumors with small extensions in each compartment. The surgical approach can be divided as per the location of the majority of the tumor ([Table 2]). Recently, there has been an increase in endoscopic approaches (EAs) and the same has been shown to have low morbidity, especially in carefully selected cases. However, in the majority of the series[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] ([Table 3]), the transcranial approaches remain the workhorse for these cases with good resection rates and minimal morbidity, especially in recent series.

Table 1 Classification systems for trigeminal schwannoma

Jefferson[3]	Samii et al[4]	Yoshida and Kawase[5]	Ramina et al[6]	Wanibuchi et al[7]
A	A	M	C	Ganglion
Middle fossa	Middle fossa	Middle fossa	Middle fossa	Tumor of ganglion (middle fossa)
B	B	P	D	Root
Posterior fossa	Posterior fossa	Posterior fossa	Posterior fossa	Tumor of trigeminal root (posterior fossa)
C	C	MP	B	Dumbbell
Combination of both middle and posterior fossa	Combination of both middle and posterior fossa	Combination of both middle and posterior fossa	Middle fossa with extracranial extension	Cavernous root (middle fossa and posterior fossa
		ME	E	Cavernous peripheral root
		Combination of both middle and extracranial fossa	Middle and posterior fossa	Middle fossa with extracranial extension
		E
		Extracranial tumors
	D	MPE	A	Peripheral
	Predominately extracranial tumor which has extended intracranially	Middle, posterior, and extracranial tumor	Extracranial tumor with extension into middle fossa	Extracranial tumor
			F
			Middle, posterior, and extracranial tumor

Abbreviations: E (extracranial) in which the tumor arises from an extracranial peripheral branch of the trigeminal nerve such as the mandibular nerve; M (middle fossa):tumor arising from the Gasserian ganglion; ME (middle fossa—extracranial) which includes tumors in the middle fossa extending into either the orbit, infratemporal fossa, or pterygopalatine fossa; MP which refers to tumor spanning the middle fossa and posterior fossa; MPE which includes tumors which are in the middle, posterior fossa and extracranial compartment); P (posterior fossa) where the origin is the root of the trigeminal nerve

Table 2 Approaches to trigeminal schwannomas according to location

Location	Approach	Remarks
Middle fossa	• Extradural frontotemporal  • Lateral basal subtemporal Modifications depending on the extent  1. Infratemporal-zygomatic arch  2. Orbit-lateral orbitotomy or orbitozygomatic approach	Prefer extra-interdural approach
Posterior fossa	• Lateral suboccipital retrosigmoid approach  • Retrosigmoid intradural suprameatal (RIST) approach	RIST increases access to the posterior middle fossa and paracavernous region.
Middle and posterior fossa	• Lateral basal subtemporal extradural approach  • Anterior transpetrosal approach.  • Retrosigmoid intradural suprameatal (RIST) approach: suprameatal tubercle above the internal acoustic meatus drilled.  • Presigmoid and combined petrosal approaches	Most cases do not need petrosal drilling, tumor can be followed through the mouth of Meckel's cave without drilling the petrous apex or sectioning the tentorium.
Meckel's cave	Endoscopic or extended endoscopy  • Combine with transclival  • Transnasal: foramen of rotundum or the inferior   orbital fissure  • Transorbital approach: Meckel's cave only	EEA risk  1. CSF leak due to the skull base defect  2. V1 neuropathy (sacrifice of the vidian nerve)  3. Rhinosinusitis Transorbital advantage:  1. Low risk of corneal keratopathy.  2. Shorter trajectory to Meckel's  3. Direct visualization of the abducens nerve

Abbreviations: CSF, cerebrospinal fluid; EEA, endoscopic endonasal approach.

Table 3 Approaches and surgical outcomes of major microsurgical series for trigeminal schwannomas

Series (year)	Cases (n)	Location	Surgical approach	Outcomes	Resection rates (%)
Samii et al. (1995)[4]	5	A Middle fossa	Frontotemporal craniotomy intradural	Mortality 0%	Gross total resection 100%
	1	B Posterior fossa	Retrosigmoid	Trigeminal function	Gross total resection 100%
	5	C Combination of middle and posterior fossa	Subtemporal + presigmoid	Slightly worse 75%	Gross total resection 100%
	1	D Predominately extracranial	Frontotemporal craniotomy, extradural	Static 25%
					Gross total resection 100%
Konovalov et al (1996)[8]	26	Posterior fossa	Suboccipital	3% Mortality	Gross total resection or near total resection in 77%
			Transtentorial
	42	Gasserian ganglion (middle fossa)	Subtemporal	Moderate or no V palsy 85%
			Frontotemporal	Same 7%
			Subfrontal	Worsened 7%	11.7% Surgery for symptomatic recurrence
	30	Dumbell (middle fossa and posterior fossa)	Temporal-suboccipital
			Presigmoid
	13	Intraorbital	Frontozygomatic
Yoshida and Kawase[5]	5	M Middle fossa	Frontotemporal epidural – interdural approach	KPS outcomes	60% Gross total resection (after introduction of skull base techniques 100%)
				> 90 92%
				< 80 8%
	5	P Posterior fossa	Lateral suboccipital or transpetrosal	Trigeminal hypesthesia	60% Gross total resection (after introduction of skull base techniques 66%)
	1	E Extracranial	Zygomatic infratemporal	46.6% worse	Gross total resection 100%
				53.4% same or better
	10	MP Middle and posterior fossa	Anterior petrosectomy		Gross total resection 80% (after introduction of skull base techniques 90%)
				Trigeminal pain
	2	MPE Middle and posterior fossa and extracranial extension	Zygomatic infratemporal and Anterior petrosectomy	Better 33%	Gross total resection 100%
	4	ME Middle fossa and extracranial extension	Orbitozygomatic infratemporal approach		Gross total resection 75%
Goel et al (2003)[9]	29	A Middle fossa	Lateral basal subtemporal	Mortality 3%	Gross total resection 69.9%
	7	B Posterior fossa	Retrosigmoid
	30	C Combination of middle and posterior fossa	Petrosal + lateral basal subtemporal
	7	D Extracranial	Frontotemporal ± orbitozygomatic
Ramina et al (2008)[6]	2	A Predominately extracranial with small extension into middle fossa	Extradural (transmaxillary or transmaxillary and extradural middle fossa)	Mortality 0%	Gross total resection 94%
				Trigeminal hypesthesia
	2	B Middle fossa with extracranial extension	Intradural middle fossa	Worsened 41.10%
				Trigeminal pain
	5	C Middle fossa	Intradural middle fossa or extradural middle fossa	Improvement 88.9%
				Preserved motor function 41%
	2	D Posterior fossa	Retrosigmoid
	4	E Middle and posterior fossa	Presigmoid or 2 stage retrosigmoid/middle fossa
	2	F Middle, posterior fossa and extracranial extension	Combination of all
Fukaya et al (2010)[10]	8	M Middle fossa	Frontotemporal/subtemporal/anterior petrosectomy/trans-zygomatic and orbitozygomatic	Mortality 0%	Gross total resection or near total resection in 81%
	12	P Posterior fossa	Lat suboccipital/anterior petrosectomy	CN V palsy 43.8%
	4	E Extracranial extension	Supraorbital/subtemporal/transzygomatic/zygomatic infratemporal fossa approach
	22	MP Middle and posterior fossa	Subtemporal/anterior petrosectomy/zygomatic transpetrosal approach
	7	ME Middle fossa and extracranial extension	Frontotemporal/orbitozygomatic
	4	MPE Middle and posterior fossa and extracranial extension	Zygomatic transpetrosal
Srinivas et al (2011)[11]	0	A. Predominately extracranial with small extension in middle fossa		0% Mortality	Gross total resection 85%
	4	B. Predominately middle fossa with extracranial extension	Frontotemporal craniotomy and orbitozygomatic craniotomy
	5	C. Middle fossa	Frontotemporal craniotomy and epidural approach
				2 CN VII palsy, 1 CN VIII
	11	D. Posterior fossa	Suboccipital craniotomy
	18	E Middle and posterior fossa	Frontotemporal craniotomy and orbitozygomatic craniotomy, frontotemporal craniotomy and orbitozygomatic craniotomy + mandibular swing, frontotemporal craniotomy and petrosectomy, combined middle/posterior fossa approach
	5	F. Middle fossa, posterior fossa, and extracranial extension
Wanibuchi et al (2012)[7]	39	Ganglion (middle fossa)	Extradural temporopolar	Mortality 0%	Gross total resection 84.6%
			Pericavernous lateral loop
				Trigeminal hypesthesia
	14	Peripheral (extracranial)			Gross total resection 78.6%
	5	V1	Temporopolar transorbital	15.9% improvement
				72.5% static
	3	V2	Preauricular transzygomatic	11.6% decline
	6	V3	Preauricular transzygomatic	Pain
				91.7% improvement
				8.3% static
	32	Dumbbell		0 worsening	Gross total resection 78.1%
	30	Cavernous root (middle fossa + posterior fossa)	Extended middle fossa rhomboid or combined petrosal
				Dysesthesia
				Improved 80%
	2	Cavernous peripheral (middle fossa and extracranial extension)	Temporopolar trans-orbital	Static 20%
			/ Infratemporal fossa approach	Weakness
				Improved 33.3%
				Static 67.7%
	22	Root (posterior fossa)	Retrosigmoid lateral suboccipital		Gross total resection 77.3%
Chen et al (2014)[12]	13	A Middle fossa	Transzygomatic/subtemporal approach	Trigeminal hypesthesia	Gross total resection 69%
					Near total resection 24%
	10	B Posterior fossa	Suboccipital approach	Improved 28%	Subtotal resection 7%
				Same 72%
	21	C Combination of middle and posterior fossa	Transzygomatic + extradural temporopolar	Trigeminal pain
				Improved in 100%
	11	D Extracranial	Transzygomatic anterior infratemporal fossa/cranio-orbital
Samii et al (2014)[13]	8	A Middle fossa	Frontotemporal ± orbitotomy	Mortality 0%	Gross total resection 75%
			Subtemporal	Trigeminal hypesthesia
			Frontotemporal + subtemporal	Improvement 15%	Near total resection 25%
				Same 80%
	1	B Posterior fossa	Retrosigmoid intradural suprameatal approach	Worsens 5%
				Trigeminal pain
	8	C Combination of middle and posterior fossa	Retrosigmoid intradural suprameatal approach + endoscope assistance	Improved 100%
			Retrosigmoid intradural suprameatal approach ± subtemporal
	3	D Extracranial	Infratemporal fossa approach
			Infratemporal fossa + subtemporal approach
Yang et al (2018)[14]	18	Orbital apex/pterygopalatine fossa/middle cranial fossa and extension into V2	Medial maxillectomy approach	Trigeminal hypesthesia
				Improved 62.5%	Gross total resection or near total resection 69%
		Inferior temporal fossa/pterygopalatine fossa/middle cranial fossa and cavernous sinus	Endoscopic endonasal assisted with sublabial transmaxillary approach	Worsened 18.8%
				Same 18.8%
	9	Middle cranial fossa/infratemporal fossa and extension into V3	Endoscopic endonasal assisted with sublabial transmaxillary approach and septectomy	Trigeminal pain
				Improved 45%
				Worsened 27%
	11	Ganglion (middle fossa)		Same 18%
		Inferior temporal fossa/middle cranial fossa/posterior cranial fossa and Meckel's cave	Endoscopic endonasal assisted with sublabial transmaxillary approach and septectomy (+ second stage in one case with posterior fossa involvement—neurosurgery procedure but approach not specified)
	1	Root (posterior fossa)	Endoscopic endonasal assisted with sublabial trans-maxillary approach and septectomy + second stage (neurosurgery procedure approach not specified)
Jeong et al (2014)[15]	9	M Middle fossa	Frontotemporal craniotomy and epidural approach	Mortality 0%	Gross total resection 95.9%
	6	MP Tumor predominately in middle fossa with a posterior fossa component	Frontotemporal craniotomy and epidural approach	Middle fossa
			Frontotemporal craniotomy and epidural approach + orbitozygomatic craniotomy	Trigeminal hypesthesia
	3	Me3 Middle fossa and extend into intracranial V3	Frontotemporal craniotomy and epidural approach + Orbitotomy	Improved 25%
				Same 75%
	1	Me1 Middle fossa and extend into intracranial V1	Frontotemporal craniotomy and epidural approach + orbitozygomatic craniotomy	Worse 0%
				Trigeminal pain
	1	Mpe3 Middle fossa, posterior fossa and extend into intracranial V3	Frontotemporal craniotomy and epidural approach + zygomatic craniotomy	Improved 75%
				Worse 25%
	16	Pm Tumor predominately in posterior fossa with a meddle fossa component	Suboccipital craniotomy	Trigeminal Weakness Improvement 60% Worsened 40%
			Suboccipital craniotomy + suprameatal approach	Posterior fossa
			Posterior petrosal	Trigeminal hypesthesia
				Improved 6.7%
	4	P Posterior fossa	Suboccipital craniotomy	Same 53.3%
	9	MP Middle and posterior fossa	Frontotemporal craniotomy and epidural approach + orbitozygomatic craniotomy	Worsened 40%
			Frontotemporal craniotomy and epidural approach + zygomatic craniotomy	Trigeminal pain
			Frontotemporal craniotomy and epidural approach	Improved 100%
			Posterior petrosal	Trigeminal weakness
			Staged op (suboccipital + frontotemporal craniotomy and epidural approach)	Same 100%
Makarenko et al (2018)[16]	2	M Middle fossa	(Not specified which tumor had which approach) Frontotemporal craniotomy and orbitozygomatic osteotomy, endoscopic resection, partial labyrinthectomy and anterior petrosectomy	Mortality 0%	Planned gross total resection 9/12 (75%)
	0	P Posterior fossa		Trigeminal hypesthesia	Gross total resection 100%
	1	E Extracranial			Remaining STR was planned
	4	MP Middle and posterior fossa		42.8% improvement
	2	MPE Middle and posterior fossa and extracranial extension		Pain
	3	ME Middle fossa and extracranial extension		100% improvement
Park et al (2020)[17]	9	M Middle fossa	Endoscopic transorbital approach/endoscopic endonasal approach	Trigeminal hypesthesia	Gross total resection 48%
				Improvement 41.4%	Near total resection 28%
	8	E Extracranial	Endoscopic transorbital approach/endoscopic endonasal approach	Same 28.5%	Gross total and near total resection 76%
					Endoscopic trans-orbital approach (Gross total and near total resection rate) 81.8%
	8	MP Middle and posterior fossa	Endoscopic transorbital approach/endoscopic endonasal approach or endoscopic trans-orbital approach ± retrosigmoid lateral suboccipital
					Endoscopic endonasal approach (Gross total and near total resection rate) 69.2%
	0	P Posterior fossa	−
Li et al (2021)[18]	8	M Middle fossa	FTSA: frontotemporal subdural approach; FTEA: frontotemporal epidural approach; STAA: subtemporal epidural anterior transpetrosal approach; STTA: subtemporal transtentorial approach	Remission rate of facial numbness 17.2%	Gross total and near total resection rate- 90.69%
	6	P Posterior fossa	SRSA: suboccipital retrosigmoid approach
	19	MP Middle and Posterior	FTSA: frontotemporal subdural approach; FTEA: frontotemporal epidural Approach; SRSA: suboccipital retrosigmoid approach; CSITA: combined supratentorial-infratentorial approach
	3	ME Middle AND extracranial extension	EEA: endoscopic endonasal approach; FTOZA: frontotemporal-orbitozygomatic approach
	2	MPE Middle and posterior fossa and extracranial extension	EEA: endoscopic endonasal approach
	5	E Extracranial extension	EEA: endoscopic endonasal approach

Methods

We performed a retrospective review of all patients who underwent surgery for trigeminal schwannomas between January 2012 and May 2021 at the institute. The clinical records of patients were analyzed according to surgical approach, preoperative and postoperative neurological and cranial nerve deficit(s) status, and surgical complications documented at follow-up visits. The extent of resection was defined by comparing pre- and postoperative 3.0 T cranial magnetic resonance imaging (MRI) using T1 ± contrast agent sequences by manual volumetric segmentation.

Statistical analysis was performed using the software—R Software (Version 4.0.3). Normal distribution was assumed according to the central limit theorem. Data in text and graphs are shown as median with interquartile range or mean ± standard deviation (SD). The following p values have been considered as significant: p < 0.05.

Results

A total of 83 cases of large trigeminal (≥3 cm) schwannomas were operated in our center from 2012 to 2021, and the clinical details are summarized in [Table 4]. As an institute policy, any schwannomas less than 3 cm was subjected to Gamma Knife radiosurgery (GKRS) unless causing debilitating symptoms like trigeminal neuralgia, long tract signs, or cranial nerve deficits. Hence, our series differs from any previous reported ones where the percentage of large (>3 cm) schwannomas ranges from 40 to 65%.[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] The median follow-up period was 6 months (2–12 months). The majority of our cases were young individuals (30–39 years) with median age of 33 years ([Table 5]) and an almost equal incidence among males and females (43:40).

Table 4 Characteristics of the study population

Total Number of patients		83
Age	Median, IQR	33, 44–26
Gender	Males	43
	Females	40
Symptoms	Seizures	1
	5th Nerve	72
	6th Nerve	19
	7th Nerve	27
	8th Nerve	24
Neurological deficits	Visual disturbances	19
	5th nerve motor deficits	62
	5th nerve sensory deficits	61
	Motor deficits	15
Dimensions (≥3cm)	Length (median, IQR)	4, 4.9–3.5
	Breadth (median, IQR)	3.5, 4.2–3.0
Consistency (n = 61)	Solid	30
	Solid cystic	29
	Cystic	2
Middle fossa involvement (n = 71)	Yes	65
	No	6
Brain stem compression (n = 71)	Yes	58
	No	13
Approach types	FTOZ	43
	RMSOC	13
	Subtemporal (ST)	21
	Combined Petrosal (CP)	1
	Endonasal (EN)	5
Follow-up	Percentage of people followed-up	66.3
	Median duration	6 m
	IQR	2–12 m
	Trigeminal motor weakness	46
	Trigeminal sensory weakness	38
	Facial paralysis	31
	Hearing loss	11
	Residue (intraoperative impression) Residue(postoperative MRI)	45 20
Mode of treatment of residue (n = 20)	GKRS	8
	Reexploration	12

Abbreviations: FTOZ, frontotemporal-zygotomy; GKRS, Gamma Knife radiosurgery; IQR, interquartile range; MRI, magnetic resonance imaging; RMSO, retrosigmoid approach.

Table 5 Age and gender distribution

Gender	Male	Fema le	Total
Age
0–9	0	0	0
10–19	5	6	11
20–29	10	4	14
30–39	18	13	31
40–49	7	11	18
50–59	2	3	5
60–69	1	3	4
Total	43	40	83

As expected the most common mode of presentation was fifth nerve symptoms (sensory and motor) followed by seventh, eighth cranial nerve, and then sixth nerve ([Table 6]). Due to the large tumor burden in most of our cases, visual disturbance due to secondary optic atrophy (postpapilledema) was seen in 19 cases (23%), and motor deficits due to long tract involvement were seen in 15 cases (18%). The follow-up was available for 71 out of 83 cases (85.5%). However, preoperative characteristics and immediate postoperative resection details were available for all ([Tables 7] and [8]).

Table 6 Clinical features of study population

S.No.	Age	Gender	Trigeminal symptoms		Other cranial nerves affected				Motor symptoms	Seizures
			Sensory symptoms	Motor symptoms	2nd	6th	7th	8th
1	40	Female	No	No	Yes	No	No	No	No	No
2	52	Female	Yes	Yes	Yes	Yes	No	No	No	No
3	21	Male	Yes	Yes	No	No	No	Yes	No	No
4	22	Male	Yes	Yes	No	No	Yes	Yes	No	No
5	30	Male	Yes	Yes	Yes	No	Yes	Yes	No	No
6	26	Female	Yes	Yes	No	No	Yes	Yes	No	No
7	26	Female	Yes	Yes	No	No	No	Yes	No	No
8	30	Female	Yes	Yes	No	No	No	No	No	No
9	30	Female	Yes	No	No	No	Yes	No	Yes	No
10	30	Male	Yes	Yes	No	No	Yes	No	Yes	No
11	36	Male	Yes	Yes	No	No	No	No	Yes	No
12	35	Female	Yes	Yes	No	Yes	Yes	Yes	No	No
13	53	Female	Yes	Yes	No	Yes	No	No	No	No
14	35	Female	Yes	Yes	Yes	No	Yes	Yes	No	No
15	63	Male	Yes	Yes	No	No	Yes	Yes	No	No
16	43	Female	No	Yes	No	No	Yes	No	No	No
17	47	Female	Yes	No	No	No	No	No	No	No
18	45	Female	Yes	Yes	Yes	Yes	No	No	No	No
19	32	Male	Yes	Yes	Yes	Yes	No	No	No	No
20	21	Male	Yes	Yes	No	Yes	Yes	Yes	Yes	No
21	21	Male	Yes	Yes	No	Yes	Yes	Yes	Yes	No
22	45	Female	Yes	Yes	No	Yes	No	No	No	No
23	46	Female	Yes	Yes	Yes	No	Yes	Yes	Yes	No
24	48	Female	Yes	Yes	−	No	Yes	Yes	Yes	No
25	40	Male	Yes	Yes	No	Yes	No	No	No	No
26	43	Male	Yes	Yes	No	No	No	No	No	No
27	17	Female	Yes	No	No	No	Yes	Yes	Yes	No
28	39	Female	Yes	Yes	Yes	No	Yes	Yes	No	No
29	48	Male	Yes	Yes	No	No	No	No	No	No
30	38	Male	Yes	Yes	No	No	No	Yes	No	No
31	47	Female	No	Yes	No	No	No	No	No	No
32	30	Male	Yes	Yes	No	Yes	No	Yes	No	No
33	31	Female	Yes	Yes	No	No	No	No	No	No
34	33	Male	Yes	No	Yes	No	Yes	No	No	No
35	33	Male	Yes	No	Yes	No	Yes	No	No	No
36	26	Male	No	Yes	−	Yes	No	No	No	No
37	26	Male	No	Yes	Yes	Yes	No	No	No	No
38	26	Male	No	Yes	−	Yes	No	No	No	No
39	38	Female	No	No	No	No	No	No	No	No
40	38	Female	Yes	Yes	No	No	No	No	No	No
41	16	Female	Yes	Yes	No	Yes	Yes	No	No	No
42	32	Male	Yes	Yes	No	No	No	No	No	No
43	31	Female	No	No	No	No	No	Yes	No	No
44	38	Female	No	Yes	Yes	No	No	No	No	No
45	63	Male	Yes	No	No	No	No	No	No	No
46	18	Male	No	No	No	No	No	No	No	No
47	18	Male	Yes	Yes	Yes	No	Yes	Yes	Yes	No
48	28	Male	No	Yes	No	No	Yes	Yes	Yes	No
49	67	Female	Yes	Yes	No	No	No	No	No	No
50	67	Female	Yes	Yes	No	No	Yes	No	No	No
51	34	Male	Yes	Yes	No	No	No	No	No	No
52	32	Male	No	Yes	No	No	No	No	No	No
53	52	Female	No	No	Yes	Yes	No	No	Yes	No
54	27	Female	No	No	No	No	No	No	No	No
55	65	Female	Yes	Yes	No	Yes	No	No	No	No
56	45	Male	No	No	No	No	No	No	No	No
57	46	Male	No	No	No	No	No	No	No	No
58	41	Female	Yes	Yes	No	No	Yes	No	Yes	No
59	24	Male	Yes	Yes	No	Yes	No	No	No	No
60	49	Male	Yes	No	No	No	Yes	Yes	No	No
61	32	Female	No	No	Yes	No	No	No	No	No
62	45	Female	Yes	Yes	Yes	Yes	Yes	Yes	No	Yes
63	32	Male	No	No	No	No	Yes	Yes	Yes	No
64	17	Male	Yes	Yes	No	No	Yes	No	Yes	No
65	19	Female	No	No	Yes	No	No	No	No	No
66	37	Male	No	Yes	No	No	Yes	Yes	No	No
67	36	Male	Yes	Yes	Yes	Yes	No	No	No	No
68	30	Male	Yes	Yes	No	No	No	No	No	No
69	18	Male	Yes	Yes	Yes	No	No	Yes	No	No
70	33	Male	Yes	No	No	No	No	No	No	No
71	66	Male	Yes	Yes	No	No	No	No	Yes	No
72	38	Male	Yes	Yes	No	No	No	No	No	No
73	38	Male	Yes	Yes	No	No	No	No	No	No
74	19	Female	Yes	Yes	No	No	No	No	No	No
75	19	Female	Yes	Yes	No	No	No	No	No	No
76	18	Male	Yes	Yes	No	No	No	No	No	No
77	31	Female	Yes	Yes	No	No	No	No	No	No
78	48	Female	Yes	No	No	No	No	No	No	No
79	20	Female	No	Yes	No	No	No	No	No	No
80	24	Male	No	Yes	No	No	No	No	No	No
81	32	Female	Yes	No	No	No	No	No	No	No
82	42	Male	Yes	Yes	No	No	No	No	No	No
83	19	Female	Yes	Yes	No	No	No	No	No	No

Table 7 Tumor characteristics in study population

S.No.	Tumor characteristics
	Consistency	Length	Breadth	Height	Brainstem compression	Middle Fossa involvement
1	Solid	−	−	−	No	Yes
2	Solid	−	−	−	No	Yes
3	Solid	−	−	−	Yes	Yes
4	Solid	−	−	−	Yes	No
5	Solid	6	5.5	5	Yes	Yes
6	Solid	−	−	−	Yes	Yes
7	Solid	−	−	−	Yes	Yes
8	Solid cystic	−	−	−	Yes	Yes
9	Solid cystic	5.2	3.4	−	Yes	Yes
10	Solid	−	−	−	Yes	Yes
11	Solid	4	3.5	3	Yes	No
12	Solid	−	−	−	No	Yes
13	Solid	−	−	−	Yes	Yes
14	Solid	−	−	−	Yes	Yes
15	Solid	−	−	−	Yes	Yes
16	Solid	3.4	1	0.6	No	Yes
17	Solid	2	3	1.8	Yes	No
18	−	−	−	−	Yes	Yes
19	Solid	−	−	−	Yes	Yes
20	Solid cystic	4.5	3.5	3.5	No	Yes
21	−	−	−	−	Yes	No
22	Solid cystic	4.2	2.1	−	Yes	Yes
23	Solid cystic	−	−	−	Yes	Yes
24	Solid cystic	−	−	−	Yes	Yes
25	Solid cystic	−	−	−	No	Yes
26	Solid	−	−	−	No	No
27	Solid cystic	−	−	−	No	Yes
28	Solid cystic	−	−	−	Yes	Yes
29	Solid cystic	−	−	−	Yes	Yes
30	Solid cystic	−	−	−	Yes	Yes
31	Solid cystic	−	−	−	Yes	Yes
32	Solid cystic	−	−	−	Yes	Yes
33	Solid cystic	−	−	−	Yes	Yes
34	Solid cystic	−	−	−	Yes	Yes
35	Solid cystic	−	−	−	Yes	Yes
36	Solid cystic	−	−	−	Yes	Yes
37	−	−	−	−	Yes	No
38	−	−	−	−	−	−
39	−	−	−	−	−	−
40	Solid cystic	−	−	−	No	Yes
41	Solid cystic	−	−	−	Yes	Yes
42	Solid cystic	−	−	−	No	Yes
43	Solid cystic	−	−	−	Yes	Yes
44	Solid cystic	−	−	−	No	Yes
45	Solid	3.7	3.5	2.9	Yes	Yes
46	−	−	−	−	−	−
47	Solid cystic	−	−	−	Yes	Yes
48	Solid cystic	4	4	3.7	Yes	Yes
49	Cystic	4.4	2.7	2.2	Yes	Yes
50	−	−	−	−	−	−
51	Cystic	4.4	2.7	2.2	Yes	Yes
52	Solid cystic	−	−	−	Yes	Yes
53	Solid	−	−	−	No	Yes
54	Solid	4.9	3.3	−	Yes	Yes
55	Solid	−	−	−	Yes	Yes
56	Solid	3.2	3.5	4.2	Yes	Yes
57	Solid	3.2	3.5	4.2	Yes	Yes
58	Solid	4	3.3	−	Yes	Yes
59	Solid	−	−	−	Yes	Yes
60	Solid cystic	3.5	3.7	3.8	−	−
61	Solid cystic	3	2.8	−	Yes	Yes
62	Solid	5.2	4.5	4.5	Yes	Yes
63	Solid	−	−	−	Yes	Yes
64	Solid cystic	3.6	6.3	−	Yes	Yes
65	Solid	−	−	−	−	−
66	Solid	−	−	−	No	Yes
67	Solid	6.5	4.6	5.4	Yes	Yes
68	Solid	−	−	−	−	−
69	Solid	−	−	−	−	−
70	Solid	−	−	−	−	−
71	Solid cystic	5.1	3.6	4	Yes	Yes
72	−	−	−	−	Yes	Yes
73	Solid cystic	−	−	−	Yes	Yes
74	Solid cystic	−	−	−	−	−
75	Solid	−	−	−	Yes	Yes
76	Solid cystic	−	−	−	Yes	Yes
77	−	−	−	−	−	−
78	Solid cystic	−	−	−	Yes	Yes
79	Solid	−	−	−	Yes	Yes
80	Solid	−	−	−	Yes	Yes
81	Solid cystic	−	−	−	Yes	Yes
82	Solid	−	−	−	−	−
83	Solid cystic	−	−	−	Yes	Yes

Table 8 Surgery and outcomes for the cases

S.No.	Approach Used	Extent of resection (per-operative impression)	Radiologically defined residue present (size cm)	Follow-up duration (days)	Gamma Knife radiosurgery	Re-exploration
1	ST	STR	3.5 × 3	4,515	No	Yes
2	FTOZ	STR	1.5 × 1	71	Yes	No
3	FTOZ	STR	3.3 × 3	303	No	Yes
4	RMSOC	GTR	No	3,330	No	No
5	FTOZ	NTR	No	86	No	No
6	RMSOC	STR	2 × 1	814	No	Yes
7	ST	GTR	No	130	No	No
8	FTOZ	GTR	No	2,731	No	No
9	ST	STR	1 × 1	3,114	No	Yes
10	ST	STR	2 × 3	100	No	Yes
11	RMSOC	GTR	No	2,449	No	No
12	FTOZ	STR	0.5 ×  0.3	2,036	Yes	No
13	FTOZ	GTR	No	2,851	No	No
14	RMSOC	GTR	No	2,697	No	No
15	FTOZ	STR	1.1 ×  1.9	1,283	Yes	No
16	FTOZ	STR	0.3 ×  0.4	1,310	Yes	No
17	EN	NTR	No	1,462	No	No
18	FTOZ	GTR	No	177	No	No
19	ST	STR	No	82	No	No
20	ST	GTR	No	14	No	No
21	FTOZ	GTR	No	0	No	No
22	ST	GTR	No	258	No	No
23	ST	STR	2.4 ×  3.1	551	No	Yes
24	FTOZ	NTR	No	649	No	No
25	FTOZ	STR	3.4 ×  1.1	1,104	No	Yes
26	EN	NTR	No	0	No	No
27	EN	GTR	No	107	No	No
28	ST	GTR	No	1,444	No	No
29	FTOZ	STR	No	60	No	No
30	RMSOC	STR	4.1 ×  1.7	603	No	Yes
31	EN	STR	1.2 ×  2.4	674	Yes	No
32	FTOZ	GTR	No	169	No	No
33	FTOZ	GTR	No	44	No	No
34	FTOZ	NTR	No	153	No	No
35	RMSOC	STR	1.3 ×  0.3	369	Yes	No
36	EN	STR	No	0	No	No
37	FTOZ	STR	No	0	No	No
38	RMSOC	NTR	No	0	No	No
39	FTOZ	STR	4.1 ×  1.4	6	No	Yes
40	ST	GTR	No	649	No	No
41	FTOZ	STR	No	150	No	No
42	RMSOC	STR	3.2 ×  2.2	120	No	Yes
43	FTOZ	STR	1.3 ×  3.1	250	No	Yes
44	FTOZ	STR	0.2 ×  0.5	246	Yes	No
45	ST	STR	No	28	No	No
46	FTOZ	NTR	No	88	No	No
47	RMSOC	STR	No	88	No	No
48	FTOZ	GTR	No	391	No	No
49	FTOZ	NTR	No	241	No	No
50	RMSOC	GTR	No	139	No	No
51	FTOZ	NTR	No	106	No	No
52	ST	NTR	No	411	No	No
53	FTOZ	GTR	No	134	No	No
54	ST	GTR	No	50	No	No
55	ST	STR	No	5	No	No
56	FTOZ	GTR	No	0	No	No
57	FTOZ	GTR	No	0	No	No
58	CP	STR	1.2 ×  1.6	242	Yes	No
59	ST	GTR	No	37	No	No
60	RMSOC	NTR	No	0	No	No
61	ST	STR	No	0	No	No
62	FTOZ	GTR	No	0	No	No
63	FTOZ	GTR	No	0	No	No
64	FTOZ	NTR	No	0	No	No
65	FTOZ	GTR	No	0	No	No
66	ST	NTR	No	0	No	No
67	ST	GTR	No	0	No	No
68	FTOZ	NTR	No	27	No	No
69	ST	GTR	No	7	No	No
70	RMSOC	STR	1.2 ×  3.4	153	No	Yes
71	FTOZ	GTR	No	0	No	No
72	FTOZ	GTR	No	0	No	No
73	FTOZ	GTR	No	0	No	No
74	FTOZ	GTR	No	0	No	No
75	FTOZ	GTR	No	0	No	No
76	FTOZ	GTR	No	0	No	No
77	FTOZ	GTR	No	0	No	No
78	ST	GTR	No	0	No	No
79	FTOZ	GTR	No	0	No	No
80	ST	GTR	No	0	No	No
81	FTOZ	NTR	No	0	No	No
82	RMSOC	STR	No	0	No	No
83	FTOZ	GTR	No	0	No	No

Abbreviations: EN, endonasal; FTOZ, frontotemporal-zygotomy; GTR, gross total resection; NTR, near total resection; RMSO, retrosigmoid approach; ST, subtemporal; STR, subtotal resection.

The large tumor volume has a reflection on the clinical results, as well as the rates of gross total removal (GTR) in the present study (75.9%) are lower compared to the previously reported series. Postoperative facial paralysis was seen in an additional four cases (27 had preoperative facial weakness) and hearing loss improved in 13 cases (24 had hearing loss preoperatively). Among the 20 cases, 12 out of 20 needed resurgery and 8 cases were amenable for GKRS.

Surgical Approaches

Frontotemporal-Zygotomy (± Orbitotomy) + Subtemporal Interdural Approach

Tumors with a predominant middle fossa component ([Fig. 1]) were operated on using the frontotemporal-zygotomy (FTOZ/FTZ) approach. Detailed descriptions of the same have been previously described. The patient is positioned supine with a large shoulder roll under the ipsilateral scapula, and the head is rotated to the opposite side so that the temporal region is parallel to the ground and the head is slightly tilted downward so that the zygomatic arch is at the highest point. The head is fixed using a Mayfield 3-pin. The head end of the operating table is elevated just above the level of the heart. A preoperative lumbar drain may be placed to aid in extradural retraction during the procedure, based on the surgeon's preference. We prefer a curvilinear scalp incision which begins at the region of the root of the zygoma just in front of the tragus (but within the hairline), extends superiorly for about 2 cm, and curves posteriorly gently above the upper level of the pinna of the ear till posterior margin of the pinna and then turns superiorly and then anteriorly in a gentle curve to end just at the hairline anteriorly. In the earlier years, while a single-piece FTOZ was routine, in the recent years, we have avoided the orbitotomy and just performed the FTZ. Also, the skin incision has changed and a linear (vertical) scalp incision is being used as per the surgeon's preference. The most important is that the squamous temporal should be drilled to be flush with the middle cranial fossa (MCF) base. The temporal lobe dura-mater is then retracted superiorly with gentle extradural dissection along the middle cranial fossa floor. At the foramen spinosum, the middle meningeal artery is coagulated and cut to allow for further medial extradural dissection. The extradural dissection ends at just lateral to foramen Ovale. This is followed by entering into the interdural plane.

Fig. 1 (A) Preoperative axial contrast-enhanced magnetic resonance images of a large dumbbell shaped trigeminal schwannoma extending through Meckel's cave. (B,C) Preoperative sagittal and coronal T2 magnetic resonance images showing the majority of the schwannoma in the middle fossa with preoperative displacement of the Basilar artery and posterior cerebral arteries. (D) Postoperative axial contrast-enhanced magnetic resonance images showing complete excision of the lesion via a left fronto-orbital approach. (E,F) Postoperative sagittal and coronal T2 magnetic resonance images showing complete excision of the lesion.

Subtemporal Interdural Approach (Video Supplement)

The trigeminal nerve runs through the interdural space between the meningeal and periosteal dura and is covered with a thin membrane called the inner layer. Tumors with almost equivalent tumor presence in the middle and posterior fossa were operated on using a subtemporal (ST) interdural approach based on this anatomical feature ([Fig. 2]). Positioning and craniotomy are similar to the previous approach with the absence of fronto-orbital and zygomatic exposure ([Video 1]). In cases with infratemporal tumor extension, the lateral portion of the middle cranial base is removed until the tumor margin is exposed. The middle meningeal artery and superficial greater petrosal nerve are treated in the same manner. The periosteal dura, which is the outer layer of the dura matter and can be dissected between the periosteal dura and the meningeal dura, and the interdural space occupied by the tumor, can be entered. By peeling and tacking the meningeal dura, the tumor can be exposed without exposing the temporal lobe. The tumor, covered by only a thin membrane known as the inner layer, should then be visible. By preserving the inner layer between the cavernous sinus, the tumor can be removed without opening the cavernous sinus because this inner layer is covered around the tumor.

Fig. 2 (A,B, and C) Preoperative axial, sagittal, and coronal contrast-enhanced magnetic resonance images of a large trigeminal schwannoma with avid contrast enhancement and brainstem compression. (D,E, and F) Postoperative axial, sagittal, and coronal contrast-enhanced magnetic resonance images showing decompression via a combined petrosal route and a small residue over the brain stem which was later irradiated with Gamma Knife

Video 1 A surgical video demonstrating the subtemporal interdural route for excision of large trigeminal schwannomas.

Generally, in trigeminal schwannomas, autopetrosectomy occurs due to the tumor growth pattern and rarely is petrosectomy required. If required, which is generally to widen the opening, drilling of the bone within this triangle constitutes anterior petrosectomy and is usually done using a diamond drill. The tumor can be followed along Meckel's cave into the posterior fossa and the tumor excised. It is important to preserve the arachnoid of the tumor as the cranial nerves in the posterior fossa lie outside this. For tumors that are adherent to the brain stem, a small portion can be left behind to avoid serious brainstem complications. The tumor is exposed and excised in a piecemeal fashion following microsurgical principles. Following tumor resection, closure of the defect is done using free fat graft and is layered with fibrin glue as closure in this region is not feasible. The convexity dura mater is approximated in the usual fashion. Care must be taken to properly wax all the bone edges for any exposed air cell. Alternatively, a pedicled muscle graft obtained by splitting the temporalis muscle can be rotated to fill the dural defect and can be loosely sutured to the adjacent subtemporal dura mater. The lumbar drain is usually continued for 2 to 3 days in the postoperative period.

The FTOZ was done to provide access to the anterior cavernous sinus and allowed greater accessibility to the structures. However, when there was no significant anterior extension, only a zygotomy was performed to provide access to the base.

Retrosigmoid Approach

The retrosigmoid approach, which remains the workhorse for cerebellopontine angle lesion, can provide adequate access for small-to-medium tumors of the petroclival region or even large cystic lesions with Meckel's cave extension ([Fig. 3]). It has the advantage of less risk of injury to the venous structures around the petrous bone. However, it involves working through small windows between neurovascular complexes of the CP angle cistern with only one angle of visualization. For large, calcified, or highly vascular lesions, significant cerebellar retraction is usually needed for adequate exposure of the tumor. Moreover, visibility of the petroclival dura mater, the ostium of Meckel's cave, and tumor relationship with the brainstem and vessels medial to the cranial nerves remain poor. Hence, this approach is ideally limited to cases with large posterior fossa components with minimal supratentorial extension.

Fig. 3 (A,B, and C) Preoperative axial, sagittal, and coronal contrast-enhanced magnetic resonance images of a large trigeminal schwannoma with maximum bulk in the posterior fossa and extension to middle fossa. (D,E, and F) Postoperative axial, sagittal, and coronal contrast-enhanced magnetic resonance images showing excision via a retro mastoid route with decompression of the middle fossa component via Meckel's cave and a small residue on the trigeminal nerve which remained indolent on follow-up.

Extended Endoscopic Approach

The extended EA has evolved to become a much refined minimally invasive technique; however, it is suitable for predominantly tumors restricted to Meckel's cave ([Fig. 4]) and clivus rather than those with significant lateral extension. Even with an adequate reconstruction of the defect using vascularized flaps, EA is associated with a high risk of cerebrospinal fluid (CSF) leak and meningitis.

Fig. 4 (A,B, and C) Preoperative axial, sagittal, and coronal contrast-enhanced magnetic resonance images of a large trigeminal schwannoma with predominant component in the midline in Meckel's cave. (D,E, and F) Postoperative axial, sagittal, and coronal contrast-enhanced magnetic resonance images showing decompression via an endoscopic endonasal route with no residue.

Although the aforementioned approaches work fine in general for a particular type of tumor location, the choice of approach should not be purely dogmatic and preferably should be tailored based on patient age, comorbidities, performance status, the anticipation of the presence or absence of arachnoid plane between the tumor and the brainstem, the pre-operative clinical status, as well as the acquaintance and experience of the operating surgeon with the particular procedure.

Radiosurgery

Only 8 out of 20 residual cases treated needed GKRS with a mean dose of 12 Gy. The timing of the same was on the discretion of the treating surgeon and usually was based on the volume of the residual tumor. In two cases, a large residue (>1 cm) was seen and early GKRS was planned (within 3 months of surgery) so as to prevent the increase in the volume. In the other six cases, a small residue was seen postoperatively (<1 cm) and time to recovery was given for the cranial nerves. Late GKRS (>12 months) was planned, and tumor growth was evident at the time of radiosurgery. This strategy allowed for maximizing recovery while preventing the tumor volume outpace radio-surgical safety.

Discussion

Classification

Our series is unique as all cases included were >3 cm in dimension and not amenable for radiosurgery. Consequently, most of our cases were multicompartmental. To decide the approach, we propose going back to the 1955 classification of Jefferson. He first classified trigeminal schwannoma into three types: type A originated from GG and was located at the middle fossa; type B originated from the trigeminal root and was located at the posterior fossa; type C was dumbbell-shaped and occupied in both middle and posterior fossa. After 31 years, Lesoin et al improved the TS classification by supplementing the type of tumor originating from three peripheral branches of the trigeminal nerve and occupying the extracranial space. This classification has been used for deciding the approach to trigeminal schwannoma in our series and helps in quick and uncomplicated management in most cases. Diverse classifications have been proposed to facilitate decision-making and assess the technical difficulty ([Table 1]). However, we believe that the choice is mainly whether the tumor has to be approached via a posterior or a middle/anterior fossa approach.

Surgical Outcomes

In our series, good clinical outcomes were achieved with visual improvement in 100% (19 out of 19), trigeminal motor improvement in 26% (16 out of 62), trigeminal sensory improvement in 38% (23 out of 61), hearing improvement in 54% (13 out of 24), and additional facial paralysis only in 4.8% cases (4 out of 83). This was achieved in large multicompartment tumors with a rate of GTR (75.9% radiological clearance) which is in line with contemporary series (vary between 48% and 100%). Our series compares very favorably with the rates of improvement in clinical symptoms in previously reported series which are generally low (7–43%).[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] Symptoms persisted in 25 to 80% of patients and worsened in 5 to 47%. The only area where our results were in line was the improvement in trigeminal sensory or trigeminal pain which has been reported to be 33 to 100%. Other cranial nerve deficits were reported involving CN IV, VI, and VII. Konovalov et al[8] reported 5% CN III, 12% CN IV and VI, and 8% CN VII palsies. Fukaya et al[10] reported CN VI palsy (6.3%), CN VII palsy (4.2%), CN IV palsy (2.1%), and CN VIII palsy (2.1%). In the Wanibuchi et al series,[7] there was 72% improvement in CN VI function in those presenting with diplopia secondary to an abducens palsy, with the remaining 28% function remaining static. Of the three patients with preoperative CN III palsy, one improved, one remained the same, and one deteriorated. The abducens nerve seems particularly vulnerable, and this may be related to its fairly complex anatomical relationship to Meckel's cave, as previously described.

While GTR is preferable, it is preferable to target functional preservation/improvement, especially in benign diseases like schwannoma.[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] Thus, our series shows a trend of lower rates of cranial nerve dysfunction (rather improvement) achieved using classic and well-known skull-base approaches. As good functional outcomes with GTR can be achieved and deterioration is avoidable, we emphasize a surgical strategy to prevent cranial nerve injury, especially in the case of huge schwannomas. The 48.1% resection rate was defined via per-operative impression where a part of capsule was left adherent to the vessels or nerves. However, when postoperative MRI was reviewed the rate of resection was 75.9%. This phenomenon has also been demonstrated in previous series.[20] Eventually, only 20 cases needed further treatment.

Approach Selection

The basic division made by Samii et al[4] puts the options in a nutshell: type A, intracranial tumor predominantly in the middle fossa; type B, intracranial tumor predominantly in the posterior fossa; type C tumors in the middle and posterior fossa; and type D extracranial tumor with intracranial extensions. Type A, C, and D tumors can be targeted via a middle fossa approach, whereas type B tumors are well managed through a retrosigmoid technique; in the case of Type C tumors, a combined approach may be necessary.

Complication Avoidance

Cranial nerve dysfunction/injury can result in either motor or sensory and contrary to popular belief both of them are equally disabling. Motor deficits can range from trigeminal nerve dysfunction in approximately 20 to 30% of cases and are persistent while sensory function invariably returns to normal in 3 to 5 months. Peeling off the tumor over the nerves and interdural dissection under nerve monitoring with careful dural incision and minimum bipolar use are some measures that can prevent these complications. Other cranial nerves can be affected depending on the tumor extension. These include trochlear nerve deficits, third nerve palsy, facial palsy, and even abducent nerve injury. Frontalis muscle weakness can result from injury of the peripheral frontal branches of the facial nerve over the temporal muscle and an interfascial method of dissection with avoidance of the fat plane can prevent these injuries.

Skull base approaches allow better exposure of these tumors, multiple working angles with minimal brain retraction, and more complete removal without increased morbidity. However, it is important to look for exuberant sinus and meticulously seal them after the drilling to avoid subsequent CSF leak, and hence, a meticulous repair is necessary to prevent this additional morbidity especially in cases with infratemporal extension.

Limitations of Microsurgery and Role of Radiosurgery

In the current era of onco-functional utopia, leaving a radio-surgical residue at the cost of preventing a new deficit is advisable. Anterior approaches provide good exposure above the internal acoustic meatus, it may not be suitable for more inferior lesions and those stuck to the brainstem/ premedullary cistern and other cranial nerves. Leaving a small residue to be later managed by radiosurgery or a second posterior approach would be desirable. These approaches initially provide a rather narrow corridor allowing limited degrees of freedom which expands as decompression is continued. Moreover, there can be a risk of injury to the vein of Labbe secondary undue temporal lobe retraction, sigmoid sinus, and jugular bulb in posterior approaches. This can be avoided by a preoperative lumbar drain placement or careful drilling respectively.

In a recent retrospective review[19] on the role of radiosurgery, the mean tumor volume was 5.5 cm³, time to follow-up was 56 months, the chance of clinical improvement was 48%, tumor control was 91%, and clinical worsening or new symptoms was 12% (most commonly trigeminal neuropathy/pain). Any comparison to surgical series has to take into consideration a larger tumor volume, the chance of a quicker and longer-lasting relief in trigeminal pain and higher preoperative dysfunction in presurgical cases when compared to radiosurgery cases. Microsurgery versus radiosurgery, in comparison, microsurgery has reduced tumor control (78 vs. 91%), and greater morbidity (35 vs. 12%), including postoperative mortality (1%) when compared to radiosurgery. While many patients had preoperative trigeminal neuropathy, 11.6 to 33% of patients experienced worsened function in the form of facial pain or hypesthesia after initial surgical removal. Radiosurgery led to a 92% tumor control rate and 94% clinical improvement or stabilization rate. Microsurgical decompression and salvage/adjuvant radiosurgery for critical areas remain the best course of action for large multicompartmental lesions.

Conclusion

Large (>3 cm) multicompartmental trigeminal schwannomas are benign lesions and are best managed by a single skull base approach which aims at excision. At the same time, functional preservation or rather improvement should be the primary goal rather than gross total excision. Since these lesions have a benign course, carefully respecting anatomy and using epidural/interdural approaches that do not violate the pial plane and aiming for maximum resection remain the dictum. Radiosurgery either upfront or on signs of growth for residual lesion can prevent morbidity.

Conflict of Interest

None declared.

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Publication History

Received: 21 December 2022

Accepted: 27 March 2023

Article published online:
17 May 2023