Keywords
PIN - posterior interosseous nerve syndrome - intraneural lipoma - lipoma
Introduction
The upper extremity is a common site of compressive neuropathies. Comprehensive knowledge of its anatomy and the relationship of each nerve with different muscles and vascular structures is vital to accurately diagnose the specific nerve and compression site involved. Although less common than carpal tunnel syndrome, posterior interosseous nerve (PIN) syndrome is a condition that requires special attention, as it may lead to misdiagnosis if not correctly considered.[1]
The radial nerve arises from the C5-T1 nerve roots and comes off the posterior cord of the brachial plexus. It travels down the arm and divides into superficial and deep branches in the proximal forearm. In most cases, the deep branch dives into the posterior compartment between the supinator heads and emerges as the PIN. Some anatomical variants are described, mainly involving the arcade of Frohse, and increased susceptibility to impingement may be observed. The PIN innervates the extensor muscles of the forearm, except for the extensor carpi radialis.[2]
Compressive neuropathies affecting the radial nerve distal to the elbow include radial tunnel syndrome (RTS), PIN syndrome, and Wartenberg syndrome. Each of these conditions has distinct symptoms that can help to establish the correct diagnosis.[1]
Posterior interosseous nerve syndrome is characterized by an insidious onset, often presenting with a variable deficit in wrist and finger extension and with some degree of radial deviation. However, because the extensor carpi radialis longus is innervated by the radial nerve, wrist extension is frequently unaffected.[3] Most of the times, it is self-limiting and treated conservatively. However, when symptoms are refractory to the nonoperative treatment, surgical decompression is usually required.[4]
The causes of PIN syndrome may be trauma, space-occupying lesions, brachial neuritis, and spontaneous compression.[3] The most common site of compression is at the arcade of Frohse (the proximal edge of the supinator). Repetitive pronation/supination activities can also cause these symptoms.[5]
Unlike carpal tunnel syndrome, PIN syndrome more often affects males than females, in a 2 to 1 ratio. It also occurs about twice as often in the right limb.[5] There is sometimes a painless presentation of PIN syndrome that may lead to misdiagnosis, most frequently with radial tunnel syndrome and tendon rupture, especially when the neurological findings are incomplete (since there is a sensitive functional impairment in RTS).
The prognosis is particularly good (for both neuropraxis and axonotmesis) in space-occupying lesions, with a good chance of complete recovery if the onset of symptoms does not exceed 12 weeks.[6]
Case Report
A 72-year-old woman with type-2 diabetes mellitus and arterial hypertension was referred to our department after being unable to extend the fingers of her right hand for 11 months. The development of symptoms was progressive and simultaneous with the appearance of a subcutaneous volar radial mass on her forearm.
Upon physical examination, there was a deficit in the extension of the fingers and a painless, mobile, soft, and regular subcutaneous tumor. There was no radial or ulnar deviation, nor sensory deficits. The wrist extension strength was mantained (score of 4/5 on the Medical Research Council Scale for Muscle Strength) ([Figs. 1]
[2]
[3]
[4]).
Fig. 1 Preoperative dorsal view.
Fig. 2 Preoperative volar view.
Fig. 3 Preoperative extension.
Fig. 4 Preoperative extension.
The ultrasound study showed an hyperechogenic mass with hypoechogenic signal, and the MRI showed homogeneous hyperintensity both in T1- and T2-weighted scans.
We chose a modified posterolateral approach with an S-shape incision. Intraoperatively, we could easily identify the PIN, and an intraneural lipoma wrapped around the nerve fibers was identified and totally excised, without major damage to the nerve ([Figs. 5]
[6]
[7]). The anatomopathological examination revealed hyperplasia of the adipocytes with some fibrous tissue, confirming our clinical diagnosis of lipoma.
Fig. 5 Intraoperative intraneural PIN lipoma.
Fig. 6 Intraoperative intraneural PIN lipoma.
Fig. 7 Lipoma.
After 3 months of rehabilitation, full recovery was achieved with complete extension of the fingers and strength against resistance mantained (score of 5/5 on the Medical Research Council Scale for Muscle Strength) ([Figs. 8]
[9]
[10]
[11] and [Video 1]).
Fig. 8 Postoperative flexion.
Fig. 9 Postoperative extension.
Fig. 10 Postoperative scar.
Fig. 11 Postoperative comparison with the left hand.
Qualität:
Discussion
The differential diagnosis for PIN syndrome includes RTS, Wartenberg syndrome and even tendon ruptures. The clinical history and physical examination are vital to establish a diagnosis. Both RTS and Wartenberg syndrome may involve the same sites of compression as those of PIN syndrome; however, they present with forearm pain without motor weakness. Tendon ruptures usually have a more abrupt presentation, with functional deficit since day one, and may be caused by trauma or by fractures (either acute or sub-acute).
Extrinsic or intrinsic compression can cause PIN syndrome. Extrinsic compression is an uncommon condition that requires early diagnosis and prompt surgical treatment. Lipomas of the peripheral nerve are benign tumors with a slow-growing pattern that are composed of white fat cells. They are usually extraneural, causing a slow onset of symptoms, and can be either intramuscular, intermuscular or paraosseous. Intraneural lipomas are very uncommon; only three clinical cases[7]
[8]
[9] have been described in the literature, and it may be misdiagnosed with fibrolipomatous hamartomas of the nerve or other similar conditions. However, these clinical entities present differently, with intraneural lipomas having a well-defined capsule and hamartomas consisting of fibrous and fatty tissue containing nerve fibers.
The best surgical approach must be tailored to each patient as it depends on the anatomical relationship of the tumor with the neurovascular structures. The most common surgical approaches are the posterolateral and anterior. Although the posterolateral approach requires lower levels of dissection, it may be more challenging since the tumors may hinder direct visualization of the PIN.
In the clinical case herein reported, we chose a modified posterolateral approach with an S-shape incision to minimize the risk of contractures and provide better exposure. We found a well encapsulated tumor that was wrapped around nerve fibers and was causing extrinsic compression of the PIN. After total excision of the lipoma, few nerve fibers had to be cut.
To the best of our knowledge, only 3 clinical cases of PIN syndrome caused by an intraneural lipoma leading to extrinsic compression have been described to date ([Table 1]).
Table 1
|
Year
|
Author
|
Age (years)
|
Gender
|
Symptoms*
|
Type of lipoma
|
Pathology
|
Treatment
|
Evolution
|
|
2019
|
Nogueira et al. (present study)
|
72
|
Female
|
0/5a of finger extension
|
Intraneural
|
Nervous fibers adherent to the capsule
|
Total ressection
|
5/5a of extension
|
|
2018
|
Patel et al.[7]
|
66
|
Male
|
0/5a of finger extension
|
Intraneural
|
Nervous fibers adherent to the capsule
|
Total ressection
|
4/5a of extension
|
|
2016
|
Yamamoto et al.[8]
|
60
|
Female
|
Assymptomatic
|
Not specified
|
Nervous fibers in the adipose tissue
|
Subtotal ressection
|
5/5a of extension
|
|
2007
|
Matsuo et al.[9]
|
60
|
Male
|
Dorsal hand paresthesia and
3/5a of finger extension
|
Intraneural
|
Nervous fibers in the adipose tissue
|
Total ressection with sacrifice of the posterior interosseous nerve and tendinous reconstruction
|
4/5a of extension
|
Despite the predictable bad functional prognosis considering the age of the patient and the duration of the symptoms (11 months), the clinical case herein reported shows a dramatic improvement in function and quality of life, without impairment in daily-life activities.
