Keywords
hypotympanum - hypotympanotomy - jugular bulb - glomus jugulare - trabeculae - crotch
- band of Seibermann - temporal bone dissection
Introduction
Hypotympanum is so far ignored space in otological surgery but it is very important
in glomus jugulare tumors and infra cochlear approaches to petrous apex. It is a space
below the annulus tympanicus containing air cells and trabecule ([Figs. 1]
[2]
[3]). The lateral wall of hypotympanum is formed by tympanic bone; the medial wall is
formed by petrous bone. Posteriorly, it is limited by the styloid complex and facial
nerve. It is actually a bony groove below the mesotympanum marked by air cells of
various sizes and bony septa called the trabeculae. The upper lateral border of hypotympanum
is formed by tympanic ring of the tympanic bone. The upper lateral border of hypotympanum
is formed by tympanic ring of the tympanic bone. The medial wall is formed by promontory,
these two structures are the only consistent ones. Medially and inferiomedially, the
floor of hypotympanum is formed by tympanic bone that meets the petrous part of temporal
bone. The shape of the hypotympanum is influenced by protrusion of jugular bulb into
tympanic cavity.[1] The other structure lying anteriomedially is the carotid canal; in majority of cases,
the anterior wall of the hypotympanum is formed by petrous bone. Actually, the hypotympanum
is developed from saccus posticus.
Fig. 1 Anatomy of hypotympanum. 1. Trabeculae longa, 2. Finiculus, 3. Trabeculae Profunda,
4. Fustis.
Fig. 2 Anterior hypotympanic sinus. 5. Anterior Hypotympanic Sinus, 6. Protinaculum.
Fig. 3 Posterior hypotympanic sinus. 7. Subfustis Sinus, 8. Posterior Hypotympanic Sinus,
9. Pyramidal Leg of Fustis, 10. Styloid Leg of Fustis.
Materials and Methods
Two hundred six cases of live operations were studied, these included 160 cases of
simple dry perforations for myringoplasty and 46 cases of stapedectomy operations.
All operations were performed under local anesthesia using a 0-degree, 4-mm endoscope.
The hypotympanum was visualized. Twenty wet temporal bones were dissected endoscopically
and complete hypotympanic details were studied.
Discussion
We examined 206 ear cases, using a 0-degree, 0.4-mm endoscope. These included cases
with chronic suppurative otitis media (CSOM), with central perforations CSOM, with
cholesteatoma, and normal ears with otosclerosis. According to our findings, the endoscopic
anatomy of hypotympanum is of three types.
Type A Hypotympanum
Type A hypotympanum is present in 45% of cases. Normally, the depth of the hypotympanum
from the inferior bony annulus to floor of hypotympanum is 8 to 10 mm. It contains
air cells and 8 to 10 trabeculae; The anterior most and the longest trabeculae is
called the trabeculae longa. It makes the Surface marking for crotch jugular carotid
septum. We frequently find anterior and the posterior hypotympanum sinuses in these
cases. The jugular bulb is very low associated with backward placed sigmoid sinus.[2] Finiculus is long and associated with infra cochlear space. If this area is well
pneumatized, one can almost see the curvature of the basal turn of the cochlea ([Fig. 4]).[3]
Fig. 4 Type A hypotympanum.
Type B Hypotympanum
Type B hypotympanic sinus is present in 25% cases; the depth of the hypotympanum is
4 to 5 mm from the inferior bony tympanic ridge. This type of sinus is associated
with sparse trabeculae with limited infracochlear space. Here, the finiculus is short
([Fig. 5]).
Fig. 5 Type B hypotympanum.
Type C Hypotympanum
In type C hypotympanum, the jugular bulb is above the inferior tympanic sulcus. It
is present in 30% of cases; no trabeculae or funiculus is present. In 10% of these
cases, the jugular bulb touches the round window.[4] There is no infra cochlear space and this type of hypotympanum is associated with
forward placed sigmoid sinus ([Fig. 6]).
Fig. 6 Type C hypotympanum.
Transmeatal Endoscopic Dissection of Hypotympanum and Jugular Foramen
Transmeatal Endoscopic Dissection of Hypotympanum and Jugular Foramen
In our 206 cases, the majority of hypotympanum were type A hypotympanum. The transmeatal
endoscopic dissection in these types of hypotympanum are of four types.
Level 1
After elevation of tympanomeatal flap superiorly, the whole hypotympanum is visualized
and the trabeculae, cells, and finiculus are identified. The Arnold nerve coming out
from the finiculus is noted and passes over the promotory. It is the area of Type
1 glomus tumor (tympano jugular). All wells of hypotympanum are noted with 0-degree
and 45-degree 0.4-mm endoscopes ([Fig. 7]).
Fig. 7 Level 1. 11. Arnold’s nerve.
Level 2
After removal of all trabeculae, hypotympanotomy is performed leaving the inferior
bony tympanic with using zero-degree endoscope transmeatally.
At this level, canalus tympanicus is exposed in the bone. The canalus tympanicus is
a bony canal transmitting the Arnold nerve and the inferior tympanicus. It is the
site for the origin of type 2 (glomus tympanicus), i.e., tympanojugular paraganglioma
([Fig. 8]).[5]
Fig. 8 Level 2. 12. Canalis tympanicus, 13. Temporomandibular joint.
Level 3
Once the level 2 dissection is over using a 5-mm Diamond burr, the inferior bone along
with the floor of the hypotympanum is drilled slowly. This exposes the Jugular bulb,
Crista, the connective tissue band of Seibermann, and the crotch ([Figs. 9]
[10]
[11]
[12]).
Fig. 9 Level 3. 14. Jugular bulb.
Fig. 10 Level 3 crista. 15. Crista.
Fig. 11 Level 3 band of Seibermann. 23. Connective tissue band of Seibermann.
Fig. 12 Level 3 crotch. 16. Crotch, 17. ICA (internal carotid artery),18. Jugular bulb.
Level 4
Once the crotch is carefully removed at the lower level, the jugular bulb posteriorly
and internal carotid artery anteriorly are exposed. Pushing the jugular bulb posteriorly
helps to visualize the glossopharyngeal nerve first, after separation of glossopharyngeal
nerve from the surrounding connective tissue. The Vagus and Accessory nerves are exposed
and the other structures of jugular foramen are visualized ([Figs. 13]
[14]
[15]).
Fig. 13 Level 4 glossopharyngeal nerve. 20. Vagus nerve, 21. Accessory nerve.
Fig. 14 Level 4 opening of jugular bulb. 19. Glossopharyngeal nerve.
Fig. 15 Level 4 vagus. 22. Opening of Jugular bulb.
Conclusion
Hypotympanum is so far a neglected space in otological surgery. Recently, after the
use of endoscopes transmeatal, the hypotympanum is clearly visualized with its minute
details, including finiculus, trabeculae, Arnold nerve, hypotympanic cells and anterior
and posterior hypotympanic sinuses. Endoscopic hypotympanum is divided into three
types: type A, type B, and type C. In our observation, Type A hypotympanum is more
common followed by type C and type B. Our endoscopic endomeatal cadaveric dissection
of hypotympanum exposes the jugular foramen with its contents transmeatally. It may
be helpful for future endoscopic otological surgeries, that is limited jugular foramen
tumors and type B glomus tympanicum tumors.
