The endolymphatic sac (ELS) is located at the end of endolymphatic duct and lies within dura of the posterior fossa. The function of the ELS is unknown but it may be involved with the production and resorption of endolymph which is found within the cochlea and semicircular canals. The ELS is located at the aperture of vestibular aqueduct. Tumors of the ELS can grow outward into the cerebellum or cerebellopontine angles mimicking other tumors more commonly found at these sites. A number of posterior fossa and cerebellopontine angle tumors reported in VHL such as choroid plexus tumors, adenomas, and adenocarcinomas may actually be ELS tumors (54,55). The ELS tumors can also erode the vestibular aqueduct to involve the inner ear structures, the semicircular canals and cochlea where they can impair sense of balance and hearing. Perineural involvement of the acoustic nerve and facial nerve can cause hearing damage and facial paresis respectively.
Histologically, these tumors differ from hemangioblastomas but are similar in histology to the papillary cystadenoma of the epididymis (56,57). They are not known to metastasize but are locally aggressive.
Imaging is performed with high resolution CT and MRI through the middle ear (Figure 5).
By nature of their location ELS tumors cause bone erosion and the petrous bone can be moth eaten on CT (57,58) (Figure 5).
Figure 5. MR depicting retinal hemangioblastomas and an endolymphatic
sac tumor. a) Precontrast T1 weighted MRI demonstrates a small left eye
and high signal intensity foci in the epidural space adjacent to the expected
location of the endolymphatic sac (arrow). b) Post contrast T1 weighted
MRI demonstrates intense enhancement of both the eye and the endolymphatic
sac tumor. c.) CT through the petrous ridge demonstrates bone erosion at
the site of the endolymphatic sac tumor, typical of the locally aggressive
behavior of this tumor (curved arrow).
ELS tumors commonly enhance intensely on CT or MRI but may have non enhancing, cystic or necrotic regions. On T1 weighted MRI before contrast media, ELS may be high in signal intensity due to hemorrhage or proteinaceous fluid within the cystic components (59) (Figure 5). Post contrast T1 weighted scans typically show enhancement in the non-cystic component of the tumor. 3-D MRI techniques may be helpful in identifying the relationship of the tumor to the semicircular canals, acoustic nerve and facial nerve. Early detection of ELS tumors is important because prompt surgical intervention may prevent further hearing loss.
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