Lateral Skull Base Abnormalities in Achondroplasia: A Survey of Pediatric and Adult Radiographs with Critical Insights for the Skull Base Surgeon

Abstract

Achondroplasia (dwarfism) can lead to many bony abnormalities, including in the temporal bone and lateral skull base. These abnormalities may present across the lifespan and be associated with skull base pathologies and abnormalities which the lateral skull base surgeon must be aware of. We focus on three main areas of maldevelopment: the lack of mastoid air cells, the high jugular bulbs, and the ossicular malrotation. The unusual lack of mastoid development results in immediate proximity of the middle ear and ear canal with the middle and posterior cranial fossas. This has implications for otitis media and cholesteatoma, in which there is no reservoir for the pathology to passively spread. Case no. 1 illustrates a 7-year-old male with an otherwise normal middle ear cholesteatoma resulting in skull base erosion and encephalocele. Surgical implications are that the traditional transmastoid approaches are not possible and middle cranial fossa surgery may be indicated for otherwise routine epitympanic disease. The high, towering jugular bulbs which protrude into the middle ear space are an important consideration for the skull base and middle ear surgeon. Case no. 2 illustrates a patient referring for tympanostomy tubes due to perceived fluid in the middle ear. This was in fact the jugular bulb and lancing this as an in-office procedure could have been disastrous. From a hearing perspective, the abnormally rotated ossicles result in unreliable landmarks in the epitympanum, as well as characteristic poor hearing, due to conductive hearing loss. Case no. 3 illustrates a young adult with severe tegmen erosion as well as severe otitis externa which resulted in acquired ear canal atresia. She did have minimal mastoid air cell development, but this ultimately required radical mastoidectomy and encephalocele repair. In summary, these collections of radiographs showcase the propensity for patients with achondroplasia to develop lateral skull base pathologies, and knowledge of these abnormalities is critical for the skull base surgeon.