Neuroradiology Case of the Week

Case 21

Larry Buadu, MD, PhD and P-L Westesson, MD, PhD, DDS

Clinical Presentation: An 11-year-old female presents for cochlear implant evaluation because of profound sensorineural hearing loss.

Radiographic Findings: Thin section CT scans shows an incomplete partition of the cochlear consistent with a true Mondini defect (Figs. 1A & B). There is also incidental note of a sclerotic mastoid on the right side.

Figure 1A	Figure 1B
Figure 1: Axial (1A) and coronal (1B) thin section CT showing incomplete development of the the right cochlear apparatus.
Figure 2A	Figure 2B
Figure 2: Axial section (2A) and coronal (2B) sections of a normal right cochlear apparatus (from another patient) is shown for comparison.

Diagnosis: Mondini Defect.

Discussion:
   Only approximately 20% of patients with congenital sensorineural hearing loss will actually have abnormalities that can be seen on images. There are multiple reasons for congenital sensorineural hearing loss such as viral infection, metabolic disease, labyrinth abnormalities and others. Congenital malformation of the cochlea is generally called a Mondini defect. There are several subgroups of this congenital defect but the incomplete partition is the true Mondini defect. This is the result of a 6-8 gestational week malformation. The hearing loss is usually moderate to variable and in the higher frequencies the hearing loss may not be as profound.
   There are a number of syndromes associated with congenital sensorineural hearing loss such as Crouzon’s disease (craniofacial dysostosis), Apert’s disease, Hurler’s syndrome, Klippel-Feil, Wildervanck, and Waardenburg syndromes [1].
   Many books use the term Mondini’s deformity to describe virtually any malformation of the osseous labyrinth short of complete aplasia. Mondini himself however, never intended this to be case. He described a case of a congenital deafness in which the cochlea was shortened to 1 1/2 turns. The true Mondini deformity occurs secondary to an arrest at the seventh week; only the basilar turn of the cochlea has undergone complete development [2]. Typically the interscalar septum or osseous spiral lamina is incomplete, resulting in a confluency of the apical and middle cochlea turns (incomplete partition) [2]. The vestibule and semicircular canal may or may not be normal.
   It has been emphasized that many patients are not completely deaf. This is probably secondary to the variable development of the membranous labyrinth.
   The development of the semicircular canal takes place later [3] and occurs between weeks six and twenty-two. The lateral (horizontal) semicircular canal is more frequently abnormal. The lateral semicircular canal was normal in this patient. Occasionally other abnormalities are associated such as a narrowing of the internal auditory canal [4] which was not seen in this patient.
   Abnormalities of the middle and external ears may occasionally be associated but it should be emphasized that these structures have mutually independent embryologic origin [5]. Therefore in most situations abnormalities of the inner ear are not associated with middle or external ear abnormalities.

References:

1. Booth JB. Medical management of SNHL: Pt. II: Musculoskeletal J Laryngol Otol 1982;96:773-795.
2. Swartz JD, Harnsberger HR. Imaging of the Temporal Bone, 2nd Ed. Thieme Medical Publishers, 1992.
3. Swartz JD, Mandell DW, Berman SE et al. Cochlear otosclerosis (otospongiosis): CT analysis with audiometric correlation. Radiology 1985;155:147-150.
4. Eelkema E, Curtin HD. Congenital anomalies of the temporal bone. Semin Ultrasound CT MR 1989;10:195-212.
5. Swartz JD, Yussen PS, Mandell DW et al. The vestibular aqueduct syndrome: computed tomographic appearance. Clin Radiol 1985;36:251-253.