Incomplete partition type I with partial rhombencephalosynapsis and arachnoid cysts

A two-year-old female child with complaints of developmental delay and hearing impairment was referred to the radiology department for imaging. BERA (Brain evoked response audiometry) showed bilateral profound sensorineural hearing loss

On imaging, both cochlea showed absent modiolus with cystic appearance along with dilated both vestibules, forming a figure of 8 appearance. Upper halves of both posterior semicircular canals were atretic. Bilateral lateral semicircular canals were dysplastic due to small bony island. Both the superior semicircular canals were normal. Bilateral vestibular aqueducts were mildly dilated.

Incidentally seen were arachnoid cysts along bilateral cerebellopontine cisterns, larger on left side, causing mass effect on the underlying left middle cerebellar peduncle in form of its medial displacement.

We also found that the posterior and inferior part of both cerebellar hemispheres appeared fused to each other with continuous cerebellar folia along the midline. Hypoplastic anterior vermis above the fusion is seen. Bilateral dentate nuclei were small. These features were suggestive of partial rhombencephalosynapsis.

Background

Incomplete partition type I (IP-1)is a cochleovestibular malformation where the cochlea appears cystic due to an absent modiolus and interscalar septae[1]. It accounts for around 20% of all inner ear malformations[2]. No specific genes have been associated with IP-1 [3] and currently proposed pathogenetic mechanisms include abnormal vascular supply from the internal auditory canal vessels leading to defective endosteum formation, speculated to be due to an insult in the fifth gestational week[4]. No syndromic association has been described yet in these cases.

Clinical Perspective

IP-1 patients have profound hearing loss. Audiologically, it is more severe than the other types of incomplete partitions and shows severe to profound sensorineural hearing loss[1]. CT and MRI help differentiate the various types of partition anomalies and help in surgical planning for cochlear implantation by defining dimensions of the cochlea, internal acoustic canal (IAC) dimensions, evaluation of cochleovestibular nerves and course of facial nerve [1].        

Imaging Perspective

On CT/MRI, empty cystic cochlea is noted, in its normal location with absence of bony modiolus. Cochlear dimensions may be normal[2]. This is usually accompanied by enlarged vestibules which gives rise to a ‘figure of 8 appearance’. Dilatation of vestibular aqueducts are rare in IP-1 and is an atypical feature[5]. IAC may be enlarged in these cases[1].

Outcome

Cochlear implantation can be performed in IP-1; however surgical challenges including altered landmarks for approach, anomalous facial nerve course, risk of peri lymphatic fluid gusher due to widened IAC and increased risk for recurrent meningitis exist[1]. Proper preoperative assessment with high resolution CT and MR helps in defining regional anatomy, helping surgeons to know what to expect intraoperatively. Post-implant results have been satisfactory, depending on neural function and adequate follow up to prevent meningitis[1].

Take Home Message

Incomplete partition type I is cystic cochleovestibular malformation with complete absence of modiolus and interscalar septae.

Written informed patient consent for publication has been obtained.