Michel Aplasia & Mondini Dysplasia - MS ENT PG Exam Notes

Overview: Classification of Inner Ear Malformations

Inner ear malformations are responsible for ~20% of congenital SNHL. The classic classification (Jackler) divides them into two broad groups:

Malformations limited to membranous labyrinth (bony labyrinth normal, >90% of congenital deafness):

Scheibe (cochleosaccular dysplasia) - most common
Siebenmann-Bing (complete membranous dysplasia)
Alexander (basal turn dysplasia)

Malformations of osseous + membranous labyrinth (radiologically detectable):

Michel aplasia (complete labyrinthine aplasia)
Cochlear aplasia
Cochlear hypoplasia
Incomplete partition (Mondini)
Common cavity

Incidence of Cochlear Malformations (Cummings Table 194.1)

Malformation	Incidence
Incomplete partition / Mondini dysplasia	55% (most common)
Common cavity	26%
Cochlear hypoplasia	15%
Cochlear aplasia	3%
Michel aplasia	1% (most severe, rarest)

MICHEL APLASIA (Complete Labyrinthine Aplasia)

Key Facts

First described by: Michel (1863)
Developmental arrest: before 3rd gestational week (before otic vesicle formation)
Result: Complete absence of all inner ear structures - cochlea, vestibule, semicircular canals; the otic capsule is entirely absent
The petrous bone itself is aplastic/severely malformed
External ear and middle ear may be unaffected
Inheritance: Autosomal dominant (AD); recessive also possible

Pathology

No otic vesicle forms; the entire membranous and osseous labyrinth is absent
Associated with anencephaly and thalidomide exposure

Radiology (HRCT temporal bone)

Complete absence of the otic capsule on axial and coronal cuts
External auditory canal and middle ear may appear normal
Must differentiate from labyrinthine ossification (ossification shows a dense otic capsule of normal dimensions; aplasia shows no capsule)

Hearing

Profound anacusis (total deafness) - bilateral
Conventional hearing aids: no benefit
Cochlear implant: NOT a candidate (no cochlea or nerve to stimulate)
Vibrotactile devices: beneficial in some patients

MONDINI DYSPLASIA (Incomplete Partition / IP-II)

Key Facts

First described by: Carlo Mondini (1791) - in a young deaf man who had died of gangrene
Developmental arrest: 6th-7th week of gestation
Most common radiologically detectable cochlear malformation (~55%)
Inheritance: Autosomal dominant; may be unilateral

The Classic Mondini Triad (IP-II)

Cochlea with only 1.5 turns (normal = 2.5 turns) - basal coil is normal; middle and apical coils fuse into a single cystic apex (scala communis / apical scala communis)
Dilated/enlarged vestibule (minimal dilation)
Enlarged vestibular aqueduct (EVA)

Pathology

Only the basal coil is clearly identifiable
Absence/deficiency of the osseous spiral lamina and interscalar septum at the apex
Endolymphatic duct is usually enlarged
Abnormal communication between endolymphatic/perilymphatic spaces and subarachnoid space (via cribriform area of the lateral IAC) - predisposes to perilymphatic fistula

Radiology (HRCT)

Cochlea smaller than normal: 5-6 mm (normal 8-10 mm vertically)
Absence of interscalar septum - the key radiological feature (not just counting turns, as coronal CT can make a normal cochlea appear to have only 1.5 turns due to the oblique angle)
Dilated vestibular aqueduct (>1.5 mm at midpoint - Valvassori criteria)

Incomplete partition (Mondini) - MRI (A) and CT (B) showing absence of interscalar septum

Cummings Otolaryngology, Fig. 194.8 - Incomplete partition (Mondini): axial MRI (A) and CT (B) showing absence of interscalar septum

Hearing

Variable SNHL - may range from mild to profound
Some residual hearing is common (neurosensory structures are present in most cases)
Early hearing rehabilitation is indicated
Cochlear implant: CANDIDATE - but with important surgical caveats

Associated Syndromes

Pendred syndrome (most important association - goiter + SNHL + EVA + Mondini)
Waardenburg syndrome
Treacher Collins syndrome
Wildervanck syndrome
CHARGE association (Mondini-type deformity + absent semicircular canals + coloboma + heart disease + choanal atresia + retarded growth + genital hypoplasia + ear anomalies)
Congenital CMV infection (accounts for >40% of deafness of unknown etiology)

Incomplete Partition Subtypes (Sennaroglu Classification)

Type	Description	Key Features
IP-I	Cystic cochlea-vestibular anomaly	No modiolus, no interscalar septa; cystic cochlea + dilated vestibule; NO EVA
IP-II	Classic Mondini	1.5 turns, absent apical interscalar septum, dilated vestibule + EVA
IP-III	X-linked deafness	Interscalar septa present but NO modiolus; associated with X-linked DFNX2 (SLC17A8)

Comparison: Michel vs. Mondini (High-Yield for MCQs)

Feature	Michel Aplasia	Mondini Dysplasia
Described by	Michel (1863)	Carlo Mondini (1791)
Developmental arrest	<3rd week (before otic vesicle)	6th-7th week
Cochlea	Completely absent	1.5 turns (basal coil present)
Vestibule/SCCs	Absent	Mildly dilated vestibule
Vestibular aqueduct	Absent	Enlarged (EVA)
Otic capsule	Absent on CT	Present
Incidence	1% (rarest)	55% (most common)
Hearing	Anacusis	Variable SNHL
CI candidacy	No	Yes (with precautions)
CSF gusher risk	N/A	Yes (perilymphatic-CSF communication)

Surgical Pearls (Cochlear Implant in Mondini)

CSF gusher is the major intraoperative hazard - due to abnormal CSF-perilymph communication through the cribriform plate defect in the IAC
Higher risk of meningitis with or without CI
MRI pre-op is mandatory to confirm cochlear nerve presence
An absent cochlear nerve is an absolute contraindication for CI
Consider short/compressed electrode arrays; insertion may be difficult

Memory Aids

Michel = Most severe, Missing everything (no inner ear at all) - arrest before 3rd week
Mondini = Most common cochlear malformation, 1.5 turns - arrest at 6-7th week
Mondini triad: 1.5 turns + dilated vestibule + EVA (Pendred syndrome is the most tested association)
Scheibe = most common of ALL congenital deafness (membranous only, normal bony labyrinth)

Sources: Cummings Otolaryngology Head and Neck Surgery, 7th ed., Ch. 194; KJ Lee's Essential Otolaryngology; Scott-Brown's Otorhinolaryngology, Vol. 2

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