Michel Aplasia and Mondini Deformity

MS ENT - 20 Marks University Exam Answer

INTRODUCTION - Inner Ear Malformations

Fig. Jackler's classification - embryogenesis of cochlear malformations. Note: Arrest at 7th week = Mondini; arrest before 3rd week = Michel aplasia.

PART I: MICHEL APLASIA (Complete Labyrinthine Aplasia)

Definition

Historical Note

Embryology / Pathogenesis

• Results from a developmental arrest before the formation of the otic vesicle - i.e., before the end of the 3rd gestational week (some sources say before week 4)
• The otic placode fails to invaginate, so no otic vesicle forms
• The entire bony and membranous labyrinth is absent
• The external and middle ear may be unaffected (they develop from separate branchial arch structures)

Pathology

• Complete agenesis of the petrous portion of the temporal bone
• No cochlea, no vestibule, no semicircular canals, no internal auditory canal (IAC)
• The otic capsule is entirely absent
• External ear canal and middle ear may be present normally

Radiology (HRCT temporal bone - key investigation)

• Complete absence of the otic capsule on axial and coronal HRCT
• Ear canal and middle ear present but inner ear structures absent
• MRI confirms absence of 8th nerve in IAC (aplasia of vestibulocochlear nerve often coexists)
• Must be differentiated from labyrinthine ossification - in ossification, the otic capsule is dense and of normal dimensions; in Michel aplasia, the otic capsule bone is absent

Audiological Features

• Profound bilateral SNHL (anacusis / total deafness) from birth
• No response on ABR (Auditory Brainstem Response)
• No otoacoustic emissions

Genetics

• Autosomal dominant (AD) inheritance has been observed
• Autosomal recessive inheritance also reported
• Inherited complete labyrinthine aplasia has been described

Management

• Conventional hearing aids - no benefit (no cochlea or auditory nerve)
• Cochlear implant (CI) - CONTRAINDICATED (no cochlea or nerve)
• Auditory Brainstem Implant (ABI) - may be attempted if cochlear nerve absent
• Vibrotactile devices - have proven beneficial in some patients (Lee's)
• Special education and rehabilitation for deaf-blind communication
• Genetic counseling for family

PART II: MONDINI DEFORMITY (Incomplete Partition)

Definition and Historical Note

Embryology / Pathogenesis

• Developmental arrest at the 7th week of gestation
• At this stage the cochlea is forming its final coils; arrest produces only 1.5 turns instead of the normal 2.5-2.75 turns
• The interscalar septum fails to develop in the apical turn
• Results in a scala communis - the middle and apical turns merge into a single cystic cavity

Pathology

1. Cochlea with only 1.5 turns (normally 2.5-2.75 turns)
2. Absent interscalar septum in the apical portion - forming an "apical scala communis"
3. Enlarged vestibular aqueduct (EVA) and endolymphatic duct/sac

• Enlarged vestibule
• Deficient or absent modiolus (bony core of cochlea)
• Variable organ of Corti development
• Variable spiral ganglion neuron population
• Perilymphatic-endolymphatic communication abnormality due to defect in the cribriform area of the lateral IAC wall - leads to perilymphatic fistula risk

Histopathology (Photomicrograph)

Fig. 194.9 - Normal cochlea (A, all turns visible) vs. Mondini dysplasia (B, 1.5 turns, apical scala communis).

Sennaroglu Classification of Incomplete Partition (3 subtypes)

Radiological Features (HRCT Temporal Bone)

• Cochlea smaller than normal: normally 8-10 mm vertically; in Mondini typically 5-6 mm vertically
• 1.5 cochlear turns (instead of 2.5-2.75)
• Absent interscalar septum (more evident on CT than MRI)
• Absent or deficient modiolus
• Enlarged vestibular aqueduct (midpoint diameter > 1.5 mm)
• Enlarged vestibule
• Semicircular canal deformities in ~20% of cases
• HRCT is the imaging of choice; MRI (T2 fast spin echo) shows internal detail

Audiological Features

• Variable hearing loss - from normal to profound SNHL
• Mean hearing threshold ~75 dB (three-tone average) in a series of 41 ears (Cummings)
• Fluctuating hearing loss - a hallmark, due to CSF-perilymph pressure changes through the fistula
• May be unilateral
• "Gusher" phenomenon - perilymph gusher on stapedotomy (due to widened IAC and abnormal communication with CSF)

Genetics and Associations

• Autosomal dominant inheritance described
• Autosomal recessive Mondini dysplasia also reported
• DFN3 locus deletions - associated with familial Mondini dysplasia (X-linked)
• Described in multiple syndromes:
  • Pendred syndrome (goiter + SNHL)
  • Waardenburg syndrome
  • Treacher Collins syndrome
  • Wildervanck syndrome (Cervico-oculo-acoustic syndrome)
  • CHARGE syndrome (Coloboma, Heart defects, choanal Atresia, Retardation, Genital hypoplasia, Ear anomalies) - Mondini-type deformity + absent SCCs
• Non-genetic: Congenital CMV infection - may account for >40% of deafness of unknown etiology

Complications

1. Recurrent meningitis - due to abnormal CSF-perilymph communication via cribriform area of IAC
2. Perilymphatic fistula - more common than in normal inner ear
3. Perilymph gusher during ear surgery
4. Progressive SNHL

Management

• Early hearing aid fitting (most have residual hearing)
• Early audiological assessment (ABR, OAE, ASSR)
• The presence of neurosensory structures in most cases warrants early rehabilitative intervention with conventional amplification

• Most patients with Mondini deformity are candidates for CI (presence of cochlear nerve is the key criterion, not cochlear anatomy)
• Requires special surgical considerations:
  • Risk of perilymph gusher (profuse CSF flow on cochleostomy) - managed by gentle packing with fascia
  • Risk of facial nerve anomalies
  • Modified insertion technique may be needed
  • Full electrode insertion is usually possible
  • Outcomes with CI are generally good

• Avoid unnecessary middle ear surgery (stapedotomy risk of perilymph gusher)
• Cover any perilymphatic fistula if suspected
• In case of recurrent meningitis - close fistula/seal IAC

• Pneumococcal and Hib vaccination recommended (as in all inner ear malformation patients)
• Antibiotic prophylaxis for recurrent meningitis may be considered

COMPARISON TABLE: Michel Aplasia vs. Mondini Deformity

ADDITIONAL CONTEXT: Classification of Inner Ear Aplasias

1. Michel aplasia - arrest before week 3; complete absence of inner ear; anacusis
2. Common cavity - arrest at week 4; cochlea + vestibule form one undifferentiated cystic space
3. Cochlear aplasia - arrest at week 5; absent cochlea, vestibule + SCCs present
4. Cochlear hypoplasia - arrest at week 6; small cochlear bud (1 turn or less); variable hearing
5. Mondini (incomplete partition) - arrest at week 7; 1.5-turn cochlea; cystic apex; most common

• Scheibe aplasia (cochleosaccular dysplasia) - most common; AR; bony labyrinth normal; associated with Usher, Refsum, Jervell-Lange-Nielsen, congenital rubella
• Alexander aplasia - basal coil organ of Corti affected; high-frequency SNHL; amplification beneficial

• Cummings Otolaryngology Head and Neck Surgery, Chapter 194 (Inner Ear Malformations)
• K.J. Lee's Essential Otolaryngology, Chapter on Inner Ear Structural Malformations