The document summarizes the development of the inner ear and classifications of congenital malformations that can occur. It describes how the otic placode invaginates during the third week to form the otic vesicle, and how this develops further over subsequent weeks to form the membranous labyrinth. It then classifies congenital malformations into two categories: those limited to the membranous labyrinth, and those involving both the osseous and membranous labyrinth. Examples of specific malformations are given such as incomplete partition of the cochlea, enlargement of the vestibular aqueduct, and abnormalities of the internal auditory canal.

1. Congenital malformation of inner ear
Development of inner ear
DEVELOPMENT OF MEMBRANOUS LABYRINTH
3rd
week- otic placode-thickening of surface ectoderm dorsal to the
first branchial groove.
Otic placode then invaginate to the underlying mesenchyme and
form to auditory pit then to otic vesicle /otocyst which is completely
separated from the ectoderm-at the same time appendage for
endolymphatic duct appear.
4th
week- two flanges (the future semicircular ducts) arise from the
otocyst.
Development then involves elongation of the otocyst and the
appearance of three deepening folds (I, II, and III), which demarcate
the utricle with its three semicircular ducts, the endolymphatic duct
and sac, and the saccule with its cochlear duct.
6th
week- the lumina of the semicircular ducts have formed, and the
macula communis (the primordial macula at the medial wall of the
otocyst) has divided into superior (utricle, superior canal, lateral
canal) and inferior segments (saccule, posterior canal).
At the same time, the cochlear duct has extended from the saccule,
completing one turn during the course of the week.
Between 8 and 16 weeks, the otic labyrinth approaches its adult
configuration, cochlea complete its 2 ½ turn.
At 20 weeks, the superior semicircular duct has reached adult size.

2. The organ of Corti is differentiated to such a degree by 20 weeks that
the fetus can “hear” and respond to fluid-borne sounds. The organ
of Corti approximates the adult structure by 25 weeks.
DEVELOPMENT OF OTIC CAPSULE
develops from the precartilage (compacted mesenchyme that is
differentiating into embryonic cartilage) surrounding it.
Started by the end of fourth week as cell density of the mesenchyme
enveloping otic capsule increases.
By 8th
week cartilaginous model of otic capsule formed.
A total of 14 ossification centers eventually appear and fuse to
complete the ossification of the otic capsule.
CONGENITAL MALFORMATION OF INNER EAR
 Due to Interruption during first trimester of pregnancy- inborn
genetic error or teratogenic.
 Genetic errors- may be either autosomal dominant or recessive
and may manifest as sensorineural hearing loss (SNHL) alone or
be associated with any of a number of syndromes
 Teratogenic - in utero viral infection (e.g,rubella), chemical
teratogens (e.g, thalidomide), and radiation exposure
 Derangement of the otic capsule ossification process alone
does not appear to be a major mechanism in congenital hearing
loss.
 Ossification of the labyrinthine lumen, however, is a common
finding in early acquired deafness, typically arising as a
consequence of meningitis.
Incidence

3.  malformations limited to the membranous labyrinth
predominate-cochlea>SCC>vestibular aqueduct. Note that otic
capsule deformity is radiographically detected but not
membranous deformity.
Classification
Key terms are
1) aplasia, complete lack of development;
2) hypoplasia, incomplete development; and
3) dysplasia, aberration in development.
Congenital anomalies of the inner ear may be considered in two
broad categories: malformations with pathologic changes limited to
the membranous labyrinth and malformations that involve both the
osseous and membranous labyrinth
I. Malformations limited to the membranous labyrinth
A. Complete membranous labyrinthine dysplasia (Siebenmann-Bing)
B. Limited membranous labyrinthine dysplasia
1. Cochleosaccular dysplasia (Scheibe)
2. Cochlear basal turn dysplasia (Alexander)
II. Malformations of the osseous and membranous labyrinth
A. Complete labyrinthine aplasia (Michel)
B. Cochlear anomalies
1. Cochlear aplasia

4. 2. Cochlear hypoplasia
3. Incomplete partition (Mondini)
4. Common cavity
C. Labyrinthine anomalies
1. Semicircular canal dysplasia
2. Semicircular canal aplasia
D. Aqueductal anomalies
1. Enlargement of the vestibular aqueduct
2. Enlargement of the cochlear aqueduct
E. Internal auditory canal abnormalities
1. Narrow internal auditory canal
2. Wide internal auditory canal.
MALFORMATION LIMITED TO MEMBRANOUS LABYRINTH
 account for over 90% of congenital deafness, the bony
labyrinth is normal
A. COMPLETE MEMBRANOUS LABYRINTHINE DYSPLASIA (BING-
SIEBENMANN)
 extremely rare, has been reported in association with the
cardioauditory and Usher's syndromes
B. LIMITED MEMBRANOUS LABYRINTHINE DYSPLASIA
COCHLEOSACCULAR DYSPLASIA (SCHEIBE)
 The cochlear duct is usually collapsed, with Reissner's
membrane adherent to the limbus.

5.  The stria vascularis is typically degenerated and may contain
colloidal inclusions.
 Cochlear changes may be severe in the base turn and gradually
lessen in intensity towards the apex, or they may be severe
throughout.
 The saccule is usually collapsed and has degenerated sensory
epithelium.
COCHLEAR BASAL TURN DYSPLASIA
 May be related to familial high frequency SNHL
Malformation of the membranous and osseous labyrinth
C. COMPLETE LABYRINTHINE APLASIA
 Extremely rare, developmental arrest before appearance of otic
vesicle.
 Associated with anencephaly and thalidomide exposure.
 Radiographucally may be confused with labyrinthine
ossification.
D. COCHLEAR ANOMALIES
Incidence
Incomplete partition (Mondini) -55%
Common cavity -26%
Cochlear hypoplasia- 15%
Cochlear aplasia- 3%
Complete labyrinthine aplasia (Michel)- 1%.
COCHLEAR APLASIA

6.  Cochlea completely absent, presumably arrest at 5th
week
gestation.
 Radiographically only vestibule and SSC present.
COCHLEAR HYPOPLASIA
 Arrest during 6th week of gestation
 Hypoplastic cochlea with single turn or less
 Radiographically a small bud (1-3mm) protrude from the
vestibule which is frequently enlarged
 Hearing is variable, may be remarkably good.
INCOMPLETE PARTITION (MONDINI DEFORMITY)
 Arrest at 7th
week gestation
 Cochlea only 1.5 turns, measuring 5-6mm (normal=8-10mm
vertically)
 Radiographically cochlea smaller than normal and partially or
completely lack interscaler septum.
 Three variants of incomplete partition
 IP1- lack the entire modiolus and interscalar septa and
demonstrate a cystic appearance
 IP2- normal basal turn but cystic apex
 IP3- deficient modiolus and partial interscalar septation at the
cochlea’s periphery.
 Hearing is variable, normal to profound SNHL
E. COMMON CAVITY
 Cochlea and vestibule are confluent forming an ovoid cystic
space without an internal structure.
F. VESTIBULAR AQUEDUCT ENLARGEMENT
 Diameter > 2mm (N=0.4-1mm)-determined radiographically

7.  VA is short and broad and lack vascularity and rugosity
 Large VA syndrome usually b/l, often associated with other
radiographic anomalies of the inner ear.
 Hearing is mild to profound SNHL, gradual deterioration of
auditory function from childhood to adulthood
 Management- steroid administration, shunt procedure,
obliteration of endolymphatic sac.
G. DEVELOPMENTASL ANOMALIES OF IAC
WIDE IAC: >10mm diameter
 Important of wide IAC is its association with spontaneous CSF
leak and the occurrence of csf gusher during stapes surgery.
NARROW IAC: <3mm diameter, may indicate failure of 8th nerve
development.