Imaging characterization of cochlear malformation is based mainly in temporal bone CT. Inner ear structures should be analyzed in the reference axial plan, parallel to the lateral semicircular canal, as well as in multiplanar reconstructions (MPR).
Cochlear measurements, taken at both reference axial plan and coronal reconstruction, are essential to distinguish CH from other types of IEM (4).
Magnetic resonance imaging (MRI) of the inner ear, espacially high-resolution heavily T2-weighted three-dimensional (3D) sequences on oblique sagittal reconstructions, enable direct visualization of the vestibulo-cochlear nerve bundles. As cochlear nerve aplasia may accompany CH, MRI assessment of each subgroup is essential for cochlear implantation decision (5).
I. Imaging findings of cochlear hypoplasia:
- Computed tomography findings:
Diagnosis of inner ear malformation is usually made on a preoperative radiological assessment of sensorineural hearing loss for cochlear implant candidate population.
Temporal bone CT's interpretation is based on both inner ear morphological analysis and cochlear measurements.
1. Cochlear measurements:
Cochlear measurements performed are illustrated on Fig. 11, based on the method described by Purcell et al. (4).
The cochlea is considered hypoplastic if at least two of this measurements are small.
Normative measurements, established by Purcell et al. are reported in table 1.
Table 1: Description of measuring technique, average values, and standard deviations for patients without sensorineural hearing loss described by Purcell et al. (4).
Structure |
Measurement |
Average (mm) |
SD (mm) |
Description |
Basal turn |
Length |
8.59 |
0.41 |
Easily identified on axial plan. |
Apical turns |
Height |
3.96 |
0.38 |
Measured from apex down to imaginary line separating basal and upper turns (base of the modiolus to apex). |
Cochlea |
Height |
5.31 |
0.42 |
On coronal plan :
Measurement taken on cut with maximum height that included basal and upper turn.
|
2. Morphological analysis : Cochlear hypoplasia's characteristics (1,2):
* Cochlear hypoplasia type 1 (CH I):
This is the more severe subtype of CH. The cochlea is like a small empty bud, arising from the internal auditory canal. There is no internal architecture (Fig. 2).
In this type of cochlear hypoplasia, the cochlea is clearly hypoplastic and there is no need to make cochlear measurements.
* Cochlear hypoplasia type 2 (CH II):
The external architecture of the cochlea is similar to that of a normal cochlea, with no internal architecture. The cochlea has smaller dimensions than normal (Fig. 3). There may be complete absence of modiolus creating a wide connection with the IAC. The vestibular aqueduct is often enlarged.
* Cochlear hypoplasia type 3 (CH III):
The internal and external architecture makes this subtype looking like a normal cochlea, but with smaller dimensions, described as a cochlea with less than two and a half turn, with a short modiolus. Cochlear measurements are of a high importance, and especially cochlear height ( Fig. 4).
* Cochlear hypoplasia type 4 (CH IV):
The cochlea has normal basal turn, but apical turns are hypoplastic and located antero-medially rather than in their central position which is characteristic of this CH subtype (Fig. 5).
- Magnetic resonance findings : Cochlear nerve assessment:
As cochlear implantation decision is directly related to cochlear nerve state, cochlear nerve assessment is mandatory.
It is performed mainly on sagittal reconstruction perpendicular to IAC plan, on the high resolution T2 MR sequence (Fig. 6) (5).
It is known that cochlear nerve deficiency is frequently observed in patients with CH(1). The rate of CN hypoplasia or aplasia is different according CH subtype. Cinar BC et al. (2) reported that in CH IV group there were no case of CN aplasia.
Figures 7 et 8 illustrate cochlear nerve aplasia associated with CH III and cochlear nerve hypoplasia associated with CH IV respectively (Fig. 7, Fig. 8).
II. Management:
* Patients with mild to moderate sensorineural hearing loss can have normal language development with hearing aids (1).
* Cochlear implantation is treatment of choice for patients with severe to profound hearing loss. If the cochlear nerve is aplastic bilaterally, brainstem implantation is the only alternative for the patient(1).
* Cochlear implantation in inner ear malformations and particularly in cochlear hypoplasia cases is challenging and there are particular features to take into account (6):
- Cerebrospinal fluid leakage (gusher) during cochleostomy is more likely to appear when cochlear implant is performed in inner ear malformations. There are some CT finding that can predict and estimate CSF gusher risks, the most important one is the presence of a defect at the lateral end of the IAC (Fig. 9).
- Facial nerve abnormalities may accompany CH malformations and are easily assessed on CT in axial plan (Fig. 10).
- As the cochlea is hypoplastic, the electrode choice should be careful to fit the shape and the size of the cochlea. It is advisable to use thin and shorter electrodes (2).
- In hypoplastic cochlea, the promontory may not present the usual shape and it may be difficult for the surgeon to identify promontory and round window through the facial recess. In these cases, an additional transcanal approach may be necessary to expose the cochlea (1).