Introduction
Congenital anomalies of the orbit and internal auditory canal (IAC) can present with vision loss, hearing loss, strabismus, or cosmetic deformity. CT and MRI are complementary:
- CT → bony anatomy.
- MRI → soft tissues, cranial nerves, brain association.
Part 1: Orbital Anomalies
1. Microphthalmos / Anophthalmos
- Microphthalmos = abnormally small globe.
- Anophthalmos = complete absence of globe (rare, may have small cystic remnant).
- CT/MRI:
- Microphthalmos → small globe, calcification possible (colobomatous cyst).
- Associated with CNS anomalies (holoprosencephaly, midline defects).
2. Coloboma
- Defect in closure of embryonic fissure.
- CT/MRI: keyhole-shaped defect, often inferonasal.
- Can be isolated or with CHARGE syndrome.
3. Dermoid / Epidermoid Cysts
- Most common orbital congenital lesion.
- CT: well-defined, fat attenuation, sometimes calcification.
- MRI: dermoid = T1 hyperintense (fat), epidermoid = diffusion restriction.
4. Craniofacial Syndromes (Apert, Crouzon, Pfeiffer)
- CT: shallow orbits → proptosis.
- Often with midface hypoplasia, craniosynostosis.
Part 2: Internal Auditory Canal (IAC) Anomalies
Normal IAC Anatomy on MRI/CT
- Contains: CN VII, VIII (vestibulocochlear), labyrinthine vessels.
- IAC abnormalities are often linked to sensorineural hearing loss.
1. IAC Stenosis / Atresia
- IAC diameter < 2 mm (normal ~ 4 mm).
- CT: narrowed or absent IAC.
- MRI: may show absent or hypoplastic CN VIII.
- Clinical: congenital deafness, cochlear implant planning.
2. Vestibulocochlear Nerve Aplasia / Hypoplasia
- MRI (3D CISS/FIESTA):
- Absent or thin vestibulocochlear nerve.
- CN VII usually present.
- Associated with cochlear hypoplasia.
3. Enlarged Vestibular Aqueduct (EVA)
- One of the most common congenital inner ear anomalies.
- CT: vestibular aqueduct > 1.5 mm.
- Often associated with sensorineural hearing loss, Pendred syndrome.
4. Cochlear Nerve Canal Deficiency
- CT: bony canal < 1.4 mm.
- Often correlates with cochlear nerve hypoplasia on MRI.
Imaging Pearls ✨
- Orbit → think size (microphthalmos/anophthalmos), shape (coloboma), and cystic lesions (dermoid).
- IAC → always check size of IAC and presence of nerves on MRI.
- CT + MRI combination is essential in surgical/implant planning.
📊 Quick Comparison: Orbital vs IAC Anomalies on Imaging
| Category | Anomaly | CT Findings | MRI Findings |
|---|---|---|---|
| Orbital Anomalies | Microphthalmos / Anophthalmos | Small or absent globe, may see cystic remnant or calcification | Hypoplastic globe, associated CNS anomalies |
| Coloboma | Keyhole-shaped defect in globe, usually inferonasal | Defect with cystic extension, better delineation of optic nerve involvement | |
| Dermoid Cyst | Well-defined, fat attenuation, sometimes calcification | T1 hyperintense (fat), no enhancement; restricted diffusion if epidermoid | |
| Craniofacial Syndromes (Apert, Crouzon) | Shallow orbits, proptosis, craniosynostosis | Confirms orbital crowding, associated intracranial anomalies | |
| IAC Anomalies | IAC Stenosis / Atresia | IAC < 2 mm or absent bony canal | Absent / hypoplastic CN VIII on high-res MRI |
| Vestibulocochlear Nerve Aplasia / Hypoplasia | May see associated cochlear canal narrowing | Absent or thin CN VIII on 3D CISS/FIESTA; CN VII usually preserved | |
| Enlarged Vestibular Aqueduct (EVA) | Aqueduct > 1.5 mm at midpoint or operculum | Confirms endolymphatic sac enlargement, better soft tissue detail | |
| Cochlear Nerve Canal Deficiency | Bony canal < 1.4 mm | Correlates with cochlear nerve hypoplasia or absence |
📝 Key Takeaway
- Orbit → size (micro/anophthalmos), shape (coloboma), and cysts (dermoid).
- IAC → always assess canal diameter + presence of CN VIII on MRI (especially before cochlear implant surgery).
- CT = bones / size, MRI = nerves / soft tissue / associations.
✅ Conclusion
- Orbital anomalies: best evaluated with CT (bones, fat, calcifications) and MRI (soft tissue, CNS associations).
- IAC anomalies: require high-resolution CT (bone canal) and MRI (nerve integrity).
- Recognizing these findings is crucial in children with visual or auditory developmental delay and in pre-operative assessment for cochlear implants.
