3.3: Normal variants in ear and nose imaging

Balaji Ayyamperumal

Introduction

It is important to recognize the variations of normal anatomy in the head and neck become perhaps more important medically and surgically. The variations discussed here in the following headings are complimented by a few other details discussed in other related chapters.

1. Sinuses
2. Temporal bone
3. Neck spaces

Sinuses

All sinus variants are discussed under the Chapter 3.24 Paranasal sinuses – Anatomy and pathology.

Critical Variants are discussed here with clinical applications:

• Critical variants are best appreciated on coronal images.
- • Cribriform plate
• Lateral lamella is the thinnest part of the cribriform plate and is at risk of fracture during FESS, especially when the olfactory fossae are deep.
• Fractures of the cribriform plate can cause a defect in the dura that lead to the early complication of ascending meningitis and later intracranial hypotension from CSF leak, meningocele, or meningoencephalocele.
- • Lamina papyracea
• Congenital or posttraumatic defect of the lamina papyracea can provide a direct route for sinus surgery instruments to cause orbital injury – medial rectus muscle laceration, orbital hematoma, and orbital fibrosis.
• The anterior ethmoidal artery courses through a bone canal in the superior lamina papyracea and serves as a surgical landmark. When it courses below the skull base in the ethmoidal air cells, the artery can be inadvertently injured. Retraction of the injured artery into the orbit can cause an orbital hematoma, requiring decompression.

Attachment of the main sphenoid septum

• The main sphenoid septum is often eccentrically resulting in asymmetry of the sphenoid sinuses.
• Identification of such variants – to avoid iatrogenic vascular or neural injury.

Hyperpneumatization in sphenoid sinus

• Focal bone dehiscence of the sphenoidal sinus places the adjacent optic nerve, internal carotid artery, and trigeminal nerve at risk of injury by surgical instruments. The optic nerve also may be at risk when Onodi cells are present.
• The artery may bulge into the sinus in 65%–72% of patients. There may be dehiscence/absence of the thin bone separating the artery and the sinus in 4%–8% of cases.
• Pterygoid canal or the groove of the maxillary nerve may project into the sphenoid sinus, which may result in trigeminal neuralgia secondary to sinusitis.
• Anterior clinoid process pneumatization is associated with type II and type III optic nerve and predisposes this nerve to injury during FESS.

Normal variants of temporal imaging (Table 3.3.1)

Facial nerve dehiscence

Mostly seen in the tympanic segment near the oval window. Severe anomalies of the course of the facial nerve occur in the tympanic and vertical portions. The horizontal segment at times is displaced inferiorly to cover the oval window or lies exposed over the promontory. The facial canal is usually rotated laterally.

TABLE 3.3.1

Enumerates Some of the Variants with Further Discussion Below

Diagnosis	Findings	Comments
Cochlear cleft	Especially in children. Lucency lateral to apical turn.	Non retrofenestral otosclerosis.
Lucent periotic zone in infants
Incudal “hole”	Lucency in incus body.	Partial volume effect.
High jugular bulb	The jugular bulb is above the caudal level of the posterior semicircular canal or above IAC.	Frequently a diverticulum of the jugular bulb.
Fatty marrow in petrous apex	High signal intensity on T1 and T2 turbo spin echo.	Misinterpreted as lesion when unilateral. Compare signal intensity with subcutaneous fat.
Bulging sigmoid sinus	Anterior impression on the posterior surface of the mastoid.	To be avoided during mastoidectomy.
Pseudofractures	Cochlear aqueduct, petromastoid canal.	Be aware of sutures.

Dural exposure

Tegmen of the mastoid and attic usually passes in a horizontal plane slightly lower than the arcuate eminence produced by the top of the superior semicircular canal. A depression of the tegmental plate is caused by the deepening of the floor of the middle cranial fossa forming a groove lateral to the attic and to the labyrinth. The low hanging dura may cover the roof of the external auditory canal. The risk of penetration of the cranial cavity during surgery should be anticipated.