Head and Neck ImagingExternal ear, pathology
Aural dysplasia
The incidence of aural dysplasia is between 1/3300 to 1/10000 births. Failure of the first branchial cleft to canalize will cause
stenosis or atresia of the external auditory canal (EAC). Very often, these patients will have an abnormal auricle. The degree of deformity of the auricle is associated to the severity of the external canal deformation; in cases of minor
microtia ,
stenosis of the EAC is most common, while in patients with major microtia, atresia is predominant.
Aural dysplasia is usually unilateral and not associated with other deformities; however, it may be associated with a number of syndromes (
| - | Treacher Collins syndrome |
| - | Crouzon syndrome |
| - | Nager syndrome |
| - | Hemifacial microsomia/Goldenhar |
| - | Klippel - Feil syndrome |
| - | Pierre Robin syndrome |
| - | Moebius syndrome |
| - | Duane retraction syndrome |
| - | Craniometaphyseal dysplasia |
| - | Osteopetrosis |
| - | Chromosome 18 deletion syndrome |
| - | Thalidomide embryopathy |
).
A narrow and/or short EAC is associated with dysplasia of the tympanic bone. The canal often runs in an abnormal direction, sloping more upwards than usual. Associated
fibrosis may further narrow or completely obstruct the EAC. The tympanic membrane may be present or absent. Imaging will show the narrow EAC, which may lack a bony floor.
When no EAC is formed, the tympanic bone is not present (aplasia). A bony plate is present where the tympanic membrane is usually located, called 'atresia plate'.
The size of the tympanic cavity is also often diminished, making surgery more difficult. The tegmen may have a low position, which may also complicate the surgical approach. The air content of the mastoid is often reduced, correlating with the degree of abnormality of the external ear.
Ossicular chain abnormalities are very frequently associated with aural dysplasia. Aplasia of the tympanic bone is associated with fusion of the neck of the
malleus to the atresia plate (
Fig. 1). Also other ossicular anomalies may be present: fusion of the
malleus and
incus , with absence of the normal malleo-incudal joint, is common, and the long process of the incus may have an abnormal orientation or be hypoplastic or absent. The most important ossicular structure to look for on
CT is the
stapes ; absence of the stapes makes surgical reconstruction more complex. Furthermore, the presence and width of the oval window should be checked; on coronal images, it should have a diameter of about 2 mm.
The tympanic segment of the
facial nerve canal is often displaced caudally, sometimes reaching as low as the round window. The second genu and mastoid portion of the facial nerve typically have a more anterior position than normal, which is important for the surgeon to know, as they may appear in the surgical pathway. The mastoid portion of the facial nerve may curve anteriorly towards the area of the
temporomandibular joint , or leave the
temporal bone in a more lateral direction. Determining the exact course of the facial nerve is of utmost importance to avoid postoperative facial paralysis.
When the tympanic bone is aplastic, the position of the
temporomandibular joint is frequently more posterior than normal, due to lack of posterior bony support. Confluence of the temporomandibular joint soft tissue with the middle ear is regarded unfavourable for surgery, as this makes maintaining support for the surgically created hearing apparatus difficult.
The
CT study should also exclude the presence of a (
congenital or acquired)
cholesteatoma .
Although rarely associated with aural dysplasia, abnormalities of the
inner ear should be looked for. Usually, these inner ear malformations are minor; the labyrinthine function is not necessarily abnormal. The internal auditory canal may appear widened and more angled than normal.
Branchiogenic anomalies
Anomalies of the branchial complex are vestigial remnants, resulting from incomplete obliteration of the branchial apparatus or buried epithelial cell rests. First branchial cleft anomalies may originate along the EAC (see
branchial apparatus ).
Blind-ending opening in the external ear, often located at or near the anterior crus of the helix (see
preauricular sinus ).
Inflammatory conditions
- external otitis: a common condition, also known as swimmer's ear. It does not require imaging in most cases.
-
malignant otitis externa : also called necrotizing external otitis. This is an aggressive infection by
Pseudomonas, which spreads beyond the confines of the EAC, both intra- and extracranially. This disease is seen in diabetic and immunocompromised individuals. The condition has a fatal outcome if untreated.
-
Ramsay-Hunt syndrome (also known as herpes zoster oticus) corresponds to acute facial neuritis, causing facial paralysis, otalgia and vesicles in the external ear.
-
Relapsing polychondritis is a rare inflammatory disease of
cartilage of unknown origin, which may involve the cartilages of the external ear.
Fractures of the tympanic bone are quite frequent; they are usually caused by the impact of a posteriorly displaced mandibular condyle.
Longitudinal fracture, temporal bone often extend into the EAC.
Soft tissue
trauma to the EAC, without recognizable
fracture, may lead to
fibrosis and
stenosis of the EAC.
Posttraumatic canal
stenosis may lead to accumulation of debris deep to the
stenosis, and formation of a
cholesteatoma . Posttraumatic cholesteatoma may also develop as a result of implantation of ear canal squamous epithelium into the
fracture line.
-
cholesteatoma is rarely seen to arise from the EAC; see
cholesteatoma . It may be acquired (post-traumatic or secondary to an EAC
stenosis), but often the pathogenesis is not clear; it is presumed to arise from ectopic epithelium.
-
keratosis obturans : as in cholesteatoma, accumulations of exfoliated keratin, but often bilateral in patients with a history of sinusitis and bronchiectasis.
- exostoses and osteomata: exostoses are multinodular bony masses developing due to prolonged irritation of the EAC, most commonly secondary to excessive contact with cold sea water ('surfer's ear') (
Fig. 2). An osteoma is a typically solitary, unilateral and pedunculated bony growth; osteomata are less common than exostoses (
Fig. 3).
-
fibrous dysplasia, head and neck may cause
stenosis of the EAC, resulting in conductive hearing loss (
Fig. 4).
Several types of malignant neoplasms can arise in the external ear, or secondarily involve it. CT is normally the first study, as assessment of bone detail is critical to treatment planning ( Fig. 5). Surrounding structures are likely to become involved with further tumour growth, such as the parotid gland and temporomandibular joint . Perineural tumour spread, head and neck along the facial or auriculotemporal nerve, and intracranial spread is possible. MRI is frequently used as an adjunct in such cases. Spread to neck lymph nodes is possible (the parotid and higher parajugular lymph nodes are at risk) and a CT study of the neck is therefore desirable.
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