36 Skull Base Osteomyelitis

Chandler1 is credited with first describing this condition that he termed “malignant external otitis.” This turned out to be a misnomer, as it was not a malignancy. It was a descriptive term used to indicate that the condition behaved like a malignancy. It spread rapidly, caused immense destruction, and was associated with a very high mortality. Since then, it has undergone an evolution in terminology. It is now known as skull base osteomyelitis (SBO). As the term indicates, there is an infection in the diploe of cancellous bone with likely involvement of the outer and inner tables of the cortical bone.

Etiopathogenesis

Waldovgel et al² noted that three factors were associated with SBO: a contiguous focus of infection, hematogenous seeding, and microvascular disease.

Factors associated with SBO are extremes of age, diabetes mellitus, immunocompromisation, and a history of chronic otitis media, leukemia, and alcoholism.

The causative organism most commonly associated with SBO is Pseudomonas aeruginosa. Other organisms are also associated with SBO. However, in these instances, the organisms are likely to be associated in situations where there is immunocompromisation. The organisms isolated include Mycobacterium tuberculosis, Actinomyces, Staphylococcus aureus, S. epidermidis, Aspergillus flavus, and A. fumigatus. Still P. aeruginosa is the isolate in 99% of the cases. Despite this, it is still imperative to identify the offending organism so that appropriate treatment can be started without delay once proof of identity of the organism has been determined.

The infection initiates at the junction between the cartilaginous and the bony junction of the external auditory canal. The infection then tracks downward through the fissures of Santorini or through the tympanomastoid fissure into the surrounding tissues. The infection also involves the mastoid and the petrous apex. As the bone gets involved progressively, venous thrombosis can occur.

Cranial nerves can be involved, cranial nerve VII usually being the first to be involved.

Diagnosis

An elderly patient suffering from diabetes mellitus who presents with otalgia should arouse suspicions of SBO. Otalgia is unrelenting and does not respond to conventional treatment. Otalgia is the cardinal symptom. Otalgia is continuous and is of a deep boring nature. The patient is unable to sleep and is frequently awakened by the pain. SBO is a clinical diagnosis. It can frequently be mistaken for otitis media. Inspection of the external ear reveals granulations. These granulations are located typically at the junction of the cartilaginous and bony external canal. Biopsy of these granulations is vital to excluding a diagnosis of cancer of the external ear. Once the histopathology report determines that cancer of the external ear is excluded, it is safe to presume that SBO is present. The tympanic membrane may appear to be normal.

Babiatzki and Sadé³

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