Skull Base Osteomyelitis
Chandler1 is credited with first describing this condition which he termed “Malignant external otitis”. This turned out to be 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.
Waldovgel2 noted that three factors were associated with SBO. The first , a contiguous focus of
infection, hematogenous seeding and microvascular disease.
Factors associated with SBO are extremes of age, diabetes mellitus, immunocompromization, a history of chronic otitis media, leukemia, alcoholism The causative organism most commonly associated with SBO is pseudomonas aeruginosa.
Other organisms are associated with SBO. However in these instances the organisms are likely to be associated I situations where there is immunocompromization. The organisms isolated include mycobacterium tuberculosis, Actinomyces, Staph Aureus, Staph Epidermidis, Aspergillus flavus and Aspergillus fumigatus. Still Pseudomonas aeruginosa is the isolate in 99% of cases. Despite this it is still imperative to identify the offending organism so that appropriate treatment can be started without delay one proof of the identity of the organism has been determined.
The infection initiates at the junction between the cartilaginous and bony junction of the external
auditory canal. The infection then tracks downwards through the fissures of Santorini or through the
tympanomastoid fissure into the surrounding tissues. The infection also involves the mastoid and petrous
apex. As the bone gets involved progressively venous thrombosis can occur.
Cranial nerves can be involved, the seventh cranial nerve usually being the first to be involved.
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, of a deep boring nature. The patient is unable to sleep and is frequently awakened by the pain. The diagnosis of 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 is excluded then it is safe to presume that SBO is present.
The tympanic membrane may appear to be normal.
Babiatzki and Sade3 summarized the criteria of identifying those patients suffering from SBO. Clinically severe otitis externa that does not resolve with conventional conservative treatment, severe otalgia that typically interferes with the patients sleep at night, mandatory granulation tissue located typically at the junction of the cartilagenous and bony external canal and a positive culture of pseudomonas aeruginosa, an elderly patient suffering from diabetes. Benecke4 devised a three stage system. Stage one: SBO limited to soft tissue and has as yet not involved bone. Stage 2:Disease limited to the mastoid. Both Gallium and technetium scans are positive.
Stage 3: Extensive disease in the mastoid and tissues of the base of the skull. Cranial nerve involvement is present. Kraus, Rehm and Kinney5 propsed a staging system to include 3 categories.
- Stage 1: Disease limited to the external ear and mastoid.fcdefefef
- Stage 2: SBO with cranial nerve involvement.
- Stage 3: Involvement of the brain and meninges.
Cranial involvement is associated with significant mortality and thus carries a poor prognosis
Imaging - CT scanning
CT scanning is very important. However CT scan findings can at times resemble otitis media. If especial care is taken to visualize the external ear carefully, erosions of the external ear canal will be seen clearly. CT scans are essential to discern extent of spread of disease and to monitor progress. As was said earlier in this chapter SBO is essentially a clinical diagnosis. Ct scan of a patient suffering from SBO shows erosion of the external auditory canal (arrows) with extension into the mastoid.
Imaging - Technitium-99m
Technetium scans provide sensitive data for monitoring osteoblastic activity present in osteomyelitis. The scan is positive in as short a time as 24 hours following involvement of the bone. However what it makes up for in sensitivity it lacks in specificity. It is not specific for SBO. Another disadvantage iis that technetium is anatomically imprecise. Despite these limitations technetium scanning does provide early identification of SBO especially when co-ordinated with clinical findings.
Imaging - Gallium-67 citrate
Similar to the technitium scan gallium scan is sensitive for identifying infection and inflammation. It too like technetium is anatomically imprecise.
Management consists of the following:
- Correct identification of SBO, biopsy of granulation tissue to exclude a malignancy
- Identifying the organism.
- Imaging to identify extent of disease.
- Administering Appropriate antibiotics parenterally.
- Control of diabetes mellitus
- Correction of immunocompromization.
- Administering Adjunctive measures like Hyperbaric Oxygen.
Treatment of SBO with a view to cure is primarily administering the appropriate antibiotic intravenously at the appropriate dosage for an appropriate period of time. Significant change in treatment has taken since Chandler’s original report. Initially the combination of an aminoglycoside with a semisynthetic penicillin was the standard of treatment.
Subsequently fluoroquinolones were added to treatment. In time resistance to fluoroquinolones occurred. At this time reports of successful ttreatment protocols involving third generation cephalosporins, ureidopenicillins combined with tazobactam have been reported to be effective. Aminoglycosides are still very effective . The major disadvantage of aminoglycosides are that they are renal toxic.
Hyperbaric Oxygen therapy
Diabetes Mellitus causes microangiopathy. This leads to low oxygen saturation. Hyperbaric oxygen
temporarily cause an increase in wound po2 levels. This in turn enhances phagocytic oxidative killing of
aerobic organisms. Hyperbaric oxygen is effective in promoting wound healing, angiogenesis, osteoneogenesiss
and in restoring antibacterial activity.
Treatment consists oof giving 100% oxygen for 90 minutes at 2.5mm absolute pressure5 days a week for 4 weeks.
The role of surgery is controversial.
Surgery is usually limited to taking a biopsy and ear toilet for removal of debris. Surgical intervention is limited to debridement, removal of bony sequestra and drainage of abscesses
Duration of treatment
Treatment usually lasts a minimum of six weeks. Parameters that should be monitored are cessation of otalgia, resolution of granulation tissue in the external ear. Cranial nerve deficits should also resolve.
SBO is an aggressive infection associated with significant mortality. Unrelenting otalgia is the cardinal symptom. SBO is a clinical diagnosis. The granulation tissue needs to be biopsied to exclude the presence of a malignancy. The commonest pathogen causing SBO is pseudomonas aeruginosa. Treatment is directed at eliminating pseudomonas through third generation cepalosporins, ureidopenicillin and quinolones. Control of diabetes Mellitus and reversal of immunocompromization is vital for a cure. Imaging is useful in determining extent of disease and can be used to monitor progress. Hyperbaric oxygen is a useful treatment adjunct. Surgery is limited to biopsy and debridement.
- Chandler JR(1968): Malignant external Otitis. Laryngoscope 78:1257-1294.
- Waldvogel FA et al (1970): Osteomyelitis: a review of clinical features, therapeutic considerations and unusual aspects.
- New England Journal of Medicine. 282:198-206
- Babiaztki A and Sade J (1987): Malignant External Otitis. Journal of Laryngology and Otology 101: 205-210.
- Benecke JE: Management of osteomyelitis of the skull base. Laryngoscope 99:1220-1223.
- Kraus DH, Rehm SJ, Kinney SE (1988): The evolving treatment of necrotizing external otitis. Laryngoscope 9:934-939