Malignant Otitis Externa Workup

Leukocyte count

The leukocyte count in malignant external otitis (MEO) is usually normal or mildly elevated. A left shift is not commonly found.

Erythrocyte sedimentation rate

The erythrocyte sedimentation rate (ESR) is invariably elevated, with an average of 87 mm/h. It begins to decrease within 2 weeks of initiating therapy but takes many months to return to normal. ESR can be used to support the clinical diagnosis since acute external otitis or ear canal malignancy usually does not cause a rate elevation in this lab test.

Serum chemistry

Patients with known diabetes need an evaluation of the serum chemistry to determine if the infection is affecting their baseline glucose intolerance. Patients without a history of diabetes should be tested for glucose intolerance.

Culture and sensitivities from the external auditory canal

Culture from the ear drainage should be performed ideally before antimicrobial therapy is initiated. The most common causative organism is P aeruginosa (95%). This organism is an aerobic, gram-negative rod. Pseudomonas species have a mucoid coating that deters phagocytosis. Exotoxins (ie, exotoxin A, collagenase, elastase) can cause tissue necrosis, and some strains produce a neurotoxin that may be partially responsible for cranial neuropathies.

Less common organisms identified include Aspergillus and Proteus species, Candida species, Staphylococcus aureus, and S epidermidis.

A study by Hobson et al concluded that vigilance for nonpseudomonal organisms is necessary in treating patients with malignant external otitis, especially those without diabetes, since methicillin-resistant S aureus (MRSA) is increasingly being diagnosed as the causative organism, and not always in the presence of diabetes. Among 20 patients with malignant external otitis, the investigators found 9 patients with P aeruginosa and 3 with MRSA. They also determined that although diabetes was present in all patients with P aeruginosa, it existed in only 1 of the patients with MRSA and in only 55% of all patients with nonpseudomonal infections. ^[14]

A study by Gruber et al indicated that in patients with refractory, culture-negative malignant external otitis, a fungal infection should be suspected. The study involved a cohort of 19 patients with malignant external otitis, three of whom had refractory, culture-negative disease. Polymerase chain reaction (PCR) assay revealed a fungal pathogen in all three, with empirical targeted therapy being used to successfully treat their condition. ^[15]

These are important adjuncts for determining the presence of osteomyelitis, the extent of disease, and response to therapy.

Technetium Tc 99 methylene diphosphate bone scanning

Technetium Tc 99 methylene diphosphonate bone scanning is based on binding to osteoblasts. This scan depicts as little as a 10% increase in osteoblastic activity. However, this test is not specific since tumors or bony dysplasias, in addition to osteomyelitis, can cause osteoblastosis.

It is useful in the initial evaluation because a positive finding in the correct clinical context can lead to confirmation of the diagnosis.

The test is not useful for assessing the response to therapy since results remain persistently positive long after clinical improvement because of continuous bone remodeling and reformation.

This test may also have limited usefulness for patients with a prior history of mastoiditis or otologic surgery.

The application of single-photon emission computed tomography (SPECT) technology has improved the poor spatial resolution traditionally associated with this test.

Gallium citrate Ga 67 scan

Gallium citrate Ga 67 scan is very sensitive but is not specific because gallium binds to actively dividing cells, including inflammatory cells, tumor cells, and osteoblasts.

Uncertainty is possible regarding whether a positive test result represents an inflammatory condition, soft tissue, or bone disease.

This test is most helpful when used as a monitor of successful treatment. Improvement of a positive test result correlates with therapeutic response.

A baseline test is usually obtained at the initial diagnosis for comparison with follow-up studies during treatment.

A quantitative comparison of the lesion to the nonlesion side may improve the interpretation of these studies for distinguishing acute external otitis from malignant external otitis (MEO) and for determining the efficacy of therapy.

The application of SPECT technology has improved the poor spatial resolution traditionally associated with this test.

Indium In 111–labeled leukocyte scan

Indium In 111–labeled leukocyte scan attempts to provide the same sensitivity as a gallium citrate Ga 67 scan but is more specific to an inflammatory process.

It does not appear to provide an improvement in scintigraphic technique for helping to establish the diagnosis. It may be better than gallium citrate Ga 67 scans for assisting in establishing the correct timing of disease resolution.

This test can be unreliable for imaging chronic osteomyelitis in other areas of the body. Thus, the accuracy of this application needs further study.

CT scanning and MRI

CT scanning and MRI are both useful for evaluating the anatomic extent of soft tissue inflammation, abscess formation, and intracranial complications.

CT scanning fails to diagnose early osteomyelitis because 30-50% of bone destruction is required to detect osteomyelitis by CT scanning.

MRI provides poor bone resolution. The soft tissue manifestations regress on CT scanning and MRI with response to therapy.

Bone changes remain persistently abnormal on CT scans for at least one year and are not well demonstrated by MRI studies. Thus, neither of the tests can be used to determine osteomyelitis resolution.

A retrospective study by Goh et al suggested that MRI findings in skull base osteomyelitis secondary to malignant external otitis (MEO) can help to differentiate the osteomyelitis from advanced nasopharyngeal carcinoma by the presence of a combination of lateral extension, increased T2 signal in adjacent soft tissues, deficiency of architectural distortion, and enhancement equaling or surpassing that of mucosa. ^[16]

Most authors advocate obtaining a CT scan with the initial evaluation for all patients, whereas Benecke advocates obtaining this test selectively for patients with cranial neuropathy, extensive bone changes on technetium scan, or poor clinical response to treatment. Grandis et al and Okpala et al support obtaining a CT scan early in the diagnostic/treatment algorithm. Peleg et al showed that there is a correlation between clinical course and the extent of anatomical areas involved as measured on initial CT scan findings. ^[17]

MRI and CT scanning are equally sensitive in detecting the soft tissue extent of the disease, but MRI is more sensitive for detecting intracranial complications.

See the list below:

• Obtain a biopsy of the external auditory canal to exclude carcinoma or other etiologies.

Nadol described the histopathology of 2 temporal bones affected by malignant external otitis (MEO). The infection did not spread through the pneumatized air tracts of the temporal bone. Rather, it spread along the vascular and fascial planes on exiting the temporal bone through the external auditory canal osseocartilaginous junction or fissures of Santorini. The otic capsule appeared to be resistant to the disease process. Linthicum described histopathologic findings in 5 temporal bones. Extensive destruction in the wall of the bony external auditory canal and osteomyelitic destruction of the wall of the fallopian canal in the descending portion of the facial nerve was seen. The infection spread beneath the otic capsule to erode the wall of the carotid canal and then extended into the central skull base.

See the list below:

• Levenson et al, Corey et al, Benecke, and Davis et al have proposed staging systems for malignant external otitis (MEO). ^{[18, 19, 20]}

  • These staging systems are generally based on extent of soft tissue/bony involvement or development of neurologic complications.

  • None of these staging systems has been widely adopted.