The patient was admitted to the hospital for further evaluation and a culture was taken from the ear canal. The patient had an MRI of the skull base that did not reveal a mass but showed significant inflammation (enhancement [bright] with contrast; Fig. 2). The previous CT scan obtained showed a sclerotic mastoid (thick mastoid cortex), indicating the patient had a history of ear infections as a child (Fig. 3). A technetium 99 scan was obtained and it showed enhancement (bright) mastoid and clivus (center of the skull; Figs. 4-5).
This patient was suffering from skull base osteomyelitis. This condition, also called malignant otitis externa or necrotizing otitis externa, is a life-threatening infection that begins in the skin of the external ear canal and spreads to the temporal bone. The infection can then spread throughout the skull base and affect the nerves that exit the skull. It can cause a loss of function in those areas. It can lead to eventual death if left untreated. Skull base osteomyelitis is a condition that generally affects patients who suffer from immune suppression or those with diabetes (Laryngoscope. 2008;118:1917 http://bit.ly/2e1MwQB). It is also seen in patients with solid organ or bone marrow transplant as well as HIV-positive patients.
These patients require intravenous antibiotics for a minimum of six weeks and sometimes several months. The patients are followed up with a blood test that measures the relative level of inflammation in the blood (erythrocyte sedimentation rate). This test is used since the most common test used to measure treatment response (white blood cell count) is often normal in these patients. These patients often do not have a normally functioning immune system, which prevents the migration of the infection from the ear canal skin to the temporal bone.
Ear canal infections (otitis externa) in patients with diabetes or any type of immune suppression should be taken very seriously and treated promptly and aggressively. While a vast majority of patients with otitis externa can be treated with topical antibiotics (ear drops), patients with diabetes or immune suppression should be treated with oral antibiotics as well and followed up closely to ensure proper response to the treatment.
Unlike patients with a normal immune system, immune suppressed or diabetics with otitis externa should have a culture taken from the ear canal to identify the causative organism to allow targeted antibiotic therapy. Often patients with skull base osteomyelitis are treated with topical antibiotics that control the ear canal infection but do not address the underlying bone infection. As a result, the ear canal becomes devoid of bacteria, and the clinician cannot sample the bacteria for identification. In cases where the culture is negative, antibiotic therapy directed to the most common bacteria is undertaken (Otol Neurotol. 2006;27:250 http://bit.ly/2eoC4Yn).
The most common bacteria causing this condition are the Pseudomonas and Staph species. These bacteria commonly live in the external ear and can enter the skin through a scratch in the ear canal. The ear canal is a perfect environment for bacterial growth as it is warm, humid, and dark. This problem becomes worse when a patient uses a hearing aid, which may plug the ear canal. Scratching or manipulating the ear by the patient can cause a break in the skin and allow the bacteria to enter. In diabetics or immune-suppressed individuals, removal of cerumen should ideally be done without the use of water and performed very gently to prevent breaks in the skin.
Patients with skull base osteomyelitis most often present with ear pain that wakes them at night. The diagnosis is made using a technetium 99 SPECT bone scan, which can pinpoint the infection. Another nuclear medicine scan (gallium 67 SPECT) is obtained upon patient follow up. The scan is then repeated to ensure the infection is completely gone before stopping antibiotics.
BONUS VIDEOS: VISUAL DIAGNOSIS
Read this month's Clinical Consultation case, then watch the accompanying videos from Hamid R. Djalilian, MD, to review the patient's imaging for yourself.
- Video 1. Axial post-gadolinium T1 MRI showing the intense inflammation of the soft tissues surrounding the infected bone of the skull base.
- Video 2. Axial CT images of the patient showing the bony anatomy of the mastoid on the left.
- Video 3. Fused axial CT images with technetium 99 SPECT showing hyperintensity in the left temporal bone and clivus.
- Video 4. Fused coronal CT images with technetium 99 SPECT demonstrating the areas of inflammation. Some inflammation in the cervical spine is also seen, which may be an artifact.
- Video 5. Fused sagittal CT images with technetium 99 SPECT showing the infected areas at the skull base in the midline and on the left.
- Video 6. Three-dimensional technetium 99 scan showing the composite image, which is figure 5A in three dimensions when the patient is rotated by 360 degrees.
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