Diagnosis: Sigmoid Sinus Diverticulum
Pulsatile tinnitus is a somewhat common condition for which a uniform workup or treatment has not been developed. Most clinicians perform a series of imaging studies and sometimes laboratory tests. Part of the problem with pulsatile tinnitus workup is that it has a very long differential diagnosis and includes many significant problems that could be life-threatening.
Most clinicians are concerned about an aneurysm and therefore obtain a magnetic resonance arteriography (MRA) to evaluate this condition. This imaging study is usually combined with magnetic resonance imaging (MRI) of the brain to look for a tumor. A CT scan of the temporal bones is not usually obtained, though it may be a valuable option.
When working up a patient with pulsatile tinnitus, the first thing to establish is whether the problem is venous or arterial in nature. Venous pulsatile tinnitus can be controlled or extinguished by gently pressing the neck or turning the head. Arterial pulsatile tinnitus, however, cannot be easily suppressed with neck pressure, as it takes a significant amount of pressure on the neck to stop arterial blood flow. Venous blood flow can be reduced by applying gentle pressure on one side of the neck to compress the internal jugular vein. Identifying the type of pulsatile tinnitus enables the clinician to limit the workup to arterial or venous abnormalities, which helps reduce the number—and cost—of imaging studies.
Our general approach to pulsatile tinnitus starts with getting an MRI with gadolinium of the internal auditory canal to rule out a mass such as a glomus jugulare. Then, depending on the arterial or venous nature of the symptoms, an MRA or magnetic resonance venography (MRV) is done to evaluate the possibility of venous sinus stenosis. We also refer the patient to a neuro-ophthalmologist for an intracranial hypertension evaluation. If both tests are negative, we consider obtaining a CT scan of the temporal bones to check for abnormalities that can cause pulsatile tinnitus. These abnormalities are usually too small to be visualized on an MRI or MRV. These include sigmoid sinus diverticulum, small dehiscences of the posterior fossa plate, superior or posterior canal dehiscence, and small dehiscences of the tegmen tympani that can cause small meningoceles, which could touch the malleus or incus.
This patient's CT of the temporal bones showed a right-sided sigmoid sinus diverticulum (Figs. 1 and 2). Interestingly, the patient also had a posterior fossa plate defect on the left side (Fig. 3). The MRI indicated the presence of bilateral Meckel's cave diverticula (Figs. 4 and 5). These findings were all suggestive of intracranial hypertension.
Sigmoid sinus diverticulum is a condition of the temporal bone that occurs in the presence of intracranial hypertension. While the exact pathophysiologic mechanism is yet to be fully worked out, it is thought that higher venous pressure leads to this phenomenon wherein a small portion of the sigmoid sinus breaks through the surrounding bony plate and forms a diverticulum. This diverticulum can further dilate and enlarge over time, leading to a large outpouching of the sigmoid sinus. The blood flow through the outpouching is turbulent, causing a pulsatile sound that can be heard via bone conduction. Pressing against the internal jugular vein on the affected side reduces the blood flow, thereby diminishing or stopping the sound.
When small, a sigmoid sinus diverticulum is not visible on an MRI or MRV. This can only be visualized using careful, ultra-thin slice CT of the temporal bones. Sigmoid sinus diverticulum has been found to be associated with the narrowing of the transverse venous sinus in the brain. This abnormality is also associated with intracranial hypertension, which is a condition that most commonly affects middle-aged women who are overweight. This condition is believed to be due to the increased central venous pressure caused by the mass of tissue around the abdomen.
The best treatment for intracranial hypertension is significant weight loss, particularly to a near-normal body mass index. However, since it is difficult to rapidly lose a significant amount of weight, patients with this condition are initially treated with acetazolamide to reduce intracranial pressure. Patients are usually seen by a neuro-ophthalmologist who can noninvasively get an approximate measure of the intracranial pressure by looking at the venules of the optic nerve head and getting the intraocular pressure. The gold standard for measuring intracranial pressure is lumbar puncture, wherein a needle is inserted in the back to measure the pressure in the spinal fluid. This procedure is reserved for cases with high suspicion of intracranial hypertension based on patient examination and imaging findings.
To treat sigmoid sinus diverticulum, the possible intracranial hypertension must be addressed first. If the symptoms improve, then no further treatment is required. If the patient's symptoms continue, surgical treatment can be done to close the defect and stop the turbulent blood flow to the diverticulum. The diverticulum is typically shrunk with bipolar cauterization, and the defect is secured using bone cement or bone shavings to prevent any diverticulum from developing in the future. Caution is needed to avoid compressing the lumen of the sigmoid sinus, which can decrease the venous outflow from the brain and cause additional intracranial pressure. After surgery, patients need to see a neuro-ophthalmologist to make sure that intracranial hypertension does not occur or worsen.
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 (horizontal) CT of the right temporal bone showing the sigmoid sinus diverticulum.
- Video 2. Axial (horizontal) CT of the left temporal bone showing the full defect of the posterior cranial fossa.
- Video 3. Coronal (parallel to the face superiorly-inferiorly) CT of the right temporal bone showing the relationship between the sigmoid and mastoid.
- Video 4. Sagittal (looking outside-in laterally) CT of the temporal bone demonstrating the sagittal anatomy of the diverticulum.
- Video 5. Axial (horizontal) T2 MRI showing bilateral Meckel's cave diverticula.
- Video 6. Coronal (parallel to the face superiorly-inferiorly) T2 MRI showing the relationship between the Meckel's cave and the skull base structures.
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