Sigmoid sinus dehiscence occurs when the protective bone covering a major vein in the skull is either absent or abnormally thin. This anatomical variation can lead to noticeable symptoms for those affected. It holds significance due to its potential impact on daily life.
Understanding Sigmoid Sinus Dehiscence
The sigmoid sinus is an S-shaped venous channel located deep within the head, just below the brain. It resides near the mastoid bone behind the ear. This paired structure collects deoxygenated blood from the brain and the back of the skull, draining it into the internal jugular vein in the neck.
Dehiscence means the thin layer of bone, known as the sigmoid plate, that typically encases and protects the sinus is missing or significantly thinned. This bony defect allows the sigmoid sinus to come into direct contact with the air-filled spaces within the mastoid bone.
Recognizing the Signs
Individuals with sigmoid sinus dehiscence often experience a range of auditory and balance-related symptoms. The most characteristic and frequently reported symptom is pulsatile tinnitus. This manifests as a rhythmic whooshing, thumping, or buzzing sound in one or both ears that is synchronized with the individual’s heartbeat. The sound can become particularly noticeable in quiet environments.
This distinctive symptom arises because the absence of the bony barrier allows the normal pulsatile flow of blood within the exposed sigmoid sinus to transmit sound directly to the nearby middle and inner ear structures. Other potential symptoms include a sensation of pressure in the ear, hearing loss, and occasionally vertigo or dizziness. These symptoms can fluctuate in intensity, sometimes worsening with physical exertion or changes in head position.
Factors Leading to Dehiscence
The development of sigmoid sinus dehiscence can be attributed to several factors, broadly categorized as congenital or acquired influences. Some individuals may be born with an inherent predisposition due to variations in skull bone development. This includes instances where the bone overlying the sigmoid sinus is naturally thinner or absent from birth, or where there are anatomical variations in the size and course of the sinus itself.
Acquired factors play a substantial role in the formation or worsening of dehiscence over time. A prominent factor is increased intracranial pressure (ICP), which refers to elevated pressure within the skull. Conditions such as idiopathic intracranial hypertension (IIH) can lead to chronic elevation of ICP. This sustained pressure can exert a remodeling force on the skull bones, gradually eroding or thinning the bone that covers the sigmoid sinus.
Vascular anomalies also contribute to the development of dehiscence. A larger and more dominant right sigmoid sinus can lead to increased blood flow and turbulence within the sinus. This turbulent flow, especially when coupled with a naturally thin or absent bony covering, can further contribute to the erosion of the bone or prevent its proper development.
Confirming the Condition
Diagnosing sigmoid sinus dehiscence primarily relies on advanced imaging techniques that provide detailed views of the skull’s bony structures. A high-resolution computed tomography (CT) scan of the temporal bone is the most reliable method for confirming the condition. This scan can precisely visualize the bony defect, often revealing an “air-on-sinus” sign, where the sinus appears directly adjacent to air cells in the mastoid bone without intervening bone.
Magnetic resonance imaging (MRI) or MR venography may be used as supplementary diagnostic tools. These imaging modalities help assess venous blood flow within the sinus and can rule out other potential causes of symptoms, such as vascular malformations or tumors. A healthcare provider will also conduct a thorough physical examination and take a detailed patient history, noting the characteristics of any reported pulsatile tinnitus, to guide imaging studies.