Idiopathic Intracranial Hypertension (IIH) is a condition characterized by elevated pressure within the skull, specifically around the brain. This increased pressure can lead to various symptoms, often mimicking other neurological disorders. Magnetic Resonance Imaging (MRI) serves a significant role in understanding the underlying causes when investigating such symptoms.
Understanding Idiopathic Intracranial Hypertension
Idiopathic Intracranial Hypertension (IIH) involves increased pressure within the skull without an identifiable cause. The term “idiopathic” indicates that the origin of the condition remains unknown. This elevated pressure results from an accumulation of cerebrospinal fluid (CSF), the protective liquid surrounding the brain and spinal cord.
Common symptoms of IIH include headaches, which can be severe and originate behind the eyes, sometimes worsening with activities like coughing or sneezing. Individuals may also experience vision changes such as blurred vision, double vision, temporary blindness, or blind spots in their peripheral vision. A characteristic symptom is pulsatile tinnitus, described as a whooshing sound in one or both ears that pulses with the heartbeat. Swelling of the optic nerve, known as papilledema, is another frequent finding due to the pressure on the nerve that connects the eye to the brain. These symptoms can closely resemble those of brain tumors, leading IIH to sometimes be called “pseudotumor cerebri” or “false tumor.”
Why MRI is Used for IIH
An MRI scan is routinely performed when Idiopathic Intracranial Hypertension (IIH) is suspected, primarily to rule out other serious conditions that can cause similar symptoms of increased intracranial pressure. This imaging technique helps distinguish IIH from issues like brain tumors, hydrocephalus (excess fluid in the brain), blood clots, or other structural abnormalities within the brain.
The non-invasive nature of MRI makes it a valuable diagnostic tool, providing clear pictures of soft tissues, which is particularly useful for assessing the brain and optic nerves. By excluding other potential causes of elevated intracranial pressure, MRI guides the diagnostic process toward IIH when no other explanation for the symptoms is found. This differential diagnosis is a primary reason for its use.
Specific MRI Findings in IIH
Radiologists look for several specific signs on an MRI scan that can suggest the presence of Idiopathic Intracranial Hypertension. One such finding is an “empty sella,” which refers to a flattened appearance of the pituitary gland within an enlarged sella turcica, the bony structure at the base of the skull where the pituitary gland resides. This occurs because chronic elevated intracranial pressure can push the arachnoid membrane, which contains CSF, into the sella, compressing the pituitary gland.
Another common sign is the distension and tortuosity of the optic nerve sheaths. The optic nerves, which transmit visual information from the eyes to the brain, are surrounded by a sheath that can swell and become wavy due to the increased pressure exerted by the cerebrospinal fluid.
Narrowing of the transverse venous sinuses, known as transverse venous sinus stenosis, is also frequently observed in IIH patients. These large veins in the brain are responsible for draining blood and cerebrospinal fluid. When they are constricted, it can impede CSF outflow.
Flattening of the posterior sclera is another indicator seen on MRI, where the back part of the eyeball appears flattened rather than its usual curved shape. This flattening is a direct result of the increased pressure pushing on the back of the eye. Subtle downward displacement of the cerebellar tonsils, a condition known as tonsillar herniation, can also be present.
Confirming the IIH Diagnosis
While MRI provides strong supportive evidence for Idiopathic Intracranial Hypertension, it is generally not the only diagnostic tool used to confirm the condition. The findings on an MRI scan, such as those indicating elevated intracranial pressure, are integrated with other clinical assessments. A definitive diagnosis typically involves a lumbar puncture, also known as a spinal tap, which directly measures the pressure of the cerebrospinal fluid.
During a lumbar puncture, a needle is inserted into the lower back to access the spinal canal and measure the opening pressure of the CSF. An elevated CSF pressure, usually above 25 cm H2O in adults, in conjunction with the characteristic clinical symptoms and the absence of other underlying causes identified by MRI, collectively leads to an IIH diagnosis.