Idiopathic Intracranial Hypertension (IIH) is a condition where the pressure of cerebrospinal fluid (CSF) surrounding the brain and spinal cord becomes elevated without an identifiable cause. This increased pressure can lead to symptoms like headaches and vision problems. Magnetic Resonance Imaging (MRI) is a valuable tool for evaluating individuals suspected of IIH. This article explains how a normal brain appears on an MRI and highlights the specific changes seen in IIH.
Understanding a Normal Brain MRI
A normal brain MRI provides a clear view of the brain’s structures. The ventricles, fluid-filled spaces within the brain, appear as dark or low-signal areas on certain MRI sequences, indicating CSF. They should be of typical size and shape, without enlargement or compression.
The optic nerves are surrounded by a thin layer of CSF within their sheaths. In a healthy scan, this perioptic subarachnoid space appears as a narrow, consistent fluid signal. The optic nerves themselves are straight and well-defined as they extend from the back of the eyeballs towards the brain.
The pituitary gland rests within a bony cavity at the base of the skull called the sella turcica. On a normal MRI, it fills most of this space and has a distinct, uniform appearance. The major venous sinuses, channels that drain blood from the brain, should appear open and unobstructed, allowing for smooth blood flow.
Key MRI Findings in Idiopathic Intracranial Hypertension
In individuals with Idiopathic Intracranial Hypertension, several specific changes can be observed on an MRI due to elevated CSF pressure. One common finding is an “empty sella,” where the pituitary gland appears flattened or compressed, and the sella turcica is partially filled with CSF. This occurs because increased intracranial pressure pushes the arachnoid membrane, which contains CSF, into the sella turcica, displacing the pituitary gland.
Another frequent sign is optic nerve sheath distension, the swelling of the fluid-filled space surrounding the optic nerves. This distension results from increased CSF pressure extending along the optic nerve sheaths. The optic nerves may also show tortuosity, appearing kinked or wavy, due to the increased pressure within their sheaths.
Posterior globe flattening is a characteristic finding, where the back of the eyeball appears flattened. This flattening is attributed to chronic pressure from elevated CSF on the posterior aspect of the globe. In some cases, there might also be an intraocular protrusion of the optic nerve head, the MRI representation of papilledema.
Transverse venous sinus stenosis, or narrowing of the large veins draining blood from the brain, is often observed. This narrowing can be bilateral or affect the dominant sinus.
The Role of MRI in Diagnosing Idiopathic Intracranial Hypertension
MRI plays a role in diagnosing IIH. Primarily, it helps rule out other potential causes of increased intracranial pressure, such as brain tumors, hydrocephalus, or blood clots in the brain’s venous sinuses. This exclusionary aspect is important because IIH symptoms can overlap with those of other neurological conditions.
The characteristic MRI findings discussed earlier, such as empty sella, optic nerve sheath distension, posterior globe flattening, and transverse venous sinus stenosis, provide evidence supporting an IIH diagnosis. The presence of these findings, particularly when several are observed together, increases the likelihood of IIH. This is important in cases where papilledema might be absent, as MRI findings can still indicate elevated intracranial pressure.
While MRI findings are suggestive, they are considered in conjunction with a patient’s clinical symptoms and lumbar puncture results. A lumbar puncture directly measures CSF pressure, which is elevated in IIH. The combination of these diagnostic tools allows healthcare professionals to arrive at a definitive diagnosis of Idiopathic Intracranial Hypertension.