Preventing misdiagnosis of intracranial pressure disorders on diagnostic imaging

The bony cranium defines the intracranial space and contains three major components: the brain, vasculature, and cerebrospinal fluid (CSF). The relative amounts of these three components must be in equilibrium; otherwise, the pressure inside the head (intracranial pressure, ICP) will become excessively high or low and result in undesirable clinical manifestations. A variety of important disease processes can abnormally increase ICP including space-occupying masses, venous sinus thrombosis, and hydrocephalus. Idiopathic intracranial hypertension (IIH, previously known as “pseudotumor cerebri”) is an additional condition that results in abnormal elevated ICP without any such identifiable cause. Conditions that cause abnormally decreased ICP typically result from leakage of CSF through defects in the skull base or the contiguous spinal canal, termed “spontaneous intracranial hypotension” (SIH) when there is no known inciting cause.

Symptoms such as headache that are commonly seen in the setting of ICP disorders are frequently nonspecific, extremely common in the general population, and most often unrelated to underlying structural intracranial pathologies. The initial step in a diagnostic workup is eliciting symptoms and evaluating clinical signs of abnormally increased or decreased ICP. Imaging with computed tomography (CT), or more commonly magnetic resonance imaging (MRI) is frequently performed as part of this diagnostic workup. While imaging was traditionally obtained to exclude key important diagnoses (such as brain tumor and hydrocephalus), numerous recent studies have shown that there are a variety of imaging findings that have high associations with chronically elevated or decreased ICP.

Imaging findings in the orbits, skull base, dural venous sinuses, posterior fossa, and spine are commonly seen in association with abnormal ICP. Increased familiarity with these imaging findings by radiologists and clinical specialists results in the potential for improved diagnosis, but also the potential for misdiagnosis and over-diagnosis. While some patients with primary headache disorders may have undiagnosed ICP disorders with findings visible on MRI, there is also high potential for isolated imaging findings, particularly when taken out of appropriate clinical context, to result in subsequent over-testing and over-diagnosis. Two imaging findings that can be seen in the setting of high ICP are a CSF-filled “empty” sella and cerebellar tonsillar ectopia.

The finding of an expanded, CSF-filled “empty” sella is generally abnormal in younger patients but becomes much less useful as patients enter their 40s and 50s, and transmission of normal ICP over time through innate weaknesses in the diaphragm sella expand the sella turcica. The presence of additional imaging findings in the orbit, skull base, and venous sinuses can provide credence to the suspicion of elevated ICP in the setting of an “empty” sella but an isolated finding has far less significance.

A second problematic imaging finding is that of cerebellar tonsillar ectopia. This appearance of the cerebellar tonsils extending further below the foramen magnum than expected may be due to a variety of innate, congenitally abnormal, or ICP-related conditions. It is important to keep in mind that there is variability in the appearance of nearly all anatomic structures, and that in some people the cerebellar tonsils may have simply developed lower than usual, if they maintain a normal rounded morphology. This is to be distinguished from a Chiari 1 malformation where the tonsils are low-lying and “peg-like” in morphology and may be associated with other findings such as a spinal cord syrinx. IIH can also result in cerebellar tonsillar ectopia presumably due to the increased ICP pushing brain structures downward, as can SIH due to loss of the buoyancy from CSF causing a “sagging” appearance of the brain. It is critically important to differentiate each of these diagnoses, as the management is substantially different.

As an example, the figure below shows a patient with undiagnosed IIH imaged in 2008 (left image) whose chronically elevated ICP on re-imaging in 2015 (right image) resulted in increasing tonsillar ectopia (large arrow) as well as an increasingly “empty” sella turcica (small arrows). The appearance in 2015 could have easily been misdiagnosed as a Chiari 1 malformation.

A few guiding principles are helpful in preventing the misdiagnosis of ICP disorders on imaging. First, the imaging findings should never be interpreted in a vacuum – the asymptomatic patient with an isolated nonspecific imaging finding is unlikely to have an ICP disorder. Any finding or constellation of imaging findings must be correlated with and fit within the context of the overall clinical picture. Second, the presence of multiple associated imaging findings associated with a disorder of ICP is more helpful than a single finding. Imaging can build a stronger case for a specific diagnosis when several findings associated with that condition are present, making it important for those interpreting the images to be aware of the full scope of imaging findings in each ICP disorder. Finally, open and constructive communication between radiologists and clinical specialists is key to correct diagnosis, starting with appropriate clinical information and ending with an appreciation by the radiologist of the clinical impact and context of the imaging diagnosis.

Featured image credit: progress, clinic, medical by sasint. CC0 Public Domain via Pixabay.