Manas Sharma
Intracranial hypotension is a neurologic syndrome with various etiologies that share a common final pathway of decreased CSF volume and pressure. The classic clinical presentation is orthostatic headache with nonspecific symptoms like nausea and vertigo.1,2 Severe cases can progress to cranial nerve palsies and coma.3 The nonspecific nature of the clinical presentation that overlaps with very different etiologies, ranging from migraine to meningitis, with drastically different treatments results in delays in work-up and diagnosis.4 Currently, no standard diagnostic test exists, nor is the site of leak always easily identified, but along with the clinical presentation, diagnosis does hinge to a large extent on imaging features suggestive of low CSF pressure. A contrast-enhanced MRI of the head is usually the first-line investigation to confirm the diagnosis and rule out mimics, but all classically described imaging features are not always present in every case and many times are nonspecific.5,6 Pachymeningeal enhancement, regarded as the most sensitive finding, can also be seen in post-lumbar puncture and postoperative situations.7 Brain stem slumping, a reasonably specific feature, is present only in 51% of cases and can be very subjective.8 All these factors result in further delay in diagnosis and can cause high morbidity. Thus, neuroimagers have often sought more objective imaging criteria reflecting low CSF volumes. In this context, our anecdotal observations led us to assess and establish that a narrowed interpeduncular angle seen and measured on routinely obtained axial T2 sequences can be a strong, objective imaging marker to support the diagnosis.9
The angle reduces with decreased CSF volume and brain stem slumping. We determined that an angle threshold of 40.5° yielded a sensitivity of 80% and a specificity of 96.7% for intracranial hypotension.9 This angle can be very easily measured at the approximate level of the mamillary bodies on axial T2 images on any routine brain MRI. It takes very little experience to measure this using standard angle measuring tools available on all PACS workstations of today. It introduces a large measure of objectivity to the otherwise often subjective assessment of brain stem slumping and can be very easily adopted in routine brain MRI reporting by all radiologists. This simple measurement can especially help diagnose in cases that may not otherwise have florid classic imaging features.