Differentiating between Chiari Malformation I (CMI) and acquired tonsillar herniation due to spontaneous or iatrogenic intracranial hypotension (IH) can be challenging. We describe a case of IH misdiagnosed as CM1 to emphasize several distinct clinical signs that can prevent misdiagnosis.  

Case Description: A 36-year-old multigravida female with history of multiple epidural analgesia during vaginal deliveries presented with 6 months of constant, poorly localized headaches. Fundoscopic exam revealed bilateral mild inferior disc edema without vessel obscuration. Contrasted magnetic resonance imaging (MRI) showed diffuse pachymeningeal enhancement with a 7mm downward herniation of the cerebellar tonsils but no syrinx in the upper cervical cord. The patient was thought to have CM1, and she was treated with supportive care. After 2 years, the patient again presented with severe intractable headaches and vomiting 7 weeks following epidural for vaginal delivery. Repeat MRI brain showed a 23 mm downward herniation of the cerebellar tonsils associated with severe effacement of the basilar and prepontine cisterns and diffuse sulcal effacement. MRI cervical spine showed a large cervical syrinx at this time. MRI thoracic and lumbar spine showed an epidural cerebrospinal fluid (CSF) collection extending from T1 to L3 levels with diffuse cord edema, confirming CSF leak. Given high risk of complications, blood patch was not attempted, and the patient ultimately underwent suboccipital decompressive craniotomy.    

This case highlights the importance of considering acquired causes of cerebellar tonsillar herniation in symptomatic adults. Headache description, chronicity, and presence/absence of syrinx cannot differentiate CMI from IH. Pachymeningeal enhancement, subdural hygromas, pituitary enlargement, distention of cerebral venous sinuses, and effacement of subarachnoid spaces along with flattening of pons are more indicative of IH.  Evidence of an extradural CSF collection in the spinal cord or a CSF leak on computerized tomography myelogram are diagnostic of IH.