A 60-year-old man presented with 4 years of progressive daytime somnolence and ‘sleep attacks’ associated with dysphagia, persistent headaches that did not alter with position, and progressive spastic paraparesis. Multiple sleep latency testing was negative for narcolepsy. An MRI brain was performed to exclude central pathology.
A gadolinium-enhanced MRI brain showed striking transtentorial herniation well below the incisural line, with inferior displacement of the thalamus and massa intermedia giving rise to a mass-like appearance in the region of the massa intermedia and cerebral peduncles. The pons appeared flattened against the clivus and there was effacement of the prepontine subarachnoid space.
MRI spine demonstrated a syrinx at the level of C2. No extradural fluid or leak was seen. A nuclear imaging study was performed, where 99m Tc DTPA tracer was injected into the thecal sac. It also did not demonstrate extradural fluid or leak.
The peg-like morphology of the tonsils, and the presence of a syrinx are features of a Chiari I malformation, however, the extent of the transtentorial herniation with collapse of the third and lateral ventricles and capacious veins raise the likelihood of severe intracranial hypotension.
Spontaneous intracranial hypotension occurs due to cerebrospinal fluid (CSF) leaks which may manifest as low-pressure headaches. Supportive imaging findings include diffuse enhancement of the meninges , enlargement of the cerebral venous sinuses  as well as enlargement of the pituitary . It can however mimic Chiari malformation by creating the appearance of cerebellar tonsillar herniation and inferior displacement of the brainstem . The diagnosis can be further characterized with dedicated spinal imaging to locate dural defects or CSF leaks, although the yield may be variable .
Type I Chiari malformations, on the other hand, are a group of disorders where there is downward displacement of the hindbrain past the foramen magnum . The clinical presentations of these conditions vary, from completely asymptomatic to respiratory failure, and are usually detected from infancy to early adulthood. The pathophysiology behind Type I Chiari malformations is not fully delineated, although multiple mechanisms have been proposed.
Neurosurgical intervention in the form of decompression surgery is often performed in symptomatic patients with Type I Chiari malformations . This patient went on to have a posterior fossa decompression with C1 laminectomy and duraplasty and experienced an initial symptomatic improvement, however subsequently developed progressive hindbrain herniation and a recrudescence of his symptoms, thereby suggesting an alternate pathophysiology behind the patient’s symptoms. The degree of transtentorial herniation and effacement of the prepontine subarachnoid space is unusual in a Chiari 1 malformation. The third and lateral ventricles were collapsed, and there was a suggestion of collapse of the optic nerve sheaths which are also atypical in Chiari malformations but could be consistent with intracranial hypotension. Furthermore, the age of presentation is also significantly later than most Type I Chiari malformations. Although initial imaging did not demonstrate subdural collections or significant dural thickening consistent with spontaneous intracranial hypotension, we note that pachymeningeal enhancement may become less prevalent over time, and the patient’s symptom chronicity at presentation may have hindered the diagnosis. The calvarial thickness is elevated at 9.9mm which has been associated in prior case studies with prolonged intracranial hypotension . A CSF pressure measurement may have been helpful in delineating the diagnosis, however, this was not obtained prior to neurosurgical intervention
We propose that the patient may have had a preexisting Type I Chiari malformation, and had subsequently developed intracranial hypotension. An alternative explanation is that the patient may have developed an acquired Chiari malformation due to significant intracranial hypotension as previously described by Atkinson et al in 1998 , although our patient differs because the degree of herniation is much more severe and marked hypersomnolence was a dominant feature.
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