Friday 30 March 2012

Intracranial hypotension

A case of mild headache with history of lumbar puncture 2 days back shows mild diffuse dural thickening with enhancement, bilateral thin layer of sub dural effusion, enlargement and rounding up of superior sagittal sinus, mild diffusion enlargement of pituitary. Consistent with clinical diagnosis of post lumbar puncture intracranial hypotension. There is no sagging of Brain stem. 

Intracranial Hypotension Imaging 

Reduction of intracranial pressure due to reduction in Csf volume.
Clinically characterized by headache marked in upright posture - postural headache. 
May have isolated abducens nerve palsy, necks stiffness, hearing loss. 
The condition may be spontaneous or secondary to lumbar puncture. Other cause include neurosurgical procedure, dehydration, trauma. Lumbar puncture is most common cause among all. 

Imaging findings in Intracranial Hypotension
- Diffuse thickening of the pachymeninges with enhancement, 
- Engorgement of dural venous sinuses. 
- Enlargement of the pituitary.
- Subdural effusion / hematoma. 
- Sagging brain stem. 

Most of these findings are the result of vascular dilation to compensate for sudden depletion of Csf volume, the explanations are based on Monro Kellie hypothesis, which states that the sum of the volumes of intracranial blood, CSF, and brain tissue remain constant in an intact cranium. Accordingly increased intracranial blood volume compensates for acute loss of CSF. Dilation of the venous side of circulation contributes a lot due to its high compliance and capacitance. 

Meningeal enhancement is thick, linear, without nodularity and involves the pachymeninges without evidence of involvement of the leptomeninges. 

Dura matter, the innermost layer composed of fibroblasts with inter digitating processes that create spaces in between. Extravasation of fluid occur into this layer, in these spaces, in response to increased dural vasculature as the dura lacks blood brain barrier and tight junctions.These extravasations explains dural thickening as well as contrast extravasation and enhancement. Tight junctions in arachnoid and pia mater prevent the similar contrast accumulation, explaining enhancement is limited to the dura. Though it is a frequent finding, abnormal meningeal enhancement is not the rule as cases are reported which are still symptomatic but enhancement that resolved earlier where as in certain typical cases MR images never revealed enhancement at any stage of disease. 

Sub dural effusions occur when the extravasation continue even after meningeal thickening and enhancement, to the point of fluid accumulation in the subdural space as supported by studies in which effusions were not seen in the absence of meningeal enhancement represent more advanced stage of the condition. These sub dural effusions are typically thin, crescentic, often bilateral.

Subdural hematoma occur when effusion get complicated with bleed in subdural space due to rupture of the bridging veins traversing sub dural space in response to traction by ongoing extravasation and effusion.

Descent of cerebellar tonsils with sagging of brain stem, an associated effacement of prepontine cistern, obliteration supra chiasmatic cistern with inferior displacement of the optic chiasm result from reduction of normal Csf buoyancy due to reduced csf volume and represent most advanced stage of disease and severe Csf volume depletion, occurs after all other compensatory mechanisms have exhausted.

Isolated 6th nerve palsy reported in considerable amount of cases. In fact it is the most common nerve among all to get affected due to its longer intracranial course. Often get encountered at inisura when there is sagging of mid brain with antero posterior elongation. 

Engorgement of dural venous sinuses seen as enlarged and round dural venous sinuses which are normally triangular in shape on cross sections.

Pituitary enlargement reflects simple compensatory venous hyperaemia.

Regression in these imaging findings often parallels clinical improvement of these, reversal of pituitary enlargement occurs first. 

Reference : Intracranial Hypotension Syndrome: A Comprehensive Review: Imaging Studies; Neurosurg Focus. 2003;15(6) © 2003 American Association of Neurological Surgeons. 

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