CVT and Intracranial hypotension

Coexistence of findings spontaneous intra cranial hypotension and chronic Dural sinus thrombosis.

A 30 yo male with long standing headache. Recurrent of episodes of severe headache with nausea and vomiting.In history of previous hospital admissions headache improves with head low. Now on admission bilateral papillodema. No neuro signs. No history of lumbar puncture.

On admission MRI:

Findings of intra cranial hypotension in this case are:
1. Thin layer of sub dural effusion overlying bilateral cerebral convexity.
2. Medialisation and compression of both the lateral ventricles.
3. Mid brain compressed and antero posterioly elongated.
4. Mid brain and Brain stem sagging down on sagittal sections.
5. Obliteration of supra sellar cistern, pre pontine  and cp angel cistern.
6. Low lying cerebellar tonsils.

Findings of thrombosed dural venous sinus in this case are:
1. Heterogeneous T2 high siginal in the region of Dural venous sinuses namely superior sagittal sinus and left lateral sinuses. Loss normal T2 flow voids,
2. No to poor flow related signal on MR Venogram in the region of sinus.
3. Tiny eccentric T2 flow void in the region of sinus on cross sections which show poor flow related signal on MR Venogram represent partially recanalised channels as a part of chronic sinus thrombosis.

In this case, Idiopathic Intra canial Hypotension (IIH, also known as Spontanous Intracranial hypotension, SIH) is complicated by CVT. Intracranial hypotension is rarely associated with CVT. It is proposed that intracranial hypotension appears as primary event followed by CVT, the engorged dural venous sinuses result in venous stasis and predisposes to Dural sinus thrombosis. So IH may constitute an additional risk factor for CVT and an unusual cause for CVT.

Spontaneous Intracranial Hypotension:
 An important cause of new onset headache, common in young and middle age, in female twice more common than male.
 Often underestimated as the condition is uncommon ( ~1 per 50,000) , lack of knowledge of condition and CT in early cases may be normal.
 Exact cause not known, appears to be caused by CSF leak at base of skull or spine secondary to weak meninges and dural defects. History of previous head injuries in few.
 History of previous lumber puncture to be ruled out fist to label it as spontaneous.
 In an intact bony calvarium the total volume is a sum of volumes of intracranial blood, CSF & cerebral tissue, this remain constant. If due to any cause csf loss occurs is compensated by increased vascular component by vascular congestion or by increased extra axial CSF, effusions or hematoma.
 Subdural hematoma is caused by tearing of bridging veins traversing the subdural space.
 Sagging of brain stem and Low lying cerebellar tonsils caused by loss of CSF buoyancy.

Intracranial hypotension MRI

A 45 yo male with headache and altered sensorium.
Here is CT and MRI study of brain axial T2w and sagittal T1w images.

This CT and MRI study of Brain shows:

Bilateral subdural collections, isodense on CT appears to be sub acute sub dural hematoma.
Both the lateral ventricles compressed and medialised.
Mid brain compressed and antero posteriorly elongated.
Brain stem sagging down on sag T1 with abnormal downward protrusion of cerebellar tonsils.

Based on above findings diagnosis of Intracranial Hypotension needs consideration clinically. 

Patient given head low showed partial clinical improvement.

Intracranial Hypotension

Syndrome of headache caused by reduced intracranial CSF pressure.
Reduction of intracranial pressure is 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 includes neurosurgical procedure, dehydration, and trauma. Lumbar puncture is most common cause among all.

Possible imaging findings in Intracranial Hypotension
- Diffuse thickening of the pachymeninges with enhancement or
- Subdural effusion / hematoma in advanced cases.
- Engorgement of dural venous sinuses.
- Enlargement of the pituitary.
- Slit like lateral ventricles.
- Antero posterior elongation of mid brain.
- Sagging brain stem.
- Protrusion of cerebellar tonsils.

Classical triad : 
1 Brain stem sagging.
2 Dilatation of veins and dural sinuses.
3 Dural thickening with enhancement, subdural fluid collections.
....not expected in every case, absence of any one of them does not preclude the diagnosis.

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.
Descend 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.

Most important is after sincerely mentioning all the findings, one must mention or suggest about the condition of so called intra cranial hypotension in the report, a frequently misdiagnosed syndrome of headache caused by reduced intra cranial pressure to alarm clinician that sagging down brain stem and tonsillar desend is due to low intra cranial pressure and not secondary to raised intra cranial pressure due to bilateral sub dural hematoma. Craniotomy for evacuation of subdural hematoma or decompression should not be attempted.

Treatment: 
Aimed at restoring CSF volume by fluid replacement.
Bed res with head low.
Active leak, Lumbar or directed epidural blood patch.Intrathecal saline infusion.
If blood patch fails dural suturing, packing with muscle pledget, Gelfoam or fribrin glue.

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

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