In many such patients with ostensibly idiopathic low CSF pressure syndromes, there has been a tear in the delicate arachnoid surrounding a nerve root, with continuous leakage of CSF. The site of the leak is difficult to ascertain except when it occurs into the paranasal sinuses (causing CSF rhinorrhea). In a series of 11 patients with spontaneous intracranial hypotension, a putative leak was found by radionuclide cisternography or CT myelography (the preferred procedure by many clinicians) in the cervical region or at the cervicothoracic junction in 5 patients, in the thoracic region in 5, and the lumbar region in 1 (Schievink et al). In the patients who underwent surgical repair, a leaking meningeal diverticulum (Tarlov cyst) was found and could be ligated. This seems to be the most common structural cause, for which reason, we usually undertake a spinal MRI as the first study in obscure cases of leak. A blood patch, as described above, may be useful and should be attempted before resorting to surgical repair of the cyst. After a blood patch there has been no recurrence in all but a few of the cases that we have encountered. Others, however, have reported repeated episodes of orthostatic headache. Sometimes a case of intracranial hypotension becomes chronic; the headache is then no longer responsive to recumbency. Mokri and colleagues (1998) have also made the point that orthostatic headache and diffuse pachymeningeal enhancement on MRI may occur in the presence of normal CSF pressures.
One of the most remarkable syndromes associated with low CSF pressure is that following spinal surgery that utilizes suction drainage of the lumbar wound. The patient may fail to awaken from anesthesia or show signs such as pupillary asymmetry or a seizure. Imaging studies show features usually associated with global brain hypoxia-ischemia including signal changes in the basal ganglia, thalamus, and deep cerebellar nuclei. This syndrome apparently occurs if there has been a dural spinal leak and a high volume of CSF drainage by the suction device (Parpaley et al). The mechanism is not clear, but cerebral venous congestion has been implicated.
As noted, a helpful diagnostic sign of low CSF pressure is prominent dural enhancement with gadolinium on the MRI (see Fig. 29-6), a phenomenon attributed by Fishman and Dillon to dural venous dilatation; this finding may extend to the pachymeninges of the posterior fossa and the cervical spine. According to Mokri and colleagues (1995), biopsy of the dura and underlying meninges in these cases shows fibroblastic proliferation and neovascularity with an amorphous subdural fluid that is hard to interpret. There may be additional subdural effusions and mass effect, either on the cerebral convexities, temporal lobes, optic chiasm, or cerebellar tonsils. Using ultrasonography, Chen and colleagues have described an enlarged superior ophthalmic vein and increased blood flow velocity in this vessel, both of which normalized after successful treatment. The pituitary gland is usually hyperemic as well.
In the most severe cases of low-CSF pressure syndrome, downward sagging of the entire cerebrum with stupor, as described in Chap. 17, or stretching of the vein of Galen and consequent impairment of venous drainage may cause life-threatening swelling of diencephalic and mesencephalic structures (see Savoiardo et al and Schievink).
The use of a one-way shunt valve in hydrocephalus may be complicated by a syndrome of low CSF pressure. Reference has already been made to this syndrome, and to the slit ventricles in children who have been treated for hydrocephalus, and to midbrain syndromes from brain displacement (see Fig. 29-5). Usually the valve setting is too low, and readjustment to maintain a higher pressure is corrective.