Spontaneous intracranial hypotension

Ever since 1938, when Schaltenbrand first described spontaneous intracranial hypotension (SIH), which he termed spontaneous aliquorrhea,1 clinicians have recognized the disorder by its clinical resemblance to post lumbar puncture symptoms. However, SIH was thought to be rare until its associated MRI abnormalities were recognized in the early 1990s. These revolutionized the diagnosis of SIH and demonstrated that the disorder is by no means rare.2-4⇓⇓ Furthermore, the clinical and imaging spectrums are far broader than had been originally appreciated. Although Schaltenbrand believed the cause of the disorder was low production of the CSF, the overwhelming majority if not all cases of SIH result from spontaneous CSF leaks.

These leaks are typically at the level of the spine, particularly the thoracic spine and cervicothoracic junction, and only rarely at the skull base. Single or multiple meningeal diverticula are frequent and a minority of patients with SIH have evidence of a connective tissue disorder.4 The common clinical features of SIH are orthostatic headaches, neck or interscapular pain, nausea, emesis, horizontal diplopia, change in hearing, blurring of vision, facial numbness, and upper limb radicular symptoms. The headaches, although typically orthostatic, may occur when seated or recumbent and evolve into chronic daily headaches. The head MRI abnormalities in CSF leaks include diffuse pachymeningeal gadolinium enhancement, MR evidence of descent of the brain (descent of cerebellar tonsils, decrease in size of prepontine and perichiasmatic cisterns, flattening of the optic chiasm, and crowding of the posterior fossa), decrease in the size of the ventricles (“ventricular collapse”), and engorgement of cerebral venous sinuses and enlargement of the pituitary gland. Spine MRI may show extra-arachnoid fluid, meningeal diverticula, meningeal enhancement, or engorgement of epidural venous plexus. It may also define the level or the site of the leak. In some of the patients with documented symptomatic CSF leaks, CSF opening pressures may be consistently normal, or the meninges may appear normal on MRI. Finally, in some patients headache may be absent.5 In SIH, CSF pressures may vary, ranging from unmeasurable to normal, and the CSF cell count and protein concentrations also vary. CSF protein concentrations as high as 100 to 200 mg/dL and lymphocytic pleocytosis of 10 to 40 cells/mm² can occur; even higher figures are not rare.4 Whereas clinical manifestations, CSF findings, and MRI abnormalities vary, the CSF volume is always decreased.5-6⇓ To emphasize this invariant feature, the term “CSF hypovolemia” has been used in the past by one of us,5 but he has come to prefer the term “CSF volume depletion,” which obviates potential confusion with “hypovolemia,” which is typically referred to as decrease in plasma volume.

The optimal treatment strategy has not been defined. Many patients improve spontaneously or coincident with noninvasive measures such as bed rest, hydration, caffeine, and corticosteroids. Epidural blood patch (EBP) is used in patients who fail noninvasive measures, but many patients may require repeated EBP. Surgical repair of the leak has been performed in cases that have failed EBP if a site of the CSF leak has been identified.4 Epidural injection of fibrin glue has preliminary encouraging results.

In this issue of Neurology, Alvarez–Linera et al. report MRI evidence of enlargement of the pituitary gland in SIH in 11 consecutive patients (7 with spontaneous leaks, 3 with post lumbar puncture leaks, and 1 with leak following spine surgery).7 The authors have studied the height and imaging morphology of the pituitary gland in mid-sagittal sections in all patients during the symptomatic phase and after recovery, and have noted a larger size of the gland during the symptomatic phase.

The size of the pituitary gland and presence or absence of asymmetry of the gland can be more accurately assessed on coronal images through the pituitary rather than on midsagittal images. Others have observed symmetric or asymmetric pituitary enlargement in spontaneous CSF leaks8 but not in every patient. In CSF volume depletion, whether due to CSF leak or CSF shunt overdrainage, according to the Monro–Kellie doctrine, the decrease in CSF volume would result in increase in intracranial blood volume.2 This is primarily reflected on the venous system by diffuse pachymeningeal enhancement and engorgement of cerebral venous sinuses. The pituitary gland is rich in vascular channels. The enlargement of the pituitary gland in SIH presumably reflects volume compensatory hyperemia. The enlargement may raise the possibility of pituitary adenoma or hyperplasia. On MRI, however, appearance of gadolinium enhancement in many pituitary microadenomas is typically slower than in the adjacent normal pituitary.9

Enlargement of pituitary gland in CSF leak, yet another MRI feature of this disorder, is reversible and should not be mistaken for pituitary hyperplasia or adenoma. Attention to the full clinical and imaging spectrum of SIH allows accurate diagnosis in virtually all cases.