Atypical spontaneous intracranial hypotension (SIH) may occur with normal cerebrospinal fluid (CSF) opening pressure and in the absence of postural headache. It has been postulated that a low CSF volume state could account for these findings and explain the efficacy of epidural blood patching in these cases. However, the pathophysiology of atypical SIH remains unclear. The association between connective tissue diseases and SIH may enhance our understanding of this phenomenon.
A 52-year-old man with no neurological history was referred for evaluation of headaches and double vision. Nine months previously, he noted the gradual onset of mild to moderate constant headaches, most prominent occipitally. There was associated photophobia and phonophobia but no fever, chills, nausea, vomiting, or other systemic symptoms. Three months after the onset of symptoms, brain MRI was reported to be unremarkable, and his headaches resolved spontaneously.
Eight months later, he awoke with headache and double vision, associated with constant phonophobia and hyperacusis. The patient described horizontal diplopia that was more prominent at distance than at near but denied eye pain, swelling, redness, or blurred vision. The headaches remained unchanged with change in position or activities, which would increase intracranial pressure. There was no history of trauma or any known violation of the epidural or thecal spaces. Interestingly, his father had multiple aneurysms including the aortic, right iliac, and bilateral popliteal arteries. Neither an occipital nerve block nor the use of beta-blockers relieved the headaches.
Neuro-ophthalmological examination revealed bilateral sixth nerve palsies with intact afferent visual function. Review of the initial MRI showed thickening and enhancement of the meninges, which was more marked on a follow-up brain MRI. There was no “sagging” of the brain, tonsillar descent, or posterior fossa crowding. Opening pressure on lumbar puncture was 140 mm H2O. CSF was light yellow and clear, with 183 red blood cells per cubic millimeter and 7 white cells per cubic millimeter, all monocytes. Protein was 132 mg/dL (normal: 15-45 mg/dL), and glucose 57 mg/dL (normal: 40-70 mg/dL). Because of the normal opening pressure, an evaluation for other causes of pachymeningitis was performed but was unrevealing. The patient was given a trial of prednisone, with no improvement in his symptoms.
Two months later, headaches increased, diplopia persisted, and the patient continued to deny any change in his headaches with changes in position or activity. A third brain MRI depicted progression of meningeal enhancement with bifrontal subdural hygromas and descent of the cerebellar tonsils and optic chiasm (Fig. 1). On examination, he had no change in his bilateral sixth nerve paresis. His opening pressure on repeat lumbar puncture was again 140 mm H2O.
Despite the lack of orthostatic headache and normal CSF pressure, the patient was felt to have intracranial hypotension. Cisternography as well as MRI of the spine were done, but neither showed any evidence of a CSF leak. He underwent an epidural blood patch at the L3-L4 level. Three days later, he reported that his headaches were minimal and that his diplopia had almost resolved. Over the next week, he became totally asymptomatic with full range of eye movements, no diplopia, and no headache.
Three months following the epidural blood patch, the patient reported complete resolution of his headaches and diplopia. Neuro-ophthalmic testing was normal including full range of extraocular movements. MRI of the brain was now normal (Fig. 2). A CT angiogram of the aorta was normal.
SIH, first described by Schaltenbrand in 1938 (1), is an uncommon disorder with an estimated prevalence of 5 per 100,000 (2). It is characterized by a triad of postural headache, diffuse pachymeningeal enhancement on contrasted MRI, and low CSF pressure.
Headaches are the presenting feature in most patients and may be acute or gradual in onset. They may be generalized but when localized most commonly affect the frontal and occipital regions. As in patients with decreased intracranial pressure of any cause, the headaches are typically made worse with standing and promptly improve with sitting or lying down. This postural component is present in 92% of patients and, when absent, makes the diagnosis more difficult (3,4). Acephalgic forms of low pressure or low volume states have been reported and are usually related to overdraining CSF shunts (5,6).
Neuro-ophthalmic complaints are common. Blurred vision or transient obscurations of vision occur in 8%-23% of patients with SIH (3,7). Diplopia is reported in up to one third of patients (7-14). Unilateral or bilateral sixth nerve palsies may be due to distortion of this cranial nerve by the downward displacement (“sagging”) of the brain (7). In a literature review of 29 studies of intracranial hypotension, unilateral abducens nerve palsies were reported in 60% of cases, while bilateral paresis was noted in 24% (9).
Other symptoms noted in cases of SIH include nausea (58%-75%), vomiting (33%-58%), and pain either nuchal (29%-33%) or interscapular (25%) (3,10,12). Tinnitus, vertigo, photophobia, and phonophobia have been associated with SIH (7-9,15,16).
In the past, the diagnosis of SIH was based on the presence of postural headache associated with low CSF pressure. MRI findings in SIH have been well described and are of critical importance in establishing the diagnosis. Diffuse thickening and enhancement of the meninges and choroid plexus are present in 83% of patients with SIH (3). These changes are thought to result from leaky meningeal vessels and small vessel rupture (8,17). Subdural hematomas and hygromas, likely secondary to the rupture of veins from downward displacement of the brain, are each seen in 8% of cases (4). More recently, venous distension of the contour of the transverse sinus seen on T1 imaging has been reported as having a sensitivity of 93% and a specificity of 94% for intracranial hypotension (18).
Although dural enhancement is very characteristic of this condition, it may be absent in 22%-25% of patients (3,19). Interestingly, those patients had a significantly poorer response to treatment than those with characteristic MRI changes (19).
SIH has been linked to CSF leakage. Identification of such a leak is helpful in confirming the diagnosis and allowing for direct treatment. The leak often results from a tear in the arachnoid, most commonly thought to be at the root sleeve of an exiting spinal nerve. Rupture of a diverticulum, usually at the cervical or thoracic roots, is also thought to account for many of the leaks (2,11,20-23). SIH may be related to minor trauma, but it is often idiopathic (24).
It has been proposed that spontaneous CSF leaks could be secondary to dural weakness as a result of connective tissue disorders. In a review of 58 patients with spontaneous CSF leaks, 9 had features of connective tissue disorder with 1 patient having a strong family history of abdominal aortic aneurysm (25). Although our patient had no evidence of connective tissue disorder, his father had a history of multiple arterial aneurysms.
A specific CSF leak can be identified in 58%-92% of patients with typical SIH, using a variety of diagnostic techniques including radioisotope cisternography, spinal MRI, and CT myelography (3,4). In a review of 12 patients (3), site of leakage was revealed by radioisotope cisternography in 6 of 11 patients and by CT myelography in 1 patient. Of the 4 patients who had spine MRI, epidural free fluid from the thoracic to lumbar region was demonstrated in 1 case (3). In a series of 6 atypical patients with nonpostural headache and normal CSF pressures despite diffuse pachymeningeal enhancement on MRI, a CSF leakage site was confirmed in only a single patient (4).
As one would expect given the presumed pathophysiology, the CSF pressure is characteristically very low and a lumbar puncture is typically diagnostic in these patients. Lumbar opening pressure is less than 60 mm H2O in more than 80% of cases (3,12). Several studies have noted higher pressures including some within the normal range. Kong et al (4) observed 6 patients with atypical presentations who had opening pressures ranging from 90 to 120 mm H2O. Mokri et al (12) studied 40 patients with orthostatic headaches, 7 having consistently normal opening pressures ranging from 65 to 140 mm H2O.
In accounting for normal CSF pressure in suspected CSF leak headache syndromes, Mokri et al (12) proposed a check valve phenomenon with a pressure- or volume-dependent CSF leakage that would only permit CSF to leak at higher pressures or volumes. This in turn would create a low-volume state. The resultant sagging of the brain and subsequent traction exerted on pain-sensitive structures would produce a postural headache. The immediate relief experienced after the epidural blood patching might be attributed to replacement of volume by displacing and compressing the dural sac, rather than occlusion of the leak (12). This theory might best explain the therapeutic effect of an epidural blood patch, in cases such as ours, where no leak could be identified.
Abnormal connective tissue may also contribute to the pathophysiology of SIH. These patients are likely to tear the meninges with even minor trauma. Abnormal meningeal connective tissue with low compliance could explain the lack of expected drop in CSF pressure despite a decrease in CSF volume.
SIH is often self-limited or may respond to conservative measures including bed rest, hydration, mineralocorticoids, caffeine, and oral analgesics. If these measures fail, the treatment of choice is an autologous epidural blood patch (3). Other options include intrathecal or epidural saline infusions, and in refractory cases, where a CSF leak can be identified, surgery for direct ligation of a leaking meningeal diverticulum or epidural packing at the site of the leak (3).
Our case emphasizes 2 important features of SIH. First, the classical triad of SIH is not always present. Second, an epidural blood patch was successful in abolishing our patient's symptoms despite the absence of any identifiable CSF leak. SIH should be included in the differential diagnosis of patients presenting with headache and horizontal diplopia due to unilateral or bilateral sixth nerve palsies.
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