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Show Journal of Ncuro- Ophthalmology 15( 2): 79- 83, 1995. © 1995 Raven Press, Ltd., New York Spontaneous Intracranial A Review Gregory S. Kosmorsky, D. O. Headaches caused by low intracranial pressure are usually associated with a dural puncture in some form, such as a diagnostic procedure, myelography, or spinal anesthesia. The syndrome of spontaneous intracranial hypotension ( SIH) was first described by Schaltenbrand in 1938 and termed " aliquorrhea" ( 1). In this syndrome patients experience symptoms such as headache relieved by recumbency, neck stiffness, nausea, vomiting, tinnitus, and vertigo without having undergone any procedure that violates the cerebrospinal fluid ( CSF) space. Schaltenbrand originally postulated three possible mechanisms for this syndrome: CSF leakage, reduced CSF production, and increased CSF absorption. The diagnosis is a straightforward one in that the symptoms exactly mimic those produced by a lumbar puncture. The etiology of this syndrome is usually believed to be secondary to a spontaneous rent in the dura anywhere within the neuraxis. These leaks are usually detected with the aid of radionuclide cisternography, although contrast myelography has been successfully used to make the diagnosis. Utilizing this technique, it has been possible to detect subarachnoid CSF leaks in such conditions such as postdu-ral puncture headaches, CSF rhinorrhea and otorrhea, and postsurgical CSF fistulae. Similarly, contrast myelography has demonstrated diagnostic value identifying the exact point of an active leak. The most characteristic feature of this syndrome is that of headache relieved by lying down. The headache tends to be intense and holocranial while the person is erect, although patients report frontal or occipital headaches and even nuchal headaches Manuscript received April 26, 1994; revised July 11, 1994. From the Cleveland Clinic Foundation, Cleveland, Ohio U. S. A. Address correspondence and reprint requests to Dr. Gregory S. Kosmorsky, Division of Ophthalmology A- 31, Cleveland Clinic Foundation, 1 Clinic Center, Cleveland, OH 44195- 5024. ension at times. The mechanisms involved in the production of pain are unclear. CSF hypotension alone is not the only factor. The most likely explanation is " settling" of the brain with traction on the pain-sensitive structures of the cranial vault ( 2). An alternative mechanism is brain swelling due to venous congestion ( 3,4). Compression of the jugular veins and the Valsalva maneuver worsen the pain even in the supine position. Both of these maneuvers directly increase intracranial pressure as well as intracranial blood volume emphasizing that CSF hypotension alone is not the only etiologic factor. Activation of adenosine receptors has also been hypothesized as a cause of this pain, as adenosine receptors are found throughout the central nervous system and within the dura. In one study, 85% of patients were relieved of headache by the use of caffeine, which is known to block adenosine receptors ( 5). The headaches of SIH are often accompanied by auditory symptoms described as roaring, buzzing, popping, or echoing ( 6). Other symptoms, such as a decreased hearing, particularly for high tones, and tinnitus have been described. In addition, noises and other symptoms relating to the vestibular apparatus- dizziness, nausea, and vomiting- have also been documented. The etiology of these symptoms is postulated to be due to a change in intralabyrinthine pressure, perhaps caused by changes in pressure across the cochlear aqueduct ( Fig. 1) ( 7). This reduction in intralabyrinthine pressure has been demonstrated in cats by Hughson ( 8). Visual symptoms have been reported in face of low CSF pressure in the context of lumbar puncture. The reported incidence of such phenomena was only 34/ 10,098 ( 0.33%) ( 6). Obviously, persistent intracranial hypotension is not an uncommon occurrence after lumbar puncture, therefore, these figures likely underestimated the real incidence of G. S. KOSMORSKY visual changes with low CSF pressure. It is also likely that patients underreport visual changes, or such reports are ignored. Such symptoms include double vision, blurring, inability to read, sensitivity to light, spots before the eyes, and trouble focusing. Although cases of sixth nerve palsy, both unilateral and bilateral, have been well documented with lumbar puncture, it is quite possible that the incidence of sixth nerve palsies is unreported, presenting as difficulty with reading or blurring. The etiology of the lateral rectus palsies are likely due to displacement of the brain interiorly with pressure placed upon the abducens nerve against the clivus as well as inferior displacement of the brainstem with stretching of the nerve anchored by Gruber's ligament. The other visual symptoms, such as spots before the eyes and silvery streaks, may have a different etiology. One young woman was reported to say " My first time on street after the operation, the air was alive with silvery streaks and black spots. These have continued to be present" ( 6). Perhaps the mechanism of downward herniation could partially explain the visual symptoms on an occipital ischemic basis. The syndrome of rostrocaudal deterioration is well known. Ropper ( 9) has recently demonstrated that a shift of only 2 mm can create enough mechanical pressure on the posterior cerebral arteries against the tentorium cerebelli to create infarction of the occipital lobes. Similar phosphenes and spots are frequently reported per-tubations in patients during the spreading depression stage of migraine and are believed to emanate from the occipital lobes. Perhaps downward herniation of the brain compresses the posterior cerebral arteries, producing a low- grade occipital ischemia and resulting in the visual phenomenon that these patients report. Horton and Fishman ( 10) have reported visual field defects occurring in patients with the SIH syndrome. They reported three patients who had visual complications. One of the patients had a sixth nerve palsy that spontaneously improved. The other two patients had transient visual obscurations associated with " odd" nasal defects on screening visual fields. These patients were feeling ill ( leading to test- taking error) and only a 120- point screening field could be performed, as opposed to a more detailed 24- 2 program. These authors showed resolution of the visual field de- FIG. 1. Cochlear aqueduct: Possible explanation for hearing symptoms in SIH, as this duct connects CSF space with the auditory apparatus. / Ncuro- Ophthalmol, Vol. 15, No. 2, 1995 SPONTANEOUS INTRACRANIAL HYPOTENSION 81 fects after the resolution of the SIH syndrome in these patients. They proposed that descent of the brain compressed the optic chiasm by settling. Resolution of the SIH reversed this compression. The settling phenomenon was convincingly demonstrated on serial magnetic resonance images. The causes of this syndrome are largely unknown. Many of the patients reported with SIH have had a history of preceding strenuous exercise or a fall landing on the buttocks as a prelude to the onset of their symptoms ( 11- 13). Surgical demonstration of nerve root sleeve tears has been documented ( 12). The complex motion of the spine and dural root sleeves leads one to believe that even everyday movement may cause dural tears ( perhaps mainly of the root sleeves) ( 14). So- called Tar-lov cysts are rarefactions of the dural root sleeves and may be particularly prone to mechanical stress forces. Spinal epidural cysts may also be present and prone to mechanical rupture. Such a case was reported by Lasater ( 11), in which a protein- rich sac was found adjacent to a meningeal defect with communication between the subarachnoid space and the adjacent cyst ( 11). A similar case was reported by Rando ( 2), in which myelographic contrast material was seen to leak out of the subarachnoid space into a cyst- like structure that was thought to represent an epidural cyst. Secondary causes involve a clear history of an invasive opening in the dura. These causes include lumbar puncture, trauma, craniotomy, or spinal root surgery. The last category, producing a low CSF pressure, is that associated with medical illness, such as dehydration, diabetic coma, hyper-pnea, meningoencephalitis, uremia, following intravenous administration of a hypertonic solution, and severe systemic infection. Radionuclide cisternography has revealed an abnormal pattern of rapid disappearance of the radioactive tracer from the subarachnoid space with accumulation in the bladder, suggesting an increase in CSF absorption ( 2). However, this abnormal pattern is also consistent with a small dural CSF leak with rapid uptake into the circulation through the epidural venous plexus. The notion of " hyperabsorption" of CSF has never been demonstrated physiologically. The rate of CSF absorption is relative to the hydrostatic pressure differential between the subarachnoid space and the draining venous channels. Theoretically, lowering the CSF pressure should decrease absorption that should tend to normalize the CSF pressure. Therefore, the leak created by either by a dural tear or some invasive procedure is a likely mechanism for maintenance of the low CSF pressure and not hyperabsorption. Magnetic resonance imaging has revealed several abnormalities that in combination are fairly characteristic of this disorder. Pannullo and colleagues ( 15) have demonstrated that meningeal enhancement is common in SIH. This enhancement has several characteristic features: ( a) involvement of peripheral meninges ( cerebral, cerebellar, inter-hemispheric fissure, and tentorial), ( b) no enhancement around the brainstem, and ( c) contiguous involvement ( i. e., no skip areas or segmentation). The linear enhancement may be interrupted by subdural effusion or hemorrhages that can occur with persistently low CSF pressure. Other MRI abnormalities are inferior displacement of the midbrain iter and impaction of the cerebellar tonsils that mimic an acquired Arnold- Chiari type I malformation. These abnormalities typically occur separately, however, when both of these abnormalities occur together or particularly in conjunction with dural enhancement they are quite characteristic of SIH. The diagnosis of SIH can be made clinically if all of the classic symptoms and signs are present, if not the diagnosis is made on lumbar puncture. The diagnosis in some cases may be missed due to a " dry tap" that is ascribed to misplacement of the needle. Indeed, the pressure in this syndrome should be below 60 mm of water ( 2). In some cases, the pressure may even be negative. In such cases, a " sucking sound" can be heard as the needle enters the subarachnoid space. When there is no spontaneous flow of CSF, the patient may either perform a Valsalva maneuver or a syringe may be used to aspirate the CSF. An elevation of protein and red blood cells is found in many cases. The etiology of this increase in protein and red blood cells is unclear; however, hyperemia of the meningeal vessels was postulated by Schaltenbrand. Certainly, the blood- brain barrier may be disrupted when bridging vessels are placed on stretch, and this may be a contributing factor. Evidence for disruption of the blood-brain barrier is provided by the finding of meningeal enhancement seen on MRI scanning ( 15). A lymphocytic pleocytosis may be present without evidence of infection. When tested, CSF cultures have been negative ( 2). The occurrence of spontaneous subdural hematomas or hygromas has been documented in patients with the SIH syndrome ( 11,16,17). These are likely to result from the rupture of bridging cortical veins as the brain descends. These are a / Neuro- Ophthalmol, Vol. 15, No, 2, 1995 82 G. S. KOSMORSKY secondary phenomena and not the primary cause of headache in this syndrome. The treatment of this disorder is still controversial. It is recognized that most cases of SIH will spontaneously resolve within 2 to 16 weeks. Analgesics are of little benefit as assuming the erect posture causes the headache regardless of pain medication therapy. Treatment for this disorder has involved bed rest, fluid replacement by mouth, oral caffeine, inhalation of carbon dioxide, intrathecal saline injections, epidural saline infusions, steroids, or epidural blood patches ( 2). Although the headache will resolve spontaneously, some form of treatment is of great benefit to the patient. The use of caffeine has been reported to work in as many as 85% of patients with multiple dosings ( 5). The mechanism of this is unclear but may be mediated through adenosine receptors. Steroids are effective in some patients and probably work by decreasing presumed brain swelling, which has been demonstrated by tight basal cisterns on imaging studies ( 3). The mechanism of brain swelling is not entirely understood and definitive evidence for brain swelling is lacking in these patients. Inhalation of carbon dioxide shrinks the brain by decreasing venous flow that has been shown to play some role in the production of the pain in this syndrome ( 18). Intrathecal saline infusions have occasionally been used; however, the risk of infection seems to be higher with an indwelling catheter, which must be left in place for several days ( 16). The use of continuous epidural saline infusion has been used with good results. Given that the epidural saline does not patch the presumed hole we might assume that compression of the epidural space with secondary compression of the thecal compartment elevates the CSF pressure and produces the immediate relief of headache. Injections of small amounts of fluid into the epidural space can immediately raise the CSF pressure to normal or supernormal levels ( 20). The use of an autologous epidural blood patch appears to be the most efficacious form of therapy ( 21- 23). Once injected, the 12- 18 ml of blood typically spread cephalad and caudal to cover 8 to 10 spinal segments. However, for a maximum benefit the injection should be at least within one level of the suspected dural rent. The relief of pain is immediate and not easily explained by sealing the leak, even though our entire volume of CSF is turned over 4 to 6 times per day. Perhaps compression of the epidural space as postulated for saline injections is a sufficient explanation for the immediate relief of headache with the long- lasting headache relief resulting from the clotted blood patching the hole. Interestingly, the blood patch is not very effective in the first 24 hours and becomes far more predictable after this time in terms of pain relief. The most common complication of blood patching is back pain at the local injection site, which occurs in approximately one- third of patients. This is usually transient; however, it has been documented to last for several months in some patients. This complication may result from inadvertent injection of the blood into the thecal space resulting in a chemical meningitis. Less common complications include radicular pain and paresthesias, as well as lumbosacral meningismus. The disorder of SIH is an unusual one; however, it is likely more common than recognized and it should be sought in appropriate circumstances. The use of cisternography to detect the leak is a valuable tool, but is not always easy to interpret with accuracy. The rapid disappearance of the tracer from the subarachnoid space with the early appearance in the bladder is suggestive of a leak within the CSF space. 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