Benign intracranial hypertension (B.I.H.) is also known as Idiopathic intracranial hypertension.It describes the syndrome of increased intracranial pressure in which intracranial mass lesions, obstruction of the cerebral ventricles, intracranial infection, hypertensive encephalopathy, and chronic retention of carbon dioxide (pulmonary encephalopathy) have been excluded. It has also been termed pseudo tumor cerebri, serous meningitis, and otitic hydrocephalus. B.I.H. includes a heterogenous group of disorders in which a number of different etiologic factors have been identified, although in most cases the cause and parthenogenesis of these syndromes are poorly understood. They. are . termed “benign” because spontaneous recovery generally occurs; however, serious threats to vision may occur.
The presenting symptoms are headache and disturbance of vision. The headache is often worse on awakening and is aggravated by coughing and straining. It is often relatively mild and may be entirely absent. The most common ocular complaint is visual blurring, a manifestation of papilledema. Some patients complain of brief, fleeting movements of dimming or complete loss of vision, occurring many times during the day (amaurosis fugax), at times accentuated or precipitated by coughing and straining. This ominous symptom indicates that the patient’s vision is in serious jeopardy.
Visual loss may be minimal despite severe chronic papilledema, including retinal hemorrhages; however, blindness rarely may develop very rapidly —in less than 24 hours. Visual fields characteristically show enlargement of the blind spots, and may show constriction of the peripheral fields and central or paracentral scotoma. Diplopia due to unilateral or bilateral sixth nerve palsy may develop as a result of increased intracranial pressure. The neurologic examination is otherwise ‘ normal. A major clinical point is that patients with B.I.H. commonly look well; Chat is, theirappearance and apparent well-being belie the ominous appearance of the papilledema.
Pathophysiology of Benign Intracranial Hypertension
The signs and symptoms of B.I.H. are due to the effects of increased intracranial pressure; pressures between 300 and 600 mm. a frequent. The intracranial pressure is normally between 50 and 180 to 200 mm. of water as measured in the lumbar sac or ventricles with the patient in the lateral recumbent position. This pressure is dependent upon the pressure-volume relationships within the intracranial and spinal cavities. The intracranial cavity contains about 1400 ml. of brain, 75 ml. of blood, and 75 ml. of CSF, and an additional 75 ml. in the spinal subarachnoid space.
The CSF pressure is directly dependent upon the intracranial venous pressure; changes in the latter pressure are readily transmitted to the CSF, and thus a sustained increase in intracranial venous pressure may result in the syndrome of chronically increased intracranial pressure. The intracranial pressure is largely independent of the systemic arterial pressure, and it is normal in essential hypertension; however, it falls with acute systemic hypotension and rises acutely with very acute increases in systemic blood pressure, e.g., with the administration of vasopressor drugs or in the syndrome of acute hypertensive encephalopathy.
Increased intracranial pressure also accompanies increased cerebral blood flow resulting from C02 retention, as in acute asphyxia or with chronic pulmonary insufficiency. In patients with B.I.H.. apart from those with obstruction of the intracranial venous system, the mechanism of the increase in intracranial pressure is unknown. Two different kinds of changes have been noted on pneumoencephalography. The first has been normal air studies except for narrowed, slitlike ventricles and little air in the cortical subarachnoid space, which has been interpreted to mean that the brain volume is increased due to “edema.”
The second has been normal air studies apart from normal or enlarged ventricles with an excessive amount of air in the cortical subarachnoid spaces, implying an increased volume of CSF. It is not clear whether these differences represent separate entities or whether the differences can be attributed to variation in time, i.e., the first type may develop into the second type after sufficient time has elapsed. Although cerebral edema has been suspected, there are few clinical manifestations of cerebral dysfunction in these patients to denote any functional changes as a result of the edema.
There are new methods available that permit measurement of the rates of formation and of removal of CSF in man, but to date these have not been applied to patients with this syndrome. (The only disease in which there ip any evidence in favor of a pathologic increase in CSF formation is papilloma of the choroid plexus). The calculation of the Ayala Index at lumbar puncture, an old clinical guide to the volume of CSF, favors an increased CSF volume in many patients with B.I.H.
5.0 to 7.0 is normal. Values greater than 7.0 denote increased CSF volume; values less than 5.0 denote decreased CSF volume). The index is almost always increased in patients with benign intracranial hypertension. Although there are clinical correlations between various endocrinopathies (vide infra) the mechanism wherein the adrenal, parathyroids, ovaries, or anterior pituitary might affect brain or CSF volumes is obscure.
Etiologic Factors and Associated Disorders.
Intracranial Venous Occlusion with Relation to infection, Trauma, and Pregnancy. Increased intracranial pressure as a result of occlusion of the intracranial venous sinuses occurs most commonly as a consequence of otitis media with extension of the infection into the petrous bone and to the wall of the lateral sinus. This syndrome has been termed mitotic hydrocephalus. It occurs as a complication of both acute or chronic infection; at times the evidence for otitis media is minimal and readily overlooked. The sixth cranial nerve may also be involved, giving rise to diplopia on lateral gaze.
Thrombosis of the superior longitudinal sinus may occur as a consequence of relatively mild closed head injury and may give rise to a pseudotumor syndrome. (Occlusion of this sinus that drains both cerebral hemispheres is more likely to result in hemorrhagic infarction in the cerebrum as the thrombosis extends into the cerebral veins, giving rise to bilateral signs. In such cases, the course is frequently fulminate and the prognosis guarded, although occasionally complete recovery may occur.) Aseptic or primary thrombosis of the superior longitudinal sinus may also be responsive for a pseudotumor syndrome.
This develops as a complication of pregnancy and has been’ reported to occur during the first two to three weeks post partum and also at the end of the first trimester of pregnancy. A disorder of the blood- clotting mechanism has been suggested as basis for these events during the postpartum period, although this has not been substantiated.
A common association is the occurrence of B.I.H. in women with a history of menstrual dysfunction. The women are frequently moderately to markedly overweight (without evidence of alveolar hypoventilation). Menstrual irregularity is common, often with amenorrhea. Galactorrhea is an uncommon associated symptom. The histories usually emphasize excessive premenstrual weight gain. Endocrine studies thus far have not revealed any abnormality of urinary gonadotrophins or estrogens in these patients, and the pathogenesis is unknown.
Idiopathic. One of the Zionist common forms of B.I.H. is its occurrence in otherwise healthy subjects in the absence of any of the etiologic factors described above. Both sexes are involved and the occurrence is most often between the ages of 10 and 50 years. These cases represent the idiopathic form of B.I.H.; its parthenogenesis is a mystery.
Diagnosis of Benign Intracranial Hypertension.
The patient with headache and papilledema without other neurologic signs must be considered to have an intracranial, mass, ventricular obstruction, or intracranial infection until proved otherwise. Although the diagnosis of B.I.H. may be suspected by the appearance of apparent well-being and by the history of some of the associated features listed above, the diagnosis is essentially one of exclusion dependent upon ruling out the more common causes of increased intracranial pressure.
Brain tumor, particularly when located in relatively silent areas such as the frontal lobes or right temporal lobe or when obstructing the ventricular system, may be manifest only by headache and papilledema. Patients with chronic subdural hematoma, without history of significant trauma, may present in the same way. Diagnostic evaluation requires skull films, electroencephalography, and arteriography and/or air studies (see Intracranial Tumors). Lumbar puncture is necessary in these patients, but is generally deferred until arteriography has revealed that the ventricular system is normal in size and location.
Laboratory studies regarding possible hypoadrenalism or hypoparathyroidism may be rewarding in rare cases of these disorders that present with’ the pseudotumor syndrome. The cerebrospinal fluid pressure is elevated,
250 and 600 mm., but the fluid is otherwise normal. The protein content is generally low normal, and lumbar CSF protein levels below 15 mg. per 100 ml. are common.
Treatment of Benign Intracranial Hypertension
In patients with lateral sinus thrombosis due to chronic infection in the petrous bone, surgical decompression is often indicated. When the pseudotumor syndrome is a manifestation of hypoallergenic or hyperparathyroidism, replacement therapy is indicated. Vitamin A intoxication disappears when administration of the vitamin is stopped. Anti-coagulation therapy has been recommended for patients with dural sinus thrombosis; however, for patients with extension of the clot into cerebral veins with infarction of tissue, anticoagulant is hazardous because it increases the likelihood of hemorrhagic infarction.
The idiopathic form of B.I.H. and its occurrence in patients with menstrual disorders and obesity require individualized management. This syndrome is self-limited in most cases, and after some weeks or months spontaneous remissions occur, making evaluation of therapy difficult. In rare instances, the illness may last as long as two years. In the very obese, weight reduction is recommended. The use of daily lumbar punctures has been advocated to lower pressure to normal levels by removing sufficient fluid; 15 to 50 ml. of fluid may be required. Sub-temporal decompression has been widely used in the past. This procedure may be necessary for patients with serious threat to vision due to pressure, although its efficacy has been questioned in a number of reports. The use of adrenal contortionists has been advocated because these drugs minimize cerebral edema of diverse causes.
However, many patients with B.I.H. appear to have a large volume of CSF, and adrenal steroids have not been shown to affect CSF volume or the rate of CSF formation. Aceta- zolamide has been used because this carbonic anhydrase inhibitor has been shown to reduce CSF formation in animals, but there are no convincing data that this drug influences B.I.H. It should not be given intravenously because this results in an acute increase in CSF pressure. Hypertonic intravenous solutions (20 per cent urea or 25 per cent mannitol) to lower intracranial pressure can be used in acute situations when there is rapidly failing vision, while one awaits neurosurgical intervention; however, prolonged lehydration therapy is impossible because of its deleterious systemic effects. Management of these patients is difficult and requires the attention of neurologists and neurosurgeons experienced in these problems.