Cerebral Ischemia · 473 11

nating”) pain on one side of the face. Trigeminal neuralgia usually responds well to medical therapy (e. g., with carbamazepine). In intractable cases, however, a neurosurgical operation can be performed in which the nerve and the vascular loop are separated by a pledget of synthetic material (“microvascular decompression,” “the Jannetta procedure”). For more on the diagnosis and treatment of trigeminal neuralgia, see Chapter 4, p. 165.

Brainstem Vascular Syndromes

The many different vascular syndromes affecting the brainstem can only be understood on the basis of a thorough knowledge of its topographical anatomy. For this reason, they are discussed in the chapter on the brainstem (Chapter 4, p. 223ff.), rather than here.

Impaired Venous Drainage from the Brain

Cerebral ischemia, as we have seen, is usually caused by an impairment of the arterial blood supply of the brain. It can also be caused by an impairment of the outflowofvenousblood,thoughthissituationismuchlesscommon.Ifacerebral vein is blocked, the blood volume and venous pressure rise in the region of the brain that it normally drains. The arteriovenous pressure difference across the cerebral capillaries falls (the inflow of blood is “choked off”), leading to diminished perfusion and thus to diminished supply of oxygen and nutrients. Simultaneously, the transcapillary pressure gradient increases, leading to increased movement of water from the capillaries into the surrounding tissue (vasogenic edema). The neurons in the affected brain tissue lose the ability to functionnormally,and,iftheproblempersists,theydie.Venousinfarctionisusuallyaccompaniedbytheruptureofsmallvessels(probablyveins)intheinfarcted zone, producing intraparenchymal hemorrhage (so-called venous hemorrhage, which has a characteristic “salt-and-pepper” appearance in a CT scan).

Acute Venous Outflow Obstruction

Acute Thromboses of the Cerebral Veins and Venous Sinuses

Causes. The most common cause of acute obstruction of venous outflow from the brain is thrombosis of the dural venous sinuses and the parenchymal veins that drain into them (venous sinus thrombosis). Factors predisposing to this occurrence include coagulopathies such as protein S and C deficiency, factor V deficiency, and cardiolipin antibodies, as well as oral contraceptive use, cigarette smoking, steroid medication, dehydration, autoimmune diseases such as Behçet disease and Crohn disease, and the puerperium.

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11 474 · 11 Blood Supply and Vascular Disorders of the Central Nervous System

Manifestations. The clinical manifestations of venous sinus thrombosis are highly varied and depend on the site and extent of venous occlusion as well as on the degree to which collateral drainage is available. In one patient a relatively circumscribed occlusion may cause a major intraparenchymal hemorrhage, while in another an extensive occlusion may remain nearly asymptomatic. Prospective assessment is usually not possible in individual cases.

The general clinical manifestations of venous sinus thrombosis are headache and epileptic seizures. If the patient also develops focal neurological deficits that progress over the course of a few hours, rather than suddenly, then venous sinus thrombosis is the likely diagnosis. The diagnosis is further supported by evidence of intracranial hypertension, e. g., papilledema.

In cases of venous sinus thrombosis, marked clinical deterioration can occur in a very short time, perhaps over the course of an hour. Such occurrences are usually due to involvement of the internal cerebral veins or to extensive intraparenchymal venous hemorrhages.

Diagnostic evaluation. The diagnosis or exclusion of venous sinus thrombosis is often difficult even when modern imaging methods are used—CT, MRI, and digital subtraction angiography (DSA).

CT. Classic, acute cases can be diagnosed by CT, particularly when CT venography with contrast medium is performed. Problems are caused by congenital variants of the vascular anatomy, by less extensive occlusions, and by thromboses of the straight sinus and internal cerebral veins. Older venous sinus thromboses, too, are difficult to assess with CT.

MRI. MRI is currently the most important diagnostic technique for evaluation of the venous outflow of the brain. It reveals the veins in multiple planes, and it is performed with flow-sensitive sequences to demonstrate intravenous flow. Its resolution is high enough that the internal cerebral veins are well seen.

MRI also enables visualization of the brain parenchyma. The site and appearance of a parenchymal lesion may provide clues to the location of venous obstruction: occlusion of the internal cerebral veins, for example, produces characteristic thalamic lesions, while transverse sinus thrombosis produces characteristic lesions in the temporal lobe. The diagnostic power of MRI is reduced, however, by anatomical variants of the cerebral blood vessels (as in CT), and also by certain flow-related effects that are incompletely understood at present. MRI thus cannot detect every case of venous sinus thrombosis, and it occasionally yields false-positive findings as well. Furthermore, MRI scanning of uncooperative or unconscious patients is sometimes very difficult, and the resulting scans may be of marginal diagnostic value. In extreme cases, patients must be scanned under general anesthesia.

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Case Presentation 8: Superior Sagittal Sinus Thrombosis

This 37-year-old secretary had a generalized epileptic seizure at work. After a postictal twilight phase lasting about 20 minutes, she regained full alertness and complained of a severe holocephalic headache. On admission to the hospital, she was awake, but abnormally slow. She continued to complain of severe headache. An MRI scan was obtained at once: the T2weighted image revealed a left frontal parenchymal lesion (Fig. 11.28a), while MR venography (Fig. 11.28b, c) revealed throm-

botic occlusion of the rostral portion of the superior sagittal sinus. The sinus thrombosis was treated with full heparinization followed by oral warfarin administration, and anticonvulsants were given to prevent further seizures. The headache improved rapidly with analgesics, and the patient was asymptomatic a few days later. Further diagnostic testing revealed no underlying hypercoagulable state. The only identifiable risk factor for venous sinus thrombosis was the use of oral contraceptives.

a

b

Fig. 11.28 Venous sinus thrombosis. a The T2weighted FLAIR image reveals a hyperintense lesion in the left frontal lobe: this is an infarct due to venous obstruction. b MR venography reveals brisk flow in the posterior portion of the superior sagittal sinus (bright) and in the large tributary veins that join it. The rostral portion of the sinus, however, is devoid of signal, indicating lack of flow. There is also poor flow in the tributary veins that enter this portion of the sinus. c The same findings are evident in lateral projection: there is flow in the posterior portion of the superior sagittal sinus, as well as in the straight sinus (arrow) and the internal cerebral veins,

c but not in the rostral portion of the superior sagittal sinus.

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Intra-arterial DSA. Intra-arterial angiography or DSA was once the only method of diagnosing venous sinus thrombosis with certainty. Unfortunately, the usefulness of this method is limited in precisely the same situations where the other methods fail to provide conclusive findings. DSA is no longer used to diagnose venous sinus thrombosis, except in rare cases, because it carries a much higher risk of complications than MRI.

Clinical course, treatment, and prognosis. The spontaneous course of venous sinus thrombosis is not clear. It was once thought that most cases were fatal, probably because most less-extensive thromboses went undetected and only the cases that turned out unfavorably were, in the end, correctly diagnosed. Occlusion of the straight sinus and/or internal cerebral veins is particularly ominous; this type of venous obstruction is still highly lethal, because it often leads to necrosis in the diencephalon to an extent that is incompatible with life. It can also cause cerebellar hemorrhage with mass effect. The straight sinus and the internal cerebral veins sometimes undergo thrombosis in isolation, but more often do so as a later stage in the progression of extensive thrombosis of the remaining venous sinuses.

The prognosis of venous sinus thrombosis has improved markedly since the introduction of therapeutic anticoagulation with heparin. Heparin is given even in the face of a parenchymal hemorrhage due to venous sinus thrombosis. In such cases, the correct interpretation of the hemorrhage as a sequela of thrombosis is essential, because it would otherwise absolutely contraindicate anticoagulation. Fibrinolytic techniques have not been shown to be of value in the treatment of venous sinus thrombosis. Surgical resection of venous hemorrhages is also not indicated.

Therapeutic anticoagulation is thought to halt the progression of venous sinus thrombosis, to promote the opening of collateral venous pathways, and to promote microcirculation. Intravenous heparin is given in the acute phase, then converted to oral anticoagulation for a further six months. Follow-up examinations are performed to detect recurrences early, particularly when known risk factors are present. Patients found to have suffered venous sinus thrombosis because of an underlying hypercoagulable disorder must be therapeutically anticoagulated for life.

Chronic Venous Outflow Obstruction

The manifestations of chronic venous outflow obstruction differ markedly from those of acute thrombosis.

The causes of chronic venous outflow obstruction are many, including medication side effects and bilateral stenosis of the venous outflow channels. In one

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