Introduction
Subarachnoid hemorrhage (SAH) is the presence of blood in the meninges and CSF. Head trauma, the leading cause of SAH, is discussed in Chapter 18. Excluding trauma, the annual incidence of spontaneous SAH is 10/100,000 and accounts for 3% of all strokes. Spontaneous SAH is uncommon in infants and children, has a mean age of onset in the sixth decade, and is rare in elderly adults over age of 75 years. Women outnumber men 3:2, and African Americans outnumber caucasians 2:1.
At least 85% of SAH is due to rupture of a sac-cular (berry) or fusiform aneurysm. Sacsac-cular
aneurysms are little outpouchings at bifurcations of mid-sized cerebral blood vessels, while fusiform aneurysms are dilated elongated segments of the vessel. Saccular aneurysms rupture much more often than fusiform aneurysms. The remaining causes include superficial arteriovenous malfor-mations of the brain and spinal cord and SAH in which no etiology is identified.
Major risk factors for rupture of an aneurysm include hypertension, smoking, heavy alcohol con-sumption, and a positive family history. Of patients with SAH, 10% have a positive family his-tory, and first-degree relatives have a 5-fold risk.
Pathophysiology
Autopsy studies estimate the prevalence of unrup-tured saccular aneurysms at 1%–2%, with 30% of these patients having multiple aneurysms. The location of saccular aneurysms is mainly at the bifurcation of larger vessels or at sites where dis-turbances of blood flow are generated, such as the anterior and posterior communicating arteries (Figure 9-5). The most common locations are the posterior communicating artery (40%), anterior communicating artery (20%), and bifurcation of the middle cerebral artery (15%). Except for a few hereditary diseases, patients with cerebral aneurysms do not have systemic aneurysms.
The pathophysiology by which saccular aneurysms develop is incompletely understood.
Evidence points to development of the aneurysm in adulthood, as children seldom experience a rup-tured aneurysm and autopsy studies of infants and children rarely find aneurysms. The origin of the aneurysm is just distal to a bifurcation where there are high shear forces. The aneurysm wall is charac-terized by (1) reduction of collagenous fibers, (2) atrophy of tunica media, and (3) loss of internal elastic lamina in addition to the expected absence of external elastic lamina. The sac of a small aneurysm is reduced to a single layer of endothe-lial cells and a thin fibrous layer. The histologic appearance of the artery wall before and after the aneurysm is normal. The role of genetic factors in the pathogenesis is unclear.
The risk of bleeding from an aneurysm increases considerably in those larger than 5-mm diameter. Patients with multiple aneurysms also are at higher risk of rupture.
96 FUNDAMENTALS OF NEUROLOGIC DISEASE 087-100_Davis09 3/2/05 4:22 PM Page 96
Major Clinical Features
Sudden, explosive headache, the cardinal feature, develops within seconds of a rupture. However, in patients presenting to an emergency room with this description, only 10% prove to have an SAH.
The others are due to a “thunderclap” headache or migraine headache. Vomiting occurs in 70%. A period of unresponsiveness occurs in over half the patients and focal neurologic signs occur in 1/3 of patients. Common neurologic signs are cranial nerve palsies, including dilated pupils, disconju-gate gaze, facial weakness, dysphagia, and dysarthria and hemiparesis. Seizures occur in 5%
of patients. Neck stiffness usually develops hours after the bleed. Papilledema from increased
intracranial pressure is commonly seen on fundo-scopic exam after 12 hours.
Occasional giant and fusiform aneurysms produce neurologic deficits by mass effect and may cause a CN III palsy or other cranial nerve deficits.
Major Laboratory Findings
The diagnosis of SAH is best made by CT, which is widely available, rapidly performed even in a restless patient, and identifies blood in the sub-arachnoid space over 80% of the time. The charac-teristic hyperdense appearance of extravasated blood in the basal cisterns is the most common finding (Figure 9-6). Collections of extravasated
CHAPTER 9—Disorders of the Cerebrovascular System 97
Pons Temporal Lobe of
Cerebrum
Frontal Lobe of Cerebrum
Cerebellum
Posterior Cerebral Artery Superior Cerebellar Artery
Anterior Inferior Cerebellar Artery Posterior Inferior Cerebellar Artery
Vertebral Artery Anterior Cerebral Artery
Middle Cerebral Artery
Posterior Communicating Artery
Basilar Artery Anterior Communicating Artery
Medulla
Anterior Choroidal Artery
Anterior Spinal Artery Pituitary
Gland
Grey Circle Size Reflects Aneurysm Frequency
Figure 9-5 Distribution of saccular aneurysms.
087-100_Davis09 3/2/05 4:22 PM Page 97
blood elsewhere may suggest the site of the bleed-ing aneurysm. In 30% of patients, there is also an intraparenchymal hematoma due to rupture of the aneurysm upward into the brain. CT only detects bloody CSF when there is an RBC concen-tration of greater than 0.5%. After 8 hours, CSF exam demonstrates blood in all tubes and xan-thochromia (yellow color) of the supernate, estab-lishing the diagnosis in the few patients missed by CT. MRI helps to detect an SAH more than several days old.
Several methods exist to identify the location of the aneurysm and whether other aneurysms coexist. The gold standard is four-vessel catheter angiography, but this method is time consum-ing, difficult to perform on a sick patient, and carries a complication rate of rebleeding in 2%–5%. CT angiography using contrast media is becoming popular because it is faster, safer, and has a sensitivity of 90% compared with arteriog-raphy. Because MRA is slower and difficult to perform in a patient on a ventilator, it is less helpful.
Principles of Management and Prognosis The goal is to maximize the quality of survival from the acute SAH and to eliminate the aneurysm, thus preventing rebleeding.
Patients are often classified as to severity and prognosis based on the Glasgow coma scale and other scales (Table 9-4). Patients should be placed in an intensive care unit as they often deteriorate during the first day. If mental status and breathing deteriorate, intubation and mechanical ventilation is required. Blood pressure should be carefully con-trolled. Pain should be controlled with narcotics.
Secondary cerebral ischemia develops in 1/3 of patients, often after several days and continuing into the second week. This ischemia can lead to secondary infarction. Arterial vasospasm (reversible narrowing of a cerebral vessel) often occurs 4 to 21 days after the bleed but does not always produce recognizable cerebral ischemia symptoms and infarctions may develop without corresponding arterial vasospasm. Nevertheless, daily administration of a calcium-channel blocker, nimodipine, from bleeding onset is associated with a modest, but significant, reduction in secondary ischemia and improvement in outcome.
Rebleeding from the aneurysm is a serious problem. Rebleeding within 24 hours of initial bleed occurs in 15% of patients. After survival of 1 day, 1/3 of patients will rebleed over the next 4 weeks, with the daily risk of bleeding being about 98 FUNDAMENTALS OF NEUROLOGIC DISEASE
Figure 9-6 Computed tomography scan of subarach-noid hemorrhage. The arrow denotes the hyperdense blood outlining the basilar cisterns. (Courtesy of Blaine Hart, MD)
Table 9-4 General Grading Systems for Ruptured Saccular Aneurysms*
Grade Clinical Characteristics 1 Alert, minimal headache, slight neck
stiff-ness, and no neurologic deficit
2 Alert, moderate-to-severe headache, stiff neck, and no neurologic deficit other than cranial nerve palsy
3 Drowsiness and mild confusion with mild neurologic deficit
4 Semicoma, moderate-to-severe hemipare-sis, and possible early decerebrate rigidity 5 Deep coma, decerebrate rigidity, and
moribund
* Includes Hunt Hess and Botterell and Lougheed scales.
087-100_Davis09 3/2/05 4:22 PM Page 98
equal. Surgical obliteration of the aneurysm has been the mainstay of treatment for decades. The surgeon is often faced with a dilemma. Operating on a comatose patient with brain edema is techni-cally difficult and carries a considerable surgical risk of death. However, waiting 1 to 2 weeks for the brain swelling to reduce and the patient to clinically improve carries the increased risk of the aneurysm rebleeding. Studies have not identified an ideal time for surgery, but many surgeons wait until the patient’s level of consciousness improves. In recent years, endovascular techniques enable placement of a detachable spring coil into the aneurysm via an arterial catheter, which triggers aneurysm clotting.
While this technique has promise for select patients, the overall outcomes have been similar to surgical clipping. After 1 month, the risk of rebleeding from an unclipped aneurysm is 1% per year for 4 years and then falls much lower.
The prognosis of a ruptured saccular aneurysm is poor. Overall, 1/3 of patients die from the acute bleed and 2/3 of survivors are left with consider-able neurologic sequelae and a diminished quality of life. Poor prognostic signs include grades 4 or 5 on the aneurysm grading scales, scores of 3 to 6 on
the Glasgow coma scale, presence of intracerebral hematoma, development of hydrocephalus, and rebleeding.
RECOMMENDED READING
Brott T, Bogousslavsky J. Treatment of acute ischemic stroke. N Engl J Med 2000;343:710–722.
(Reviews how ischemia damages neurons and the treatment options in an acute stroke.)
Johnston SC. Transient ischemic attack. N Engl J Med 2002;347:1687–1692. (Good review, with focus on treatment.)
Qureshi AI, Tuhrim S, Broderick JP, et al. Sponta-neous intracerebral hemorrhage. N Engl J Med 2001;344:1450–1460. (Current review of causes and management options.)
Staph C, Mohr JP. Ischemic stroke therapy. Annu Rev Med 2002;53:453–475. (Current review of clinical picture, workup, and treatment options.) van Gijn J, Rinkel GJE. Subarachnoid
haemor-rhage: diagnosis, causes and management.
Brain 2001;124:249–278. (Excellent review of all causes, with attention to saccular aneurysms.)
CHAPTER 9—Disorders of the Cerebrovascular System 99
087-100_Davis09 3/2/05 4:22 PM Page 99