Epidemiology

Cerebral aneurysm has been recognized as a cause of human illness since the end of the 19th century, but the actual incidence is difficult to estimate. In autopsy studies, the prevalence ranges from 0.2% to 7.9%. Other studies indicate a prevalence of 5%.³ The ratio of ruptured aneurysm to unruptured (incidental) aneurysm is 5:3 to 5:6 (rough estimate is 1:1; i.e., 50% of these aneurysms rupture).⁴ Only 2% of aneurysms manifest during childhood.⁵

Etiology

The exact pathophysiology of the development of aneurysms is controversial, but abnormal vascular anatomy and flow dynamics are important factors. In contrast to extracranial blood vessels, there is less elastic in the tunica media and adventitia of cerebral blood vessels, the media has less muscle, the adventitia is thinner, and the internal elastic lamina is more prominent.^6,7 Outpouching is more likely to occur because of this, but it is also noteworthy that aneurysms are usually found at vessel bifurcations at sites of greatest flow. This, together with the fact that large cerebral blood vessels lie within the subarachnoid space with little supporting connective tissue,^8,8a may predispose these vessels to the development of saccular aneurysms. Aneurysm formation is more common in women and in smokers. Beyond the third decade in particular, SAH is usually caused by rupture of a cerebral aneurysm. Other than size of aneurysm alone, the “aspect ratio” (aneurysm depth to aneurysm neck) has also been shown to be a potentially useful predictor of rupture (Figs. 43-1 and 43-2).⁹

Figure 43-1 Plot 1 with markers. Microdialysis markers of 64-year-old woman who presented with a World Federation of Neurosurgical Societies grade 5 subarachnoid hemorrhage. The patient recovered to the point of full consciousness, but she had to be sedated and paralyzed for the management of a chest infection. Time is on the x axis (one tick = 24 hours). The vertical red line on the left marks the beginning of a gradual deterioration of the brain metabolic state, illustrated by a surge in lactate/pyruvate (L/P) ratio and glycerol level above normal values, in the absence of any other clinical signs. A few hours later, the patient became hypertensive, and her pupils became fixed and dilated (vertical red line on the right). A computed tomographic scan confirmed the development of a large infarct in the right middle cerebral territory, ipsilateral to the microdialysis probe.

Figure 43-2 Plot 2 with markers. Microdialysis markers of 53-year-old woman who presented with a World Federation of Neurosurgical Societies grade 5 subarachnoid hemorrhage. The patient died several days later without regaining consciousness. Values of lactate/pyruvate (L/P) ratio, glycerol (μmol/L), glutamate (μmol/L) and transcranial Doppler (TCD) imaging velocities (cm/second) of the ipsilateral middle cerebral artery are plotted against time (one tick = 24 hours). Values above the dotted horizontal lines are considered abnormal. The plot shows persistent high levels of microdialysis markers, indicating a prolonged metabolic crisis of the brain. Many TCD imaging values are above the dotted line, which suggests the presence of cerebral vasospasm, and appear to parallel glutamate levels closely.

The etiology of aneurysms may be any of the following:

Congenital predisposition (e.g., defect in the muscular layer of the arterial wall)

“Atherosclerotic” or hypertensive, the presumed etiology of most saccular aneurysms; probably interacts with congenital predisposition

Embolic: as in atrial myxoma

Infectious (“mycotic aneurysms”)

Traumatic

Associated with other conditions

Location related

Saccular, or berry, aneurysms are usually located at bifurcations of the major cerebral arteries. Aneurysms located more peripherally do occur, but these tend to be associated with infection (mycotic aneurysms) or trauma. Fusiform aneurysms are more common in the vertebrobasilar system.

Of all saccular aneurysms, 85% to 95% are located in the carotid system, with the following three most common locations: (1) the anterior communicating artery (ACom) for 30% (aneurysms in the ACom and anterior cerebral artery are more common in male patients); (2) the posterior communicating artery for 25%; and (3) the middle cerebral artery for 20%. Of the remaining saccular aneurysms, 5% to 15% are located in posterior circulation (vertebrobasilar); 10%, at the basilar tip (most common), followed by the basilar artery-superior cerebellar artery junction, basilar artery-vertebral artery junction, and anterior inferior cerebellar artery; and 5%, on the vertebral artery (vertebral artery-posterior inferior cerebellar artery junction most common). Twenty percent to 30% of patients with aneurysm have multiple aneurysms.¹⁰

Manifestation of Aneurysms

Major rupture is the most frequent manifestation leading to SAH. Clinical features may vary from sudden headache to a moribund comatose state. Neurological deficits, if present, depend on location and size of the hematoma. Signs of meningism with photophobia, neck stiffness, and vomiting are often seen. Progressive neurological deficit or decreasing con scious level may occur as a result of mass effect from a growing hematoma or the development of hydrocephalus. Ruptured cerebral aneurysm may be accompanied by the following:

Intracerebral hemorrhage, which occurs in 20% to 40% of affected patients (more common with aneurysms distal to the circle of Willis; e.g., middle cerebral artery aneurysms).

Intraventricular hemorrhage, which occurs in 13% to 28%.¹¹

Subdural hemorrhage, which occurs in 2% to 5%.

Intraventricular hemorrhage occurs with 13% to 28% of ruptured aneurysms in clinical series (higher in autopsy series)¹¹ and may lead to obstructive hydrocephalus. The presence of intraventricular hemorrhage is an indicator of poor prognosis¹¹ (64% rate of mortality); ventricular size on admission is the most important prognostic factor (large ventricles being worse). Patterns that may occur include the following:

ACom aneurysm, which tends to produce intraventricular hemorrhage by rupture through the lamina terminalis into the lateral or anterior third ventricles.

Distal basilar or carotid terminus aneurysms, which may rupture through the floor of the third ventricle.

Distal posterior inferior cerebellar artery aneurysms, which may rupture directly into the fourth ventricle through the foramen of Luschka.¹²

Other Possible Presentations

Mass effect can give rise to possible “warning signs”: for example, giant aneurysms can include brainstem compression, which produces hemiparesis and cranial neuropathies; cranial neuropathy (average latency from symptom to SAH was 110 days) can include non-pupil-sparing third nerve palsy produced by expanding posterior communicating artery aneurysm; rarer cranial neuropathies produce visual loss as a result of compressive optic neuropathy¹² (ophthalmic artery aneurysms) and chiasmal syndromes (ophthalmic, ACom, or basilar apex aneurysms). Facial pain syndromes in the ophthalmic or maxillary nerve distribution that may mimic trigeminal neuralgia can occur with intracavernous or supraclinoid aneurysms.^13,14

Also, intrasellar aneurysm may produce disturbances in the endocrine system.¹⁵ Warning or sentinel hemorrhage (minor bleeding) may occur; this condition has the shortest latency (10 days) between symptom onset and SAH.¹⁶ Headache without hemorrhage may occur; this may be acute, severe, and “thunderclap” in nature, sometimes described as the “worst headache of my life.” It may be caused by aneurysmal expansion, thrombosis, or intramural bleeding¹⁷ (all without rupture). Alternatively, it can be chronic, having usually been present for more than 2 weeks, is unilateral (often retro-orbital or periorbital) in about one half of cases, and is possibly caused by dural irritation. In the other one half of cases, it is diffuse or bilateral, possibly caused by mass effect that produces raised intracranial pressure. Asymptomatic aneurysm may be discovered incidentally on angiography, computed tomographic (CT) scan, or magnetic resonance imaging obtained for other reasons.

Conditions associated with aneurysms include the following:

Autosomal dominant polycystic kidney disease

Fibromuscular dysplasia

Arteriovenous malformations

Connective tissue disorders: Ehlers-Danlos syndrome type IV, Marfan’s syndrome

Osler-Weber-Rendu syndrome

Family history of intracranial aneurysms

Bacterial endocarditis

Coarctation of the aorta

Diagnosis and Investigations

Diagnosis of aneurysmal SAH is crucial, and there are two main components to this: identification of blood on CT scan and demonstration of blood breakdown products on spectroscopy.

Computed tomography is 98% sensitive for detection of SAH in the first 12 hours after the ictus, but this rate declines to approximately 70% by day 3. Thus, without other diagnostic facilities, CT scanning alone may miss SAH. Lumbar puncture performed 12 hours after the onset of symptoms is therefore vital for diagnosis if CT scan results are negative or equivocal. The presence of bilirubin in the cerebrospinal fluid confirms SAH.

Cerebral angiography is the “gold standard” for demonstrating the cerebral vasculature and is mandatory in SAH. An adequate study should reveal the causative aneurysm.

Computed tomographic angiography is being increasingly used to diagnose and provide information for treatment of intracranial aneurysms. Conventional angiography is usually carried out by a neuroradiologist through a femoral artery puncture with local anesthetic, whereas computed tomographic angiography can be performed with injection of contrast medium into a forearm vein. This can be done in the computed tomography suite immediately after the diagnostic CT scan.

Treatment