Objectives
• Describe the etiology and pathophysiology of selected neurological disorders.
• Identify the clinical manifestations of selected neurological disorders.
• Explain the treatment of selected neurological disorders.
• Discuss the nursing priorities for managing a patient with selected neurological disorders.
• Discuss the concept of cerebral autoregulation.
• Describe the therapies commonly used to treat intracranial hypertension.
Be sure to check out the bonus material, including free self-assessment exercises, on the Evolve web site at
http://evolve.elsevier.com/Urden/priorities/.
To accurately anticipate and plan nursing interventions, the critical care nurse must have an understanding of the disease pathology, determine the areas of focused assessment, and be well acquainted with the medical management of the neurological patient. Fortunately, despite a wide array of neurological disorders, only a few routinely require the critical care environment.
Coma
Normal consciousness requires awareness and arousal. Awareness is the combination of cognition (mental and intellectual) and affect (mood) that can be construed based on the patient’s interaction with the environment.1 Alterations of consciousness may be the result of deficits in awareness, arousal, or both.2 There are four discrete disorders of consciousness: coma, vegetative state, minimally conscious state, and locked-in syndrome. Coma is characterized by the absence of both wakefulness and awareness, whereas a vegetative state is characterized by the presence of wakefulness with the absence of awareness. In the minimally conscious state, wakefulness is presence and awareness is severely diminished but not absent. Locked-in syndrome is characterized by the presence of wakefulness and awareness, but with quadriplegia and the inability to communicate verbally; thus the patient appears to be unconscious.3 Box 18-1 lists the disorders of consciousness in descending order of wakefulness.
Coma is the deepest state of unconsciousness; arousal and awareness are lacking.1–3 The patient cannot be aroused and does not demonstrate any purposeful response to the surrounding environment.4 Coma is a symptom rather than a disease, and it occurs as a result of some underlying process.1,2 The incidence of coma is difficult to ascertain because a wide variety of conditions can induce coma.1,2 This state of unconsciousness is unfortunately very common in critical care, and it is the focus of the following discussion.
Etiology
The causes of coma can be divided into two general categories: structural or surgical and metabolic or medical. Structural causes of coma include ischemic stroke, intracerebral hemorrhage (ICH), trauma, and brain tumors.5 Metabolic causes of coma include drug overdose, infectious diseases, endocrine disorders, and poisonings.5 Coma demands immediate attention, resulting in a high percentage of admissions to all hospital services.6 Table 18-1 provides a list of the possible causes of coma.
TABLE 18-1
| STRUCTURAL OR SURGICAL COMA | METABOLIC OR MEDICAL COMA |
| Trauma | Infection |
| Epidural hematoma | Meningitis |
| Subdural hematoma | Encephalitis |
| Metabolic encephalopathy | |
| Diffuse axonal injury | Metabolic conditions |
| Hypoglycemia | |
| Brain contusion | Hyperglycemia |
| Intracerebral hemorrhage | Hyperosmolar states |
| Uremia | |
| Subarachnoid hemorrhage | Hepatic encephalopathy |
| Hypertensive encephalopathy | |
| Posterior fossa hemorrhage | Hypoxic encephalopathy |
| Hyponatremia | |
| Supratentorial hemorrhage | Hypercalcemia |
| Myxedema | |
| Hydrocephalus | Intoxication |
| Ischemic stroke | Opioid overdose |
| Tumor | Alcohol |
| Other causes | Poisonings |
| Psychogenic causes |
Pathophysiology
Consciousness involves arousal, or wakefulness, and awareness. Neither of these functions is present in the patient in coma. Ascending fibers of the reticular activating system (ARAS) in the pons, hypothalamus, and thalamus maintain arousal as an autonomic function. Neurons in the cerebral cortex are responsible for awareness. Diffuse dysfunction of both cerebral hemispheres and diffuse or focal dysfunction of the reticular activating system can produce coma.1,6,7 Structural causes usually produce compression or dysfunction in the area of the ARAS, whereas most medical causes lead to general dysfunction of both cerebral hemispheres.8 Trauma, hemorrhage, and tumor can damage the ARAS, leading to coma. Destruction of large regions of bilateral cerebral hemispheres can be the result of seizures or viral agents. Toxic drugs, toxins, or metabolic abnormalities can suppress cerebral function.5–7
Assessment and Diagnosis
The clinical diagnosis of the coma state is readily established by assessment of the level of consciousness. However, determining the full nature and cause of coma requires a thorough history and physical examination. A medical history is essential, because events immediately preceding the change in level of consciousness can often provide valuable clues to the origin of the coma. When limited information is available and the coma is profound, the response of the patient to emergency treatment may provide clues to the underlying diagnosis; for example, the patient who becomes responsive with the administration of naloxone can be presumed to have ingested some type of opiate.6
Detailed serial neurological examinations are essential for all patients in coma. Assessment of pupillary size and reaction to light (normal, sluggish, or fixed), extraocular eye movements (normal, asymmetrical, or absent), motor response to pain (normal, decorticate, decerebrate, or flaccid), and breathing pattern yields important clues for determining whether the cause of the coma is structural or metabolic.1,6
The areas of the brainstem that control consciousness and pupillary responses are anatomically adjacent. The sympathetic and parasympathetic nervous systems control pupillary dilation and constriction, respectively. The anatomic directions of these pathways are known, and changes in pupillary responses can help identify where a lesion may be located. For example, if damage occurs in the midbrain region, pupils are slightly enlarged and unresponsive to light. Lesions that compress the third nerve result in a fixed and dilated pupil on the same side as the neurological insult. Pupillary responses are usually preserved when the cause of the coma is metabolic in origin. Pupillary light responses are often the key to differentiating between structural and metabolic causes of coma.1,6,7,9
Areas of the brainstem adjacent to those responsible for consciousness also control the oculomotor eye movement. The ability to maintain conjugate gaze requires preservation of the internuclear connections of cranial nerves III, VI, and VIII by means of the medial longitudinal fasciculus (MLF).9 As with pupillary responses, structural lesions that impinge on these pathways cause oculomotor dysfunction such as a disconjugate gaze. Deficits in extraocular eye movements usually accompany a structural cause.1,5,9
Focal or asymmetric motor deficits usually indicate structural lesions.1,5 Abnormal motor movements may also help pinpoint the location of a lesion. Decorticate posturing (abnormal flexion) can be seen with damage to the diencephalon. Decerebrate posturing (abnormal extension) can be seen with damage to the midbrain and pons. Flaccid posturing is an ominous sign and can be seen with damage to the medulla.9
Abnormal breathing patterns may also assist in differentiating structural from metabolic causes of coma. Cheyne-Stokes respirations are seen in patients with cerebral hemispheric dysfunction or metabolic suppression. Central neurogenic hyperventilation, or Kussmaul breathing, occurs with metabolic acidosis or damage to the midbrain and upper pons. Apneustic breathing may occur with damage to the pons, hypoglycemia, and anoxia. Ataxic breathing occurs with damage to the medulla. Agonal breathing occurs with failure of the respiratory centers in the medulla.6,9
In addition to physical assessment, laboratory studies and diagnostic procedures are done. Structural causes of coma are usually readily apparent with computed tomography (CT) or magnetic resonance imaging (MRI).4,8 Evoked potentials are also useful in facilitating a differential diagnosis between the disorders of consciousness and in evaluating a patient’s prognosis.3 Generally a patient in a coma with an absence of brainstem auditory evoked responses (BAERs) is considered to have a poor prognosis of recovery.3 Laboratory studies are also used to identify metabolic or endocrine abnormalities.7 Occasionally, the cause of coma is never clearly determined.
Medical Management
The goal of medical management of the patient in a coma is identification and treatment of the underlying cause of the condition. Initial medical management includes emergency measures to support vital functions and prevent further neurological deterioration. Protection of the airway and ventilatory assistance are often needed. Administration of thiamine (at least 100 mg), glucose, and an opioid antagonist is suggested when the cause of coma is not immediately known.1,6 Thiamine is administered before glucose, because the coma produced by thiamine deficiency, Wernicke’s encephalopathy, can be precipitated by a glucose load.1
The patient who remains in a coma after emergency treatment requires supportive measures to maintain physiological body functions and prevent complications. Intubation for continued airway protection and nutritional support are essential. Fluid and electrolyte management is often complex because of alterations in the neurohormonal system. Anticonvulsant therapy may be necessary to prevent further ischemic damage to the brain.1,5,6
The health care team and the patient’s family make decisions jointly regarding the level of medical management to be provided. Family members require informational support in terms of probable cause of the coma and prognosis for recovery of consciousness and function. Prognosis depends on the cause of the coma and the length of time unconsciousness persists. Only 15% of patients in nontraumatic coma make a satisfactory recovery.7 Metabolic coma usually has a better prognosis than coma caused by a structural lesion, and traumatic coma usually has a better outcome than nontraumatic coma.5,7
Nursing Management
Nursing management of the patient in a coma incorporates a variety of nursing diagnoses (Nursing Diagnosis Priorities box on Coma) and is directed by the specific cause of the coma, although some common interventions are used. The patient in a coma totally depends on the health care team. Nursing priorities are directed toward (1) monitoring for changes in neurological status and clues to the origin of the coma, (2) supporting all body functions, (3) maintaining surveillance for complications, (4) providing comfort and emotional support, and (5) initiating rehabilitation measures. Measures to support body functions include promoting pulmonary hygiene, maintaining skin integrity, initiating range-of-motion exercises, managing bowel and bladder functions, and ensuring adequate nutritional support.1
Eye Care
The blink reflex is often diminished or absent in the comatose patient. The eyelids may be flaccid and may depend on body positioning to remain in a closed position, and edema may prevent complete closure. Loss of these protective mechanisms results in drying and ulceration of the cornea, which can lead to permanent scarring and blindness.1
Two interventions that are commonly used to protect the eyes are instilling saline or methylcellulose lubricating drops and taping the eyelids in the shut position. Evidence suggests that an alternative technique may be more effective in preventing corneal epithelial breakdown. In addition to instillation of saline drops every 2 hours, a polyethylene film is taped over the eyes, extending beyond the orbits and eyebrows. The film creates a moisture chamber around the cornea and assists in keeping the eyes moist and in the closed position. This technique also prevents damage to the eyes that results from placement of tape or gauze directly on the delicate skin of the eyelids.10
Collaborative management of the patient in a coma is outlined in the Collaborative Management box on Coma.
Stroke
Stroke is a descriptive term for the sudden onset of acute neurological deficit persisting for more than 24 hours and caused by the interruption of blood flow to the brain. Stroke is the third leading cause of death in the United States, preceded by heart disease and cancer, and the leading cause of adult disability.11 Approximately 795,000 people have a stroke each year; 610,000 of these are first attacks, and 185,000 are recurrent attacks.11
Strokes are classified as ischemic or hemorrhagic (Concept Map on Stroke). Hemorrhagic strokes can be further categorized as subarachnoid hemorrhages (SAHs) and intracerebral hemorrhages. Approximately 87% of all strokes are ischemic, 10% are ICHs, and 3% are SAHs.11 Although less common, hemorrhagic strokes (ICHs and SAHs) have a higher mortality rate than ischemic strokes. Approximately 8% to 12% of ischemic strokes and 37% to 45% of hemorrhagic strokes result in death within 30 days.11 The annual cost for care and loss of productivity was estimated to be $73.7 billion in 2009.11
Ischemic Stroke
Ischemic stroke results from interruption of blood flow to the brain and accounts for 80% to 85% of all strokes. The interruption can be the result of a thrombotic or embolic event. Thrombosis can form in large vessels (large-vessel thrombotic strokes) or small vessels (small-vessel thrombotic strokes). Embolic sources include the heart (cardioembolic strokes) and atherosclerotic plaques in larger vessels (atheroembolic strokes). In 30% of the cases, the underlying cause of the stroke is unknown (cryptogenic strokes).12
Strokes are preventable. Most thrombotic strokes are the result of the accumulation of atherosclerotic plaque in the vessel lumen, especially at bifurcations or curves of the vessel. The pathogenesis of cerebrovascular disease is identical to that of coronary vasculature disease. The greatest risk factor for ischemic stroke is hypertension.12,13 Other risk factors are dyslipidemia, diabetes, smoking, and carotid atherosclerotic disease.11,14 Common sites of atherosclerotic plaque are the bifurcation of the common carotid artery, the origins of the middle and anterior cerebral arteries, and the origins of the vertebral arteries.13 Ischemic strokes resulting from vertebral artery dissection have been reported after chiropractic manipulation of the cervical spine.15
Etiology
An embolic stroke occurs when an embolus from the heart or lower circulation travels distally and lodges in a small vessel, obstructing the blood supply. At least 20% of ischemic strokes are attributed to a cardioembolic phenomenon.12 The most common cause of cardiac emboli is atrial fibrillation. It is responsible for about 50% of all cardiac emboli.16 Other sources of cardiac emboli are mitral stenosis, mechanical valves, atrial myxoma, endocarditis, and recent myocardial infarction.13 Researchers hypothesize that a patent foramen ovale or atrial septal aneurysms may be the cause of cryptogenic stroke.17
Pathophysiology
Ischemic stroke is a cerebral hemodynamic insult. When cerebral blood flow is reduced to a level insufficient to maintain neuronal viability, ischemic injury occurs. In focal stroke, an area of hypoperfused tissue, the ischemic penumbra, surrounds a core of ischemic cells. The ischemic penumbra can be salvaged with return of blood flow. However, sustained anoxic insult initiates a chain of biochemical events leading to apoptosis, or cellular death.18
The phenomenon of a focal ischemic stroke is identical to that associated with myocardial infarction, which is why brain attack is used in public education strategies. Often, a history of transient ischemic attacks (TIAs), brief episodes of neurological symptoms that last less than 24 hours, offers a warning that stroke is likely to occur. Sudden onset indicates embolism as the final insult to flow.12,13 The size of the stroke depends on the size and location of the occluded vessel and the availability of collateral blood flow.12 Global ischemia results when severe hypotension or cardiopulmonary arrest provokes a transient drop in blood flow to all areas of the brain.18
Cerebral edema sufficient to produce clinical deterioration develops in 10% to 20% of patients with ischemic stroke and can result in intracranial hypertension. The edema results from a loss of normal metabolic function of the cells and peaks at 4 days.12 This process is commonly the cause of death during the first week after a stroke.19 Secondary hemorrhage at the site of the stroke lesion, known as hemorrhagic conversion,19 and seizures20 are the two other major acute neurological complications of ischemic stroke.
Assessment and Diagnosis
The characteristic sign of an ischemic stroke is the sudden onset of focal neurological signs persisting for more than 24 hours.12 These signs usually occur in combination. Box 18-2 lists common patterns of neurological symptoms associated with an ischemic stroke. Hemiparesis, aphasia, and hemianopia are common. Changes in the level of consciousness usually occur only with brainstem or cerebellar involvement, seizure, hypoxia, hemorrhage, or elevated intracranial pressure (ICP). These changes may be exhibited as stupor, coma, confusion, and agitation.1 The reported frequency of seizures in patients with ischemic stroke ranges from 3% to 8%. If seizures occur, they are usually seen within 24 hours of an insult.20
The National Institutes of Health Stroke Scale (NIHSS) is often used as the basis of the focused neurological examination. The score ranges from 0 to 42 points; the higher the score, the more neurologically impaired the patient. A change of 4 points on the scale indicates significant neurological change. The components of the NIHSS include level of consciousness (LOC); LOC questions; LOC commands; gaze; visual fields; face, arm, and leg strength; sensation; limb ataxia; and language function.12 A copy of the NIHSS with complete instructions can be retrieved at http://www.ninds.nih.gov/disorders/stroke/strokescales.htm.
Confirmation of the diagnosis of ischemic stroke is the first step in the emergency evaluation of these patients. Differentiation from intracranial hemorrhage is vital. Noncontrast CT scanning is the method of choice for this purpose and is considered the most important initial diagnostic study. In addition to excluding intracranial hemorrhage, CT can assist in identifying early neurological complications and the cause of the insult.12 MRI can demonstrate infarction of cerebral tissue earlier than CT but is less useful in the emergency differential diagnosis.21 Because of the strong correlation between acute ischemic stroke and heart disease, 12-lead electrocardiography, chest radiography, and continuous cardiac monitoring are suggested to detect a cardiac cause or coexisting condition. Echocardiography is valuable in identifying a cardioembolic phenomenon when a sufficient index of suspicion warrants its use.1 Laboratory evaluation of hematological function, electrolyte and glucose levels, and renal and hepatic function is also recommended. Arterial blood gas analysis is performed if hypoxia is suspected, and an electroencephalogram is obtained if seizures are suspected. Lumbar puncture is performed only if SAH is suspected and the CT appearance is normal.1
Medical Management
Major changes have taken place in the medical management of ischemic stroke since 1996. On the basis of results of the National Institute of Neurologic Disorders and Stroke (NINDS) rtPA Stroke Study, thrombolytic therapy with intravenous recombinant tissue-type plasminogen activator (rtPA) was initially recommended within 3 hours of onset of ischemic stroke. This time frame has now been expanded from 3 hours to 4.5 hours.22 Patients who should be considered for thrombolysis are listed in Box 18-3. Confirmation of diagnosis with CT must be accomplished before rtPA administration. The recommended dose of rtPA is 0.9 mg/kg up to a maximum dose of 90 mg. Ten percent of the total dose is administered as an initial intravenous bolus, and the remaining 90% is administered by intravenous infusion over 60 minutes.17,21
The desired result of thrombolytic therapy is to dissolve the clot and reperfuse the ischemic brain. The goal is to reverse or minimize the effects of stroke. The major risk and complication of rtPA therapy is bleeding, especially intracranial hemorrhage. Unlike thrombolytic protocols for acute myocardial infarction, subsequent therapy with anticoagulant or antiplatelet agents is not recommended after rtPA administration in ischemic stroke. Patients receiving thrombolytic therapy for stroke should not receive aspirin, heparin, or warfarin for at least 24 hours after treatment.12,17
The major barriers to effective application of thrombolytic therapy for ischemic stroke are prehospital and in-hospital delays. To help decrease delays, the public needs to be educated about stroke symptoms and activation of the emergency medical system (EMS). EMS responders need adequate education and training on managing a patient with an acute ischemic stroke, focusing on stabilization and quick transport of the patient to the emergency department. The receiving hospital should ideally be primary stroke certified and have expert staff and the infrastructure to care for the complex stroke patient.21
Other emergency care of the patient with ischemic stroke must include airway protection and ventilatory assistance to maintain adequate tissue oxygenation.19 Hypertension is often present in the early period as a compensatory response, and in most cases, blood pressure (BP) must not be lowered (Table 18-2). For the patient who has not received thrombolytic therapy, antihypertensive therapy is considered only if the diastolic blood pressure is greater than 120 mm Hg or the systolic blood pressure is greater than 220 mm Hg.12 Criteria are different for patients who have received rtPA. Their blood pressure is kept below 180/105 mm Hg to prevent intracranial hemorrhage. Intravenous labetalol or nicardipine is used to achieve blood pressure control. If these agents are not effective, nitroprusside, hydralazine, or enalaprilat should be considered.12 Body temperature and glucose levels also must be normalized.12,19
TABLE 18-2
BLOOD PRESSURE MANAGEMENT FOR STROKE ACCORDING TO THE AMERICAN STROKE ASSOCIATION GUIDELINES
| BLOOD PRESSURE* | TREATMENT |
| Nonthrombolytic Candidates | |
| DBP >140 mm Hg | Sodium nitroprusside (0.5 mcg/kg/min); aim for 10%-20% reduction in DBP |
| SBP >220 mm Hg, DBP 121-140 mm Hg, or MAP† >130 mm Hg | 10-20 mg of labetalol‡ given by IVP over 1-2 min; may repeat or double labetalol every 20 min to a maximum dose of 300 mg |
| SBP <220 mm Hg, DBP = 120 mm Hg, or MAP† <130 mm Hg | Emergency antihypertensive therapy is deferred in the absence of aortic dissection, acute myocardial infarction, severe congestive heart failure, or hypertensive encephalopathy |
| Thrombolytic Candidates | |
| Pretreatment | |
| SBP >185 mm Hg or DBP >110 mm Hg | 1-2 inches of nitroglycerine paste (Nitropaste) or 1-2 doses of 10-20 mg of labetalol‡ given by IVP; if BP is not reduced and maintained to <185/110 mm Hg, the patient should not be treated with tPA |
| During and After Treatment | |
| Monitor BP | BP is monitored every 15 min for 2 hr, then every 30 min for 6 hr, and then hourly for 16 hr |
| DBP >140 mm Hg | Sodium nitroprusside (0.5 mcg/kg/min) |
| SBP >230 mm Hg or DBP 121-140 mm Hg | 10 mg of labetalol‡ given by IVP over 1-2 min; may repeat or double labetalol every 10 min to a maximum dose of 300 mg or give initial labetalol bolus and then start a labetalol drip at 2-8 mg/min |
| If BP not controlled by labetalol, consider sodium nitroprusside | |
| SBP 180-230 mm Hg or DBP 105-120 mm Hg | 10 mg of labetalol‡ given by IVP; may repeat or double labetalol every 10-20 min to a maximum dose of 300 mg or give initial labetalol bolus and then start a labetalol drip at 2-8 mg/min |
*All initial blood pressures should be verified before treatment by repeating reading in 5 minutes.
†As estimated by one third of the sum of systolic and double diastolic pressure.
‡Labetalol should be avoided in patients with asthma, cardiac failure, or severe abnormalities in cardiac conduction. For refractory hypertension, alternative therapy with sodium nitroprusside or enalapril may be considered.
From Bader MK, Littlejohns LR: AANN core curriculum for neuroscience nursing, ed 4, St Louis, 2004, Elsevier.
Medical management also includes the identification and treatment of acute complications such as cerebral edema and seizure activity. Prophylaxis for these complications is not recommended. Deep vein thrombosis (DVT) prophylaxis, however, should be initiated to decrease the risk of pulmonary embolism.12 One study demonstrated that improved outcomes for ischemic stroke patients can be achieved by managing swallowing issues, initiating DVT prophylaxis, and treating hypoxemia.23 Surgical decompression is recommended if a large cerebellar infarction compresses the brainstem.19
Subarachnoid Hemorrhage
Subarachnoid hemorrhage is bleeding into the subarachnoid space, which usually is caused by rupture of a cerebral aneurysm or arteriovenous malformation (AVM).19 At the time of autopsy, approximately 4% of the population has been found to have one or more aneurysms.24 Aneurysmal SAH is associated with a mortality rate of 25% to 50%, with most patients dying on the first day after the insult.24 Hemorrhage due to AVM rupture has a better chance of survival and is associated with an overall mortality rate of 10% to 15%.25
Etiology
Cerebral aneurysm rupture accounts for approximately 85% of all cases of spontaneous SAH.24 An aneurysm is an outpouching of the wall of a blood vessel that results from weakening of the wall of the vessel (Table 18-3).24 Ninety percent of aneurysms are congenital—the cause of which is unknown. The other 10% can be the result of traumatic injury (that stretches and tears the muscular middle layer of the arterial vessel) or infectious material (most often from infectious vegetation on valves of the left side of the heart after bacterial endocarditis) that lodges against a vessel wall and erodes the muscular layer, or they are of undetermined cause.26 Multiple aneurysms occur in approximately 30% of the cases and often are bilateral, occurring in the same location on both sides of the cerebral vascular system.27
TABLE 18-3
ANEURYSM CLASSIFICATION ACCORDING TO TYPE, SHAPE, LOCATION, AND COMMON CHARACTERISTICS
| TYPES OF ANEURYSMS | CHARACTERISTICS |
Berry or saccular
 |
Most common type, usually congenital; appears at a bifurcation in the anterior circulation, primarily at the base of the brain or the circle of Willis and its branches; grows from the base of the arterial wall with a neck or stem; contains blood; thinned dome is usually the site of rupture |
Giant or fusiform
 |
Can have an irregular shape and can be larger than 2.5 cm and atherosclerotic; involves mainly the internal carotid or vertebrobasilar artery; rarely ruptures; has no stem; can act like a space-occupying lesion in the brain; difficult to manage |
Mycotic
 |
Rare form; usually occurs from septic emboli, usually results from bacterial infection, which weaken the vessel wall, causing dilation involving the distal branches of the middle cerebral arteries |
Dissecting
 |
May occur during angiography; caused by trauma, syphilis, or arteriosclerosis, or when blood is forced between layers of the arterial wall; intima is pulled away from the medial layer, allowing blood to enter |
| Traumatic | Sometimes called a pseudoaneurysm, which may resolve after trauma |
| Charcot-Bouchard | Small aneurysm that can be seen in the area of the basal ganglia or the brainstem in individuals with a history of hypertension; chronic hypertension causes fibrinoid necrosis in the penetrating and subcortical arteries, weakening the arterial walls and causing formation of small aneurysmal outpouching |
AVM rupture is responsible for roughly 6% of all SAHs.27An arteriovenous malformation is a tangled mass of arterial and venous blood vessels that shunt blood directly from the arterial side into the venous side, bypassing the capillary system. They may be small, focal lesions or large, diffuse lesions that occupy almost an entire hemisphere.26 AVMs are always congenital, although the exact embryonic cause for these malformations is unknown. They also occur in the spinal cord and the renal, gastrointestinal, and integumentary systems.27 In contrast to SAH from aneurysm, which occurs in the middle-aged population, SAH from an AVM usually occurs in the second to fourth decades of life.27
Pathophysiology
The pathophysiology of the two most common causes of SAH is distinctly different.
Cerebral Aneurysm.
As the individual with a congenital cerebral aneurysm matures, blood pressure rises, and more stress is placed on the poorly developed and thin vessel wall. Ballooning of the vessel occurs, giving the aneurysm a berry-like appearance. Most cerebral aneurysms are saccular or berry-like with a stem or neck. Aneurysms are usually small, 2 to 7 mm in diameter, and often occur at the base of the brain on the circle of Willis.28 Figure 18-1 illustrates the usual distribution between the vessels. Most cerebral aneurysms occur at the bifurcations of blood vessels.24,25,28
The aneurysm becomes clinically significant when the vessel wall becomes so thin that it ruptures, sending arterial blood at a high pressure into the subarachnoid space. For a brief moment after the aneurysm ruptures, ICP is thought to approach mean arterial pressure (MAP), and cerebral perfusion decreases.28 In other situations, the unruptured aneurysm expands and places pressure on surrounding structures. This is particularly true with posterior communicating artery aneurysms, because they put pressure on the oculomotor nerve (cranial nerve III), causing ipsilateral pupil dilation and ptosis.27
Arteriovenous Malformation.
The pathophysiologic features of an AVM are related to the size and location of the malformation. One or more cerebral arteries, also known as feeders, supply an AVM. These feeder arteries tend to enlarge over time, increasing both the volume of blood shunted through the malformation and the overall mass effect. Large, dilated, tortuous draining veins develop as a result of increasing arterial blood flow being delivered at a higher than normal pressure. Normal vascular flow has an MAP of 70 to 80 mm Hg, a mean arteriole pressure of 35 to 45 mm Hg, and a mean capillary pressure that drops from 35 to 10 mm Hg as it connects with the venous side. Lack of this capillary bridge allows blood with an MAP of 35 to 45 mm Hg to flow into the venous system. Unlike arteries, veins have no muscular layer, and the veins become extremely engorged and rupture easily. Some patients with AVMs also have cerebral atrophy. It is the result of chronic ischemia because of the shunting of blood through the AVM and away from normal cerebral circulation.29
Assessment and Diagnosis
The patient with an SAH characteristically has an abrupt onset of pain, described as the “worst headache of my life.” A brief loss of consciousness, nausea, vomiting, focal neurological deficits, and a stiff neck may accompany the headache.24,26–28 The SAH may result in coma or death.
The patient’s history may reveal one or more incidences of sudden onset of headache with vomiting in the weeks preceding a major SAH. These are small “warning leaks” of an aneurysm in which small amounts of blood ooze from the aneurysm into the subarachnoid space. The presence of blood is an irritant to the meninges, particularly the arachnoid membrane, and the irritation causes headache, stiff neck, and photophobia. These warning leaks seldom are detected because the condition is not severe enough for the patient to seek medical attention.28 If a neurological deficit, such as third cranial nerve palsy, develops before aneurysm rupture, medical intervention is sought, and the aneurysm may be surgically secured before the devastation of a rupture can occur. Symptoms of unruptured AVM—headaches with dizziness or syncope or fleeting neurological deficits—also may be found in the history.27
Diagnosis of SAH is based on clinical presentation, CT findings, and lumbar puncture results. Noncontrast CT is the cornerstone of definitive SAH diagnosis. In 95% of the cases, CT can demonstrate blood in the subarachnoid space if performed within 48 hours of the hemorrhage.24,28 On the basis of the appearance and the location of the SAH, diagnosis of the cause—aneurysm or AVM—may be made from the CT scan.26 MRI is not routinely used, but it may provide greater sensitivity for detecting areas of SAH clot and potential location of bleed.28
If the initial CT finding is negative, a lumbar puncture is performed to obtain cerebrospinal fluid (CSF) for analysis. CSF after SAH appears bloody and has a red blood cell count greater than 1000 cells/mm3. If the lumbar puncture is performed more than 5 days after the SAH, the CSF fluid is xanthochromic (dark amber) because the blood products have broken down.29 Cloudy CSF usually indicates some type of infectious process, such as bacterial meningitis, not SAH.28
After the SAH has been documented, cerebral angiography is necessary to identify the exact cause of the hemorrhage. If a cerebral aneurysm rupture is the cause, angiography is essential for identifying the exact location of the aneurysm in preparation for surgery.27,29,30 After the aneurysm has been located, it is graded using the Hunt and Hess classification scale. This scale categorizes the patient on the basis of the severity of the neurological deficits associated with the hemorrhage (Box 18-4).31 If AVM rupture is the cause, angiography is necessary to identify the feeding arteries and draining veins of the malformation.26
Medical Management
SAH is a medical emergency, and time is of the essence. Preservation of neurological function is the goal, and early diagnosis is crucial. Initial treatment must always support vital functions. Airway management and ventilatory assistance may be necessary.19 A ventriculostomy is performed to control ICP if the patient’s level of consciousness is depressed.30
Evidence suggests that only 19% of the deaths attributable to aneurysmal SAH are related to the direct effects of the initial hemorrhage.32 Rebleeding accounts for 22% of deaths from aneurysmal SAH, cerebral vasospasm for 23%, and nonneurological medical complications for 23%.32 Principal nonneurological causes of death are systemic inflammatory response syndrome (SIRS) and secondary organ dysfunction.33 After initial intervention has provided necessary support for vital physiologic functions, medical management of acute SAH is aimed primarily toward prevention and treatment of the complications of SAH that can produce further neurological damage and death.28
Rebleeding.
Rebleeding is the occurrence of a second SAH in an unsecured aneurysm or, less commonly, an AVM.6 The incidence of rebleeding during the first 24 hours after the first bleed is 4%, with a 1% to 2% chance per day for the following month. The mortality rate associated with aneurysmal rebleeding is approximately 70%.26,27
Historically, conservative measures to prevent rebleeding have included blood pressure control and SAH precautions (see “Nursing Management”). An elevation in blood pressure is a normal compensatory response to maintain adequate cerebral perfusion after a neurological insult. In the belief that hypertension contributes to rebleeding, intravenous antihypertensive agents are used to maintain a systolic blood pressure no greater than 140 mm Hg.28 Individualized guidelines must be determined on the basis of the clinical condition and preexisting values of the patient. Evidence suggests that rebleeding has more to do with variations in blood pressure than it does with absolute values and that blood pressure control does not lower the incidence of rebleeding.30
Surgical Clipping of Aneurysms.
Definitive treatment for the prevention of rebleeding is surgical clipping or endovascular coiling with complete obliteration of the aneurysm.27,28 Timing of the operation is a key medical management issue. Since the introduction of microsurgery and improved surgical techniques, patients are commonly taken to the operating room within the first 48 hours after rupture.28 This early surgical intervention to secure the aneurysm eliminates the risk of rebleeding and allows more aggressive therapy to be used in the postoperative period for the treatment of vasospasm.26 Early surgery also allows the neurosurgeon to flush out the excess blood and clots from the basal cisterns (reservoir of CSF around the base of the brain and circle of Willis) to reduce the risk of vasospasm.33 Careful consideration of the patient’s clinical situation is necessary in determining the optimal time for surgery.
The surgical procedure involves a craniotomy to expose and isolate the area of aneurysm. A clip is placed over the neck of the aneurysm to eliminate the area of weakness (Figure 18-2). This is a technically difficult procedure that requires the skill of an experienced neurosurgeon. It is not uncommon, particularly in early surgery, for the clot to break away from the aneurysm as it is surgically exposed. Extensive hemorrhage into the craniotomy site results, and cessation of the hemorrhage often causes increased neurological deficits. Deficits also may occur as a result of surgical manipulation to gain access to the site of the aneurysm.28
Surgical Excision of Arteriovenous Malformations.
Management of AVMs has traditionally involved surgical excision or conservative management of such symptoms as seizures and headache. The decision for surgical excision depends on the location and size of the AVM. Some malformations are located so deep in the cerebral structures (thalamus or midbrain) that attempts to remove the AVM would cause severe neurological deficits. History of a previous hemorrhage and the patient’s age and overall condition are also taken into account in the decision regarding surgical intervention.28
Surgical excision of large AVMs includes the risk of reperfusion bleeding. As feeding arteries of the AVM are clamped off, the arterial blood that usually flowed into the AVM is diverted into the surrounding circulation. In many cases, the surrounding tissue has been in a state of chronic ischemia, and the arterial vessels feeding these areas are maximally dilated. As arterial blood begins to flow at a higher volume and pressure into these dilated arteries, blood may seep from the vessels. Evidence of reperfusion bleeding in the operating room is an indication that no more arterial blood can be diverted from the AVM without risk of serious ICH. In the postoperative phase, a low blood pressure is maintained to prevent further reperfusion bleeding. For large AVMs, two to four stages of surgery may be required over 6 to 12 months.28
Embolization.
Embolization is used to secure a cerebral aneurysm or AVM that is surgically inaccessible because of size or location or because of the medical instability of the patient. Embolization involves several new interventional neuroradiology techniques. All of the techniques use a percutaneous transfemoral approach in a manner similar to an angiogram. Under fluoroscopic guidance, the catheter is threaded up to the internal carotid artery. Specially developed microcatheters are then manipulated into the area of the vascular anomaly, and embolic materials are placed endovascularly. Three embolization techniques are used, depending on the underlying pathologic derangement.26
The first type of embolization is used to embolize an AVM. Small polymeric silicone (Silastic) beads or glue is slowly introduced into the vessels feeding the AVM. Blood flow carries the material to the site, and embolization is achieved. This procedure may be used in combination with surgery. One to three sessions of embolization of the feeding vessels are performed to reduce the size of the lesion before a craniotomy is performed for total excision. The primary risk of this procedure is lodging of the embolic substance in a vessel that feeds normal tissue, which creates an embolic stroke with the immediate onset of neurological symptoms.26
The second type of embolization involves placement of one or more detachable coils into an aneurysm to produce an endovascular thrombus (Figure 18-3). The advantage of this technique is that an electrical current creates a positive charge on the coil, which induces electrothrombosis. Complications include embolic stroke, coil migration, overproduction of the clot, subtotal occlusion and intraprocedural rupture of the vasculature, and death.26
