Management of Increased Intracranial Pressure and Intracranial Shunts

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Chapter 59

Management of Increased Intracranial Pressure and Intracranial Shunts

Jessica L. Osterman and Megan L. Rischall

Headache and head injury are encountered commonly in the emergency department (ED). If either is accompanied by vomiting, decreased level of consciousness, or abnormal vital signs, the possibility of increased intracranial pressure (ICP) should be considered. Acutely increased ICP is a neurologic emergency that must be managed quickly before further brain damage and death ensue. In some cases the accompanying clinical symptoms may be vague or subtle and make diagnosis difficult. Familiarity with the pathophysiology of increased ICP facilitates its diagnosis and management.

Pathophysiology of ICP

Alexander Monro, an anatomist in the 18th century, described the intracranial contents as containing a fixed volume. The fixed-volume theory was supported by George Kellie a few years later and became known as the Monro-Kellie doctrine. This doctrine has since guided our understanding of intracranial dynamics and the principles of autoregulation.

The components of the calvaria are the brain parenchyma, cerebrospinal fluid (CSF), the venous blood supply, and the arterial blood supply (Fig. 59-1). CSF and the venous blood supply have the greatest ability to change their volume to compensate for increases in pressure. These dynamic changes in the relative proportion of the cranial content may not affect the patient if ICP is not excessive. However, if a pathologic process overwhelms the compensatory mechanisms, the result will be a nearly exponentially increase in ICP (Fig. 59-2).

Figure 59-1 Intracranial contents and their volumes in healthy adults. CSF, cerebrospinal fluid; ECF, extracellular fluid.

Figure 59-2 The intracranial volume-pressure relationship demonstrates the limits of compensatory mechanisms. ICP, intracranial pressure.

Normal supine ICP ranges from 5 to 15 mm Hg. Transient increases in ICP as high as 80 to 100 mm Hg occur with coughing or straining. Other factors that can transiently increase ICP are movement, pain, and fever. A space-occupying lesion such as a tumor, hematoma, abscess, or foreign body can also raise ICP. Figure 59-3 demonstrates that the area and cause of the increased ICP will determine where shifts occur to result in brain herniation.

Figure 59-3 Intracranial shifts as a result of supratentorial lesions. A, Relationships of the various supratentorial compartments as seen in a coronal section. B, Herniation of the cingulate gyrus under the falx (1); herniation of the temporal lobe into the tentorial notch (2); compression of the opposite cerebral peduncle against the unyielding tentorium, which produces Kernohan’s notch (3); and downward displacement of the brainstem through the tentorial notch (4). (A and B, From Plum F, Posner JB. The Diagnosis of Stupor and Coma. 2nd ed. Philadelphia: Davis; 1972. Reproduced by permission.)

Brain

Brain volume can be increased by edema, idiopathic intracranial hypertension (IIH), tumor, or bleeding. The three types of edema are vasogenic, cytotoxic, and interstitial. Vasogenic edema results from increased permeability of the capillaries, which leads to passage of excess fluid into the extracellular space. Cytotoxic edema is due to accumulation of intracellular fluid in brain tissue (neurons and glia) secondary to dysfunction of the adenosine triphosphatase pump. Interstitial edema occurs when fluid accumulates as a result of blockage of CSF absorption.

IIH, formerly known as pseudotumor cerebri, is a chronic condition characterized by increased CSF pressure not caused by a tumor, edema, hydrocephalus, or change in CSF composition. It occurs most frequently in obese women. Symptoms may include headache, nausea, and blurry vision. The headache is typically worse on waking or with exertion. In general, patients with IIH have normal findings on neurologic examination except for the frequent presence of papilledema. Although the precise pathophysiology remains unclear, severe cases can lead to permanent loss of vision. Commonly associated conditions are listed in Box 59-1. IIH is a diagnosis of exclusion. Because of its often subtle symptoms and normal findings on computed tomography (CT), it is often not suspected or diagnosed on initial clinical evaluation.

Box 59-1 Clinical Conditions and Factors Associated with Idiopathic or Benign Intracranial Hypertension (Pseudotumor Cerebri)

Hematologic Disorders

Iron deficiency anemia

Endocrine Conditions and Disorders

Addison’s disease

Menstrual irregularities, menstrual cycle

Pregnancy

Medical/Surgical Conditions with Impaired Cerebral Venous Drainage

Otitis media, mastoiditis

Idiopathic dural sinus thrombosis

Radical neck surgery

Chronic pulmonary disease with venous hypertension

Heart failure with venous hypertension

Congenital heart disease

Renal failure

High-flow arteriovenous malformation

Chronic obstructive pulmonary disease

Systemic steroid withdrawal

Topical steroid withdrawal (infants)

Oral contraceptives

Tetracycline/minocycline

Isotretinoin (Accutane)

Nonsteroidal antiinflammatory drugs

Tamoxifen

Cyclosporine

Dietary Considerations

Brain tumors encompass neoplasms that originate in the brain itself (primary brain tumors) or involve the brain as a metastatic site (secondary brain tumors). Primary brain tumors include tumors of the brain parenchyma, meninges, cranial nerves, and other intracranial structures (the pituitary and pineal glands). Primary central nervous system (CNS) lymphoma refers to non-Hodgkin’s lymphoma confined to the CNS. The site of origin of this type of tumor remains unknown. Secondary brain tumors, the most common type, originate elsewhere in the body and metastasize to the intracranial compartment.

Bleeding in the brain can occur spontaneously (as in the case of hemorrhagic stroke or spontaneous subarachnoid hemorrhage) or can be a result of trauma. In addition to the mass effect of the blood itself, the associated edema in both instances contributes to further increases in ICP. Diffuse axonal injury (DAI) may occur in isolation or in conjunction with intracerebral bleeding. With DAI it is believed that the axons are not actually torn, but instead suffer significant injury that may lead to edema (shearing effect).¹

CSF

CSF is produced by the choroid plexus at a daily rate of 500 mL. It flows from the ventricles into the cisternae of the subarachnoid space and is drained by the arachnoid villi of the dural sinuses to maintain a constant volume of 100 to 150 mL. Obstructive hydrocephalus occurs when flow is blocked at any point in the ventricular system by clotted blood, tumor, colloid cyst, edema, or primary stenosis. Communicating hydrocephalus is due to impedance of flow beyond the ventricular system at the level of the basal cisternae or lack of absorption by the arachnoid villi. Communicating hydrocephalus can occur with both infection and subarachnoid hemorrhage (Fig. 59-4).

Figure 59-4 Cerebrospinal fluid production and flow. (From Rengachary SS, Wilkins RH, eds. Principles of Neurosurgery. Philadelphia: Mosby; 1994.)

Blood

Up to a certain range, cerebral blood flow (CBF) is maintained by an autoregulatory mechanism despite fluctuations in cerebral perfusion pressure (CPP) (Fig. 59-5). Constant CBF can typically be maintained at any CPP between 60 and 160 mm Hg. Once CPP is out of the autoregulatory zone, CBF is linearly related to CPP. CPP lower than 60 mm Hg can lead to ischemia, whereas CPP higher 160 mm Hg can result in hypertensive encephalopathy.

Figure 59-5 Cerebral autoregulation. CPP, cerebral perfusion pressure; ICP, intracranial pressure.

Signs and Symptoms

Findings on neurologic examination can be normal in a patient with a mild increase in ICP because of the brain’s compensatory mechanisms. Patients with a complaint of headache or head injury may not initially manifest the more dramatic and worrisome symptoms of increased ICP such as vomiting, syncope, altered mentation, or Cushing’s reflex (bradycardia, increased blood pressure, and irregular respirations). ICP correlates poorly with clinical symptomatology. One of the earliest clinical signs is decreased venous pulsation on funduscopic examination, but this may be difficult to appreciate in an acutely ill patient in a busy ED. Moreover, the initial findings on head CT might not reveal the true extent of injury, especially with early stroke or when DAI is involved. However, as compensatory mechanisms fail, CT findings, as well as clinical symptoms, will become more obvious.

Signs and symptoms of severely increased ICP include a decreasing level of consciousness, papilledema, cranial nerve palsies, and lateralizing neurologic deficits. When any of these are noted, particularly when CT confirms the presence of a mass effect such as hydrocephalus or a midline shift, urgent intervention is necessary (Fig. 59-6). Neurosurgical consultation for possible invasive means of reducing ICP and monitoring is indicated. Medical management of increased ICP should also proceed without delay.

Figure 59-6 Acute subdural hematoma. The large hyperdense (i.e., white) collection on the right side of the brain is an acute subdural hematoma. The finding of hypodense areas (black arrow) within the subdural is referred to as the “swirl” sign, which suggests active bleeding. Also seen is about 3 cm of midline shift (white arrow), which indicates subfalcine herniation. Immediate reduction of intracranial pressure and surgical intervention are required.

Medical Treatment of Increased ICP

Oxygenation

Management of the airway and breathing is paramount in patients with brain injury. If the patient is hypoxic, supplemental oxygen is necessary to prevent further ischemia. Patients with a Glasgow Coma Scale (GCS) score of 9 or lower or with impending signs of inadequate respiratory status should undergo rapid-sequence intubation (RSI) to protect the airway and better control blood levels of oxygen and carbon dioxide (Po₂ and Pco₂).

Sedation and Paralytics

Rapid assessment of findings on the patient’s neurologic examination should be performed before sedation and paralysis (RSI). If time allows, premedicate the patient with lidocaine, 1 to 1.5 mg/kg, 3 minutes before intubation while preoxygenating the patient. Lidocaine has been reported to blunt the rise in ICP associated with laryngoscopy and may protect against some hypoxia-related dysrhythmias; however, its true value is unproven (see Chapters 4 and 5). Fentanyl is an excellent drug for control of pain, which if untreated, can lead to increased ICP. Fentanyl can also be given during the pretreatment phase of RSI (3 minutes before administration of the paralytic agent) if time allows. Caution should be exercised, however, to avoid precipitous drops in blood pressure, which can actually threaten CBF and exacerbate the brain injury. It is for this reason that we recommend titrating fentanyl up to the target dose rather than giving a full dose (typically 3 µg/kg) up front. We also recommend caution in brain-injured patients with concomitant trauma, who may have exaggerated episodes of hypotension with narcotics and sedatives. A defasciculating dose of the intubating dose of a nondepolarizing neuromuscular blocking agent may also be administered as pretreatment. If given 3 minutes before a paralyzing dose of succinylcholine, it counteracts the transient fasciculations and increase in ICP that occur with the administration of succinylcholine. It is important to note, however, that practice variations are common, and though theoretically attractive, none of these pretreatment options have been demonstrated to improve patient-oriented outcomes.²

Induction (sedation) may be achieved with drugs that block the rise in ICP, such as etomidate (0.3 mg/kg intravenously [IV]), thiopental (50 to 100 mg IV or 3 to 5 mg/kg IV), or propofol (1 mg/kg IV).3–5 Etomidate is preferred over thiopental in patients with unstable hemodynamic status. Etomidate has two significant benefits—it has minimal effect on systemic blood pressure and does not appear to increase ICP.^3,4 Propofol has gained acceptance as a sedative in patients with increased ICP because of its short duration of action and depression of cerebral metabolism and oxygen consumption. This may have a neuroprotective effect. However, propofol can cause profound decreases in systemic blood pressure and with higher doses and extended periods of use can be associated with significant morbidity.⁵

The paralytic agent of choice is succinylcholine (1.5 mg/kg in adults and up to 2.5 mg/kg in pediatric patients). Nondepolarizing agents are appropriate when contraindications to succinylcholine exist (see Chapter 5). Sedatives and paralytics should be short acting to facilitate close monitoring of the patient’s neurologic status.

Oxygenation and Hyperventilation

As the airway is secured, adequate supplemental oxygen should be provided, with titration down rapidly from the initial Fio₂ of 1.0 used for RSI to ensure oxygen saturation greater than 90%.¹

In general, avoid hyperventilation in patients with brain injury because low Pco₂ levels cause cerebral vasoconstriction, which results in decreased CBF in the critical hours following injury.1,6 Hyperventilation is also associated with poor survival and neurologic outcomes.^6,7 For the majority of brain-injured patients, the target is thus eucapnia with a Pco₂ of 35 to 40 mm Hg.⁶ Nevertheless, if a patient displays evolving signs of brain herniation (e.g., anisocoria, hemiparesis, asymmetric posturing, Cushing’s reflex, or rapid deterioration in GCS score), hyperventilation may be necessary to arrest the process. Current recommendations target a Pco₂ of 28 to 35 mm Hg in these scenarios as a temporizing measure until surgical intervention to lower ICP can occur.⁶

Head Position

If not in shock, elevate the patient’s head to decrease ICP. This position allows drainage of cerebral veins. Feldman,⁸ Ng,⁹ and their colleagues demonstrated that elevation of the head to 30 degrees significantly reduces ICP in most patients without impairing CBF, CPP, or cerebral metabolism. Raising the head of a patient with hypotension, however, exacerbates any decrease in the patient’s mean arterial pressure (MAP) and hence lowers CPP. If head elevation is to be used, MAP must be maintained above 90 mm Hg to facilitate a CPP of approximately 60 mm Hg.¹⁰ It is also important to avoid neck rotation and flexion or any other intervention that could result in compression of the jugular vein. If the jugular veins are compressed, venous outflow from the head can be further compromised.

Fluid Management

The goal in fluid management is to maintain euvolemia; head-injured patients with hypotension have double the mortality of normotensive patients. Patients with increased ICP may be hypovolemic from profuse vomiting, decreased fluid intake, or hemorrhage. Distributive shock may also be present and occurs as a result of loss of vasomotor sympathetic tone, especially in patients with concomitant cervical spine injury. Only isotonic or hypertonic fluids should be used (normal or hypertonic saline) to avoid worsening the brain edema. Vasopressors may also be necessary. A target CPP of 60 mm Hg and MAP of greater than 90 mm Hg are recommended.¹¹

Diuresis

Mannitol effectively reduces ICP. Mannitol can be given at doses of 0.25 to 1 g/kg infused every 2 to 6 hours. Current recommendations favor bolus therapy over continuous infusion. Mannitol can cause a precipitous drop in blood pressure (and hence CPP) in patients who are hypovolemic. Thus, mannitol is contraindicated in those with preexisting hypotension (typically defined as blood pressure lower than 90 mm Hg systolic). The effect of mannitol on ICP is transient; repeated doses lose their ability to decrease ICP over time. It also may cause renal damage as serum osmolarity increases. Similar to hyperventilation, mannitol should be viewed as a bridge to definitive neurosurgical intervention and reserve its use for patients with signs of impending transtentorial herniation.

Mannitol has two properties—it initially acts as a volume expander and then serves as an osmotic agent. On administration of mannitol, intravascular volume expands and blood viscosity decreases, which results in augmentation of CBF. Once volume expansion has occurred, osmotic movement of fluid from the cellular compartment to the intravascular compartment begins and results in a decrease in ICP. The osmotic effect usually occurs within 15 minutes. The half-life of mannitol ranges from 90 minutes to 6 hours.¹¹ Its effect is most pronounced in patients with CPP lower than 70 mm Hg.¹¹

Hypertonic saline is an alternative to mannitol in patients who are hypotensive. Theoretically, hypertonic saline is an ideal resuscitation fluid for patients with concomitant head injury and hemorrhagic shock because it can effectively expand intravascular volume while causing osmotic diuresis of the brain. However, its efficacy in human studies remains uncertain, so its use should be reserved for patients with contraindications to mannitol. Although hypertonic saline is generally administered in boluses of 100 to 250 mL of 3% to 7% saline, there is no consensus on the preferred concentration and volume of administration.11,12

Seizure Prophylaxis

Controlling seizure activity is important in the early stages of traumatic brain injury to avoid hypoxia and aspiration, as well as elevation of ICP and possible herniation. In the absence of a seizure, it is currently controversial whether or not prophylactic anticonvulsant medication should be routinely administered after spontaneous intracranial hemorrhage. If a seizure occurs, anticonvulsant therapy is warranted. However, current literature supports the use of prophylactic anticonvulsant therapy for the first week following traumatic brain injury, with the recognition that this has no effect on the occurrence of late seizures.¹³ Rapidly acting benzodiazepines such as lorazepam and diazepam are first-line therapy, followed by anticonvulsant agents such as pentobarbital or phenytoin. Fosphenytoin can be given more rapidly (up to 150 mg/min IV by infusion) than phenytoin, which can be administered no faster than 50 mg/min IV. Phenytoin can induce hypotension and lower CPP, even at rates below 50 mg/min. Levetiracetam has also been used in cases of severe head injury and stroke. Although beneficial effects of prophylactic anticonvulsants on outcomes have not been established, in the ED setting the risks associated with the administration of anticonvulsants are probably small in comparison to the potentially devastating secondary injury that results from uncontrolled seizure activity.

Patients who are paralyzed, either chemically or by their neurologic disease, are difficult to monitor for seizure activity without the use of electroencephalography (EEG). These patients may manifest seizures by subtle rhythmic movements of the extremities, tonic gaze deviation, an elevation in heart rate or blood pressure, or spikes on an ICP monitor (if one is placed by the neurosurgeon). Whenever possible, avoid giving paralytic agents so that seizure activity can be monitored. If seizures are not controlled despite upward titration of benzodiazepines, phenytoin, or other agents, endotracheal intubation and induction of a barbiturate or propofol coma may be necessary. Pentobarbital has been used to treat uncontrolled elevations in ICP when other medical and surgical treatments have failed. It decreases CBF, metabolism, oxygen consumption, and cerebral edema. It also scavenges free radicals.¹⁴ Pentobarbital is given at a loading dose of 10 mg/kg over a 30-minute period followed by infusion at 1 to 3 mg/kg/hr with an EEG monitor available for burst suppression. Intensive monitoring of the patient’s hemodynamic status is necessary because of pentobarbital’s hypotensive effect.

Steroids

Steroids have been shown to be beneficial in patients with vasogenic edema—edema associated with brain tumors. Steroids decrease CSF production, stabilize membranes, and restore normal membrane permeability.15–17 Dexamethasone is usually administered at 10 to 20 mg IV as a loading dose, followed by 4 to 10 mg every 6 hours. No studies support the use of steroids in head-injured patients; in fact, steroid use in this population was found to increase mortality in a recent randomized trial.^18,19

Skull trephination or burr hole placement is a technique that has been traced back to the Neolithic age.²² Though rarely performed by nonsurgeons, it can be a lifesaving procedure for severely head-injured patients. In a patient with an acute epidural hematoma and deteriorating findings on neurologic examination (decreasing level of consciousness, anisocoria, or hemiparesis), skull trephination has been shown to improve both short- and long-term neurologic outcomes.^23,24 Trephination is an appropriate treatment of acute subdural hematoma as well, but this requires incision through the dura mater and is less realistic for non-neurosurgeons (Figs. 59-7 and 59-8). Trephination may be lifesaving if performed before transport in selected patients from remote practice environments without neurosurgical capability. This heroic procedure is not standard nor is it an expected skill for most emergency clinicians; nonetheless, it may be attempted when the situation is dire.

Figure 59-7 Epidural hematoma (left): rupture of a meningeal artery (usually associated with a skull fracture) leads to accumulation of arterial blood between the dura and skull. Subdural hematoma (right): damage to bridging veins between the brain and superior sagittal sinus leads to accumulation of blood between the dura and the arachnoid. (From Mitchell R, Kumar V, Fausto N, et al. Pocket Companion to Robbins & Cotran Pathological Basis of Disease. 8th ed. Philadelphia: Saunders; 2011.)

Figure 59-8 Epidural and subdural hematomas. A, Acute epidural hematoma. The hyperdense (white) appearance of this lesion indicates that it is acute. The inner margins of epidurals (arrows) have a convex appearance, which results in a lens-shaped hematoma. Note that the hematoma does not cross the suture lines (temporal-sphenoid anteriorly, temporal-parietal posteriorly.) B, Acute-on-chronic subdural hematoma. The heterogeneous density of this lesion (hypodense anteriorly [double asterisk], hyperdense posteriorly [asterisk]) suggests an acute-on-chronic hemorrhage. The inner margins of subdurals (arrows) are concave, which results in a crescent-shaped hematoma. Subdural hematomas may cross suture lines (note that this hematoma extends from the frontal to the parietal regions). Both these examples demonstrate effacement of the lateral ventricles and midline shift, which are indicators of increased intracranial pressure.

Trephination in the ED should be performed only in the temporal regions. Trephination in the parietal and occipital regions is associated with a much higher risk for hemorrhage and air embolism because of their proximity to the dural venous sinuses. These approaches should generally be performed only after the results of CT are available for guidance.²³

In practice environments without access to CT, it is not unreasonable to attempt this procedure before transport of the patient when all other measures to lower ICP have failed and signs of evolving brain herniation are present. The blind approach for temporal trephination is depicted in Figure 59-9 and involves locating an area 2 cm anterior and 2 cm superior to the tragus on the side of the suspected hematoma.²³ In the majority of cases the appropriate side for trephination is ipsilateral to pupillary dilation and contralateral to motor paresis.

Figure 59-9 Emergency skull trephination. This technique is for patients with epidural hematomas in the temporal region. It should not be used for parietal or occipital hematomas. If a subdural hematoma is present, the dura must be opened. If no blood is encountered on a blind procedure done on the same side as a blown pupil, repeat it on the opposite side. CT, computed tomography.

The temporal area (between the ear and the orbit) should be shaved and prepared with chlorhexidine or Betadine via sterile technique. Local anesthetic can be used to infiltrate the area of the planned incision; however, this is not necessary in an unconsciousness patient and adds time to the procedure.

Once the proper location is identified, either with a blind approach or by using the results of CT, make a 4-cm vertical incision through the skin and temporalis fascia. Divide the temporalis muscle. Once the periosteum is identified, incise it. Expose the skull by elevating the periosteum (with a periosteal elevator if available). Insert a trephine or hand drill into the area underneath the periosteum.²³ While holding the drill perpendicular to the skull, apply continuous gentle downward pressure and rotate the drill in an alternating clockwise-counterclockwise motion. As progress is made with the hand drill, gradually reduce pressure to avoid inadvertent “plunging” into the brain parenchyma. The operator will know when penetration through both the outer and inner tables of the skull has been accomplished once resistance against the drill is no longer felt. After skull penetration has been accomplished, remove the round piece of bone that has been cored out (with the diameter of the drill) and place it in saline.

In many cases, epidural blood and clots under pressure will extrude from the site on full penetration of the skull. Insertion of a suction catheter into the trephinated space may be necessary, however, for full evacuation of the clotted material. If easily identified, the bleeding artery (usually the middle meningeal artery) may be clamped.²³ After successful trephination, the patient’s status should immediately be reassessed for signs of improvement. In a significant minority of patients, false localizing signs may lead the clinician to suspect a hematoma on the wrong side. Thus, if no improvement is noted with trephination on the side of the suspected hematoma, the procedure may be repeated on the opposite side. However, in all cases the delay in definitive neurosurgical care caused by attempts at trephination must be weighed against the possible benefits of the procedure. Moreover, trephination should ideally be performed after consultation with the accepting neurosurgeon.²⁴

Complications of the procedure include bleeding, infection, and injury to the brain parenchyma.

Operative Management

Operative management is the definitive treatment of increased ICP secondary to a space-occupying lesion. This entails either craniotomy or ventriculostomy with drainage of blood and CSF.

Intracranial Shunts

Intracranial shunts are used for the long-term management of increased ICP. A recurrent elevation in ICP because of shunt malfunction can occur days to years after shunt placement.

Intracranial shunt malfunctions are often diagnosed in the ED. In many cases the patient’s significant other or caretaker provides the subtle clues that lead to the diagnosis. Depending on the age of the patient, recurrent symptoms of increased ICP may include changes in behavior, headache, nausea, vomiting, and visual disturbances. In children, lethargy, poor feeding, vomiting, ataxia, decreased or increased activity level, fever, diaphoresis, and fussiness can all be indications of shunt malfunction. Accordingly, in children, lethargy and shunt site swelling are the most predictive signs of shunt malfunction.²⁵ Failure of upward gaze is another sensitive sign of nonacute shunt malfunction (sunset eyes). Cranial nerve and visual field examination should be routine in the assessment of older, cooperative patients with intracranial shunts.

The essential elements of any shunt system include the proximal and distal catheters, a valve, and a reservoir. A wide array of valves, catheters, and other devices are available for use in shunt systems. The valve allows unidirectional flow, incorporates a pumping chamber, and regulates the pressure at which flow will occur across it. The reservoir is usually located between two valves. The proximal valve allows flow from the ventricles to the reservoir, whereas the distal valve allows flow from the reservoir to the distal catheter (Fig. 59-10). Many different types of shunt systems incorporating a variety of designs are available (Fig. 59-11). Some have unique characteristics, such as a double dome, whereas in others, valves are absent altogether.

Figure 59-10 The basic tripartite ventricular shunt system is composed of a ventricular catheter, valve mechanism, and distal tubing. A slit valve may be used in the far end of the distal tubing instead of a more proximally placed valve, as shown.

Figure 59-11 Shunt components. (From Rengachary SS, Wilkins RH, eds. Principles of Neurosurgery. Philadelphia: Mosby; 1994.)

In most cases the reservoir gives access to the system for measurement of pressure, testing of patency, fluid sampling, and injection of medication or contrast material. In rare cases, other equipment is incorporated into the shunt system for specific purposes, including an on-off switch, a telemetric pressure sensor, and an antisiphon device.

In the ventriculoperitoneal (VP) shunt system, a ventricular catheter is placed in the right frontal horn of the lateral ventricle (nondominant hemisphere) and connected to a subcutaneous valve traversing the temporal aspect (Fig. 59-12). This valve is then connected to a distal catheter threaded subcutaneously into the neck and finally into the peritoneum or another body compartment (Fig. 59-13). Ventricular catheters can be either straight or angled, with the latter having the option of a reservoir component attachment. Valves come in four different types (ball, diaphragm, miter, slit), each with unique flow characteristics. The distal catheter has either an open or closed end. Identifying the type of shunt in place is often difficult unless the patient or caretaker has the information available. Moreover, the skin overlying the subcutaneous component of the shunt in the temporal regions can scar, thus rendering palpation of the shunt type impossible.

Figure 59-12 Cross-section of a Pudenz flushing valve (American Heyer-Schulte, Santa Barbara, CA) illustrating the diaphragm valve. The proximal inlet tube and silicone base are placed in the burr hole so that only the reservoir (silicon dome) protrudes above the skull. (Courtesy of PS Medical, Goleta, CA.)

Figure 59-13 Ventriculoperitoneal shunt and alternative occipital placement. AP, anteroposterior. (From Rengachary SS, Wilkins RH, eds. Principles of Neurosurgery. Philadelphia: Mosby; 1994.)

The principle of any extracranial shunting technique is to divert CSF into a body cavity from which it can readily be eliminated and drained. VP shunts are at present the mainstay for treatment of hydrocephalus in infants and children because of their ease of insertion and reliable long-term function. Other types of extracranial shunts include ventriculovenous, ventriculoatrial, ventriculopleural, and lumboperitoneal. These other shunts are usually reserved for circumstances in which VP shunting has failed or when the patient has a history of multiple abdominal surgeries or peritoneal infections.

Shunt Assessment

The usual cause of shunt malfunction is catheter obstruction. Proximal blockage may be due to tissue debris or choroid plexus within the ventricular catheter. Distal obstruction of venous shunts may result from thrombus or venous occlusion (such as in a ventriculovenous shunt). Peritoneal shunts may be associated with infection (peritonitis) or mechanical obstruction (e.g., omental blockade). Some evidence suggests that delayed hypersensitivity to the shunt material is also an occasional cause of obstruction. Symptoms of shunt malfunction may be difficult to interpret, particularly if the symptoms are atypical or nonspecific or if they occur in young children. Likewise, asymptomatic shunt obstruction can take place in children in whom shunt independence has developed. Clinical evaluation of a child may not always be diagnostic of a shunt malfunction.

The characteristics of valve pumping may be useful in diagnosing shunt malfunction in some cases.²⁶ Partial shunt valve compression should be done because full depression and release may lead to suction of the choroid plexus. If the CT scan shows narrow (slit) ventricles, indicative of overdrainage, further valve compression may cause blockage of the shunt by suction of the choroid plexus. When palpating the shunt, a normal refill time of 15 to 30 seconds should be observed. If the valve fills more slowly than this but can be compressed easily, the obstruction is proximal to the valve. If the valve is not compressible, the blockage is either at the valve or distal to it. Proximal obstruction is more common than distal obstruction. Other sources of proximal blockage include blood clots or debris related to the surgical procedure. Sources of distal occlusion include malposition, infection, shunt disconnection, and pseudocyst formation. The entire shunt tract and surgical incisions should be examined for signs of wound infection, disruption of the tubing, or CSF leakage around the tract. Definitive treatment usually requires shunt revision.

Infection is usually due to skin organisms overlying the valve that may ulcerate the skin and colonize the shunt.²⁷ Typical organisms include Staphylococcus epidermidis and Staphylococcus aureus, as well as gram-negative bacilli, which are thought to be introduced into the system during manipulation of the site.²⁸ Approximately 70% of these infections are seen within 2 months of shunt placement (see later under “Special Considerations—Postoperative Shunt Complications”). Other risk factors associated with shunt infection include perioperative infection and dental or urologic instrumentation. Two studies have shown that approximately 8% of neurosurgical patients with implanted shunts acquire infections.^29,30

Radiographic evaluation of shunt function in the ED begins with a non–contrast-enhanced head CT scan and shunt series radiographs. Shunt series radiographs consist of anteroposterior (AP) and lateral skull, AP chest, and AP abdominal radiographs (Figs. 59-14 and 59-15). These films may reveal kinking, breakage, or disconnection of the catheter (Fig. 59-16). Head CT has a sensitivity of 83% in detecting shunt obstruction and a negative predictive value of 93%. Shunt series radiographs have a sensitivity of 20% and a negative predictive value of 22%. When combined, the two tests have a sensitivity of 88% and a negative predictive value of 95%. Because they are complementary, it is important to obtain both studies.^31–33 It should be noted, however, that the finding of enlarged ventricles on CT reveals little about shunt function unless comparison with previous images or serial scans show progressive ventricular expansion.

Figure 59-14 Computed tomography (CT) evaluation of a ventriculoperitoneal shunt. Axial (top row) and coronal (middle row) CT images reveal the intracranial portion of the shunt. Mild ventriculomegaly is noted; however, this reveals little about shunt function without comparison to previous scans. The shunt (arrows) can also be visualized on the CT scout images (bottom row, left and middle). A companion chest radiograph (bottom row, right) demonstrates continuity of the shunt (arrows) through to the peritoneal cavity.

Figure 59-15 Conventional radiography standard shunt series. A standard shunt series includes skull, chest, and abdominal radiographs. A, This anteroposterior (AP) skull radiograph reveals the proximal intracranial portion of the shunt and a Rickham reservoir (arrow) over the burr hole. A companion non–contrast-enhanced head CT scan (see Fig. 59-14) is usually performed to further evaluate shunt function. B, The lateral skull radiograph better reveals a cylindrical Holter valve (long arrow) several centimeters distal to the Rickham reservoir (short arrow). C, This AP chest and abdominal radiograph reveals the distal portion of the shunt (arrow). Note the length of the catheter to allow patient growth. D, The lateral chest and abdominal view redemonstrates the course of the shunt.

Figure 59-16 Before and after series of a shunt malfunction. The left column (A-C) demonstrates a malfunctioning shunt. The right column (D-F) is the same patient after shunt revision. A, The shunt has become disconnected at the valve; no efferent catheter tubing is seen coming from the inferior portion of the valve (arrow). B, The catheter has migrated inferiorly and has coiled in the pelvis (arrow). C, Head computed tomography (CT) scan showing ventriculomegaly (arrow), a direct result of the shunt malfunction. D, After shunt revision, the lateral skull radiograph shows that a catheter is now connected to the valve (arrows, difficult to appreciate on this film). E, The catheter is seen coursing through the abdomen (arrows). F, Repeated head CT shows resolution of the ventriculomegaly.

If the distal catheter is in the peritoneum and a distal obstruction is suspected or if the patient complains of abdominal pain, abdominal ultrasound should be obtained. Ultrasound may reveal a pseudocyst at the distal portion of the catheter or an abnormal fluid collection.

Finally, CSF analysis is important whenever an assessment of infection or shunt function is undertaken. Shunt evaluation should include tapping the reservoir proximal to the distal valve to allow percutaneous testing of proximal shunt patency. Although this is useful in most instances, it can be difficult to assess the rate of inflow or runoff adequately with a small-bore needle, particularly if the patient is a child or is unable to cooperate. The potential for infection is a disadvantage of percutaneous tapping. Other invasive techniques include injection of either radionuclide or contrast material into the shunt as a marker of flow. If ventricular fluid pressure is low, there may be little evidence of flow, thus giving a false indication of shunt malfunction.

Various noninvasive techniques have been devised for assessing shunt function. Changes in visual evoked potentials associated with elevated ICP have been suggested as a means of determining function. Thermographic and Doppler detection of shunt flow is also possible. Magnetic resonance imaging techniques can likewise assess CSF flow and shunt patency.

Shunt Tapping

Clip the scalp hair over and around the reservoir, prepare the skin with a surgical scrub brush for 10 minutes, followed by the application of a povidone-iodine solution, and then allow it to fully dry. After appropriate draping, infiltrate the skin with 1% plain lidocaine to a level of adequate local anesthesia. Use a 25-gauge butterfly needle with tubing and enter the reservoir percutaneously at a 20- to 30-degree angle (Fig. 59-17). Lack of CSF flow from the reservoir indicates a proximal obstruction unless the ventricles are completely deflated (slit ventricles syndrome). Even so, a small amount of CSF should be obtained within the tubing, thus ensuring that entry into the lumen of the reservoir has been accomplished. If the ventricles are deflated and only a small amount of CSF is aspirated, hold the end of the tubing 5 to 10 cm below the level of the reservoir to see whether CSF will fill the tubing and eventually begin to drip at the rate of 2 to 3 drops/min. If CSF is readily aspirated from the reservoir, hold the tubing vertically to obtain an indication of intraventricular pressure. This pressure reading, as well as the ease with which CSF is aspirated, will give some indication of proximal obstruction. To assess runoff or patency of the distal end, apply pressure to the tubing proximal to the reservoir and then deflate the reservoir without any resistance. If any resistance is encountered when deflating the reservoir, suspect a distal obstruction.

Figure 59-17 Tapping a ventriculoperitoneal shunt. Use a 25-gauge butterfly needle to puncture the reservoir. To avoid damage to the reservoir, the angle should be approximately 20 to 30 degrees. Note that the dome reservoir is under the skin. Before passing the butterfly needle, the skin is anesthetized, sterilized with povidone-iodine, and nicked with a No. 11 scalpel blade or a larger needle. Fluid is not aspirated but is allowed to drain passively.

It is essential to send any aspirated CSF for laboratory analysis, including a cell count, protein, glucose, and most importantly, Gram stain and culture to evaluate the system for infection.

Special Considerations—Postoperative Shunt Complications

Major complications in the immediate postoperative period include hemorrhage, overdrainage, migration, obstruction, malpositioning, fractured tubing, infection, radiculopathy, and seizures.

Hemorrhage

Bleeding can occur after placement of a shunt. Both intracerebral and subdural hemorrhages have been noted. Intracerebral hemorrhage is due to trauma to the brain parenchyma as the catheter is passed through the ventricles. Subdural hematoma is due to sudden decompression of the ventricles, which leads to tearing of the bridging veins as the brain pulls away from the dura. Acute or chronic subdural bleeding can also be encountered in shunts with overdrainage problems.

Shunt Malfunction

The ventricles usually begin to diminish in size within a week after shunt placement in patients with high-pressure hydrocephalus, so continued enlargement suggests (but does not confirm) shunt malfunction. The shunt series images confirm the continuity of the drainage system from the insertion site through the proximal catheter, reservoir, and distal catheter to the receiving draining cavity, most commonly the peritoneum. It is important to know that with normal-pressure hydrocephalus, the ventricles may remain large despite good functional shunting. The timing and indications for shunt placement are important factors for the neurosurgeon when assessing the urgency of management.

Seizures

Seizures are an infrequent complication of shunt placement and occur in about 5% of patients. Technically, occipital catheter placement may lessen the incidence of seizures because this location is far from the motor cortex. In patients thought to be at risk for a seizure, antiepileptic medication may be indicated.

Shunt Infection—Treatment and Prevention

Of all the potential complications associated with shunting procedures, infection is the most notorious and occurs in 2% to 10% of cases.²⁶ Most shunt infections appear within the first 2 months after surgery. The diagnosis may be obvious in patients with variable systematic signs, wound infection, meningitis, peritonitis, or septicemia. It is also possible that the shunt may harbor an indolent infection without symptoms or signs. Culture of CSF obtained from a shunt tap may be negative even when the shunt is ultimately shown to be infected.^34,35

Most shunt infections are caused by otherwise nonvirulent bacteria such as S. epidermidis. In the presence of the shunt, these organisms exhibit unusual virulence. This reflects a variety of factors, including the ability to adhere to shunt surfaces, as well as production of mucoid substances that protect the bacteria from host defenses. The Silastic shunt material itself has an adverse effect on the immune system. Specifically, leukocytes cannot adhere to such surfaces as well as bacteria can. Controversy exists over the need for shunt removal in the treatment of infection. Although systemic antibiotics alone are of uncertain benefit, the addition of intraventricular drugs has allowed successful treatment without removal of a functioning shunt. Nevertheless, many neurosurgeons still recommend replacement of the entire shunt after the infection is eradicated.

If a shunt infection is suspected, systemic antibiotics should be given as soon as possible in the ED. Consultation with a neurosurgeon and infectious disease specialist is recommended before administering antibiotics directly into the CSF.