Hydrocephalus

David F. Long

GENERAL PRINCIPLES

Definition and Epidemiology

• Hydrocephalus is defined as “an active distension of the ventricular system of the brain related to inadequate passage of cerebrospinal fluid (CSF) from its point of production within the ventricular system to its point of absorption into the systemic circulation” [1].

• It is the most common treatable neurosurgical complication in traumatic brain injury (TBI) rehabilitation; occurs in up to 45% of severe TBI patients while in inpatient rehabilitation [2].

• Dynamic hydrocephalus can be difficult to distinguish from ex vacuo ventriculomegaly, a hallmark of severe diffuse brain injury [2,3].

Classification

• Communicating hydrocephalus—all ventricles are interconnected with free exit of CSF to subarachnoid space [1]; most posttraumatic hydrocephalus cases.

• Noncommunicating hydrocephalus—obstruction between ventricles or preventing outflow from ventricles [1]; consider aqueductal stenosis decompensated by TBI when lateral and third ventricles are large but fourth ventricle is small or normal; lumbar puncture is contraindicated [2].

Pathophysiology of Hydrocephalus

• The processes causing the development of hydrocephalus are complex. Hydrocephalus is not necessarily associated with increased pressure, because when ventricles enlarge, the expanding force is distributed over a larger area, reducing the pressure. Also, the size of the ventricles reflects the pressure within them relative to that of the surrounding tissues (just as the size of a balloon depends on the pressure inside compared with that outside) [2,3].

DIAGNOSIS

Clinical Presentation

• Risk factors—subarachnoid or intraventricular hemorrhage, meningitis, craniectomy [1,4].

• Acute hydrocephalus may present with signs of increased intracranial pressure, including headache, nausea, vomiting, lethargy, papilledema, bulging craniectomy flap, and Cushing’s triad (hypertension, bradycardia, and hypoventilation) [2].

• Normal-pressure hydrocephalus (NPH) may present with triad of gait “apraxia” (shuffling magnetic quality with reduced cadence, decreased step height, loss of counter-rotation), “subcortical” cognitive impairment, and urinary incontinence [5]. Of these three symptoms, gait impairment is the most indicative of hydrocephalus and the most likely to respond to shunting.

• Decompensated aqueductal stenosis may present with Parinaud’s/pretectal syndrome (loss of upgaze, lid retraction, impaired pupillary reactivity, convergence retraction nystagmus).

• Other presentations of hydrocephalus include akinetic mutism, bradykinesia, Parkinsonian syndrome, and nonspecific deterioration in neurological status [2,5].

Computerized Tomography and Magnetic Resonance Imaging

• Typical appearance—progressive ventriculomegaly with convex frontal horns, enlarged temporal horns, and third ventricle [2].

• Sulci are typically less prominent than ventricles, especially in high convexity, but the presence of enlarged sulci does not exclude hydrocephalus [2,5,6].

• Transependymal fluid may be present—smooth periventricular signal (MRI) or lucency (CT); predictive of a good response to shunting. Differential considerations—frontal contusions, cerebral infarctions, or demyelination—are usually more irregular and asymmetric [2].

Supplemental and Invasive Assessment

• CSF tap—clinical improvement, especially in gait, after removal of 40 to 50 mL of CSF by lumbar puncture predicts good response to shunting (a positive lumbar drainage trial was predictive of positive shunt response in 96% of patients in one study [7]), but low test sensitivity (26%–61%) means that a negative result cannot be used as an exclusionary test for hydrocephalus; many patients with a negative test can respond to shunting [8].

• Placement of a continuous external lumbar drain for 3 to 5 days, if available in a specialized center, is both more sensitive and specific for diagnosing NPH than CSF tap [2,8].

• Cisternography—does not add to accuracy of diagnosis of hydrocephalus [9].

TREATMENT

Initial Management

Surgical Options

• Ventriculoperitoneal shunt—CSF drains from ventricular catheter, out of skull through a burr hole, through a one-way valve. Catheter passes under the skin, terminating in the peritoneal space; the standard procedure for managing communicating hydrocephalus [2,10]

• Third ventriculostomy—a hole can be created between the floor of the third ventricle and the adjacent cistern to allow passage of CSF without requiring a shunt [2]. Particularly effective for aqueductal stenosis but also has been used in other types of hydrocephalus and in cases of shunt failure [11]

• Other shunt types—include ventriculoatrial, ventriculopleural (with intra-abdominal process or to get lower pressure), lumboperitoneal (communicating hydrocephalus only) [2,10]

Basic Shunt Concepts

• Shunts typically have a one-way valve, either with fixed setting or programmable with an external magnet; very large ventricles may need a particularly low-pressure valve setting [2].

• A palpable reservoir or pumping chamber will generate forward flow if there is a valve between it and the ventricle; if the valve is further downstream than the reservoir, pumping can generate retrograde flow into the ventricle, such as for intrathecal medication administration [2,12].

• Clinical improvement and reduction in ventricular size after shunting do not correlate well. Recent efforts with combined programmable and gravitational shunts seem to indicate that good clinical results can sometimes be obtained from shunting with little associated reduction in ventricular size [2].

Shunt Complications

Shunt Failure

• Incidence of shunt revision in adults is approximately 30% [2].

• Shunt failure symptoms include irritability, confusion, lethargy, headache, or acute neurologic change.

• Shunt palpation may show excessive resistance (distal occlusion) or inadequate refill (proximal obstruction). Unfortunately, one cannot determine with certainty whether a shunt is working by bedside palpation [12].

• Perform CT or MRI and look for increased ventricular size compared with prior scan.

• Distal shunt occlusion may also be associated with fluid loculation or pseudocyst on abdominal CT [13].

• Shuntogram is most definitive—A needle is inserted into a safely perforable part of shunt, pressure measured, and contrast or isotope injected and followed down into peritoneal space [10,14].

Shunt Infection

• Insidious presentation—low grade fever, malaise, irritability; erythema over shunt; 70% occur in the first 2 months after insertion; Staphylococcus epidermidis is most common; diagnosis is by shunt tap, not lumbar puncture (unreliable) [2].

• Treatment—intravenous antibiotics and either shunt removal or externalization [2].

• Prevention—antibiotic impregnated catheters have been shown to reduce overall incidence of shunt infection, but not for methicillin resistant Staphylococcus aureus (MRSA) or gram-negative infections [15].

Overdrainage

• Gravitational pressure of column of CSF between the valve and the distal end of the catheter frequently causes siphoning of CSF, resulting in overdrainage [2,10].

• Acute overdrainage symptoms can include orthostatic headache, dizziness, vomiting, lethargy, and diplopia. Chronic overdrainage causes slit ventricle syndrome with nonpostural headache and intermittent proximal shunt malfunction [2].

• Overdrainage predisposes to development of subdural hematomas and hygromas—CSF drainage from ventricles creates increased potential subdural space. Subdural collections occur in 4.5% to 28% of shunted patients and are more likely with very large ventricles before shunting [2].

Additional Considerations

Programmable Shunts—Concepts and Use

• These allow bedside adjustment of the opening pressure of shunt valves by use of an external magnet to prevent underdrainage (poor clinical response) or overdrainage [2,10,13,16].

• Cost effective (often avoids reoperation), and adjustments can improve clinical course [12,17].

• Inadvertent valve resetting by MRI, magnets, valve filliping, or transcranial magnetic stimulation may occur [2,18,19]; MRI scans are not contraindicated, but valve settings need to be rechecked after MRI; some valves require confirmatory x-ray to verify setting [2,18,19].

Antisiphon Devices and Gravitational Shunts

• Antisiphon devices are added to a shunt system in an attempt to prevent excessive CSF flow induced by siphoning [2,20]. Some of these work through a subcutaneous membrane; others have flow regulated or gravitational units.

• A gravitational unit in series with a programmable valve can decrease siphoning and allow adjustment with an external magnet [20]. The incidence of overdrainage at 6 months was significantly decreased with valve systems including a gravitational valve component compared with those without [21].

Specific Programmable Shunts

• Codman Hakim Programmable Valve—first in United States, multiple clinical trials, 18 settings at 10 mm increments from 30 to 200 mm H₂O; MRI can reset valve, and determination of setting requires x-ray [2,16,18]

• Medtronic PS Medical Strata Valve—five settings from 0.5 to 2.5 for opening pressures 15 to 170 mm H₂O; MRI can reset, but valve can be read and adjusted at bedside [2,18]

• Sophysa Programmable Valves—new Polaris model has five positions and locking mechanism designed to prevent changes in valve by MRI [18,20]

• Aesculap-Miethke proGAV programmable shunt system—combines a gravitational unit in series with a programmable valve with a brake to prevent inadvertent MRI valve change; settings can be checked at bedside without x-ray [20]

• Codman Certas Adjustable Valve—seven settings for opening pressures from 26 to 247 mm H₂O plus a “virtual off” setting for treating shunt associated subdural collections; designed to be resistant to MRI reprogramming even at 3 Tesla [22].

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