Slit Ventricle Syndrome
Slit ventricle syndrome (SVS) is a complication that occurs after cerebrospinal fluid shunting with a ventriculoperitoneal (VP) shunt. SVS is a poorly defined syndrome characterized by cerebrospinal fluid shunt-related symptoms in the setting of small ventricles on neuroimaging.
Epidemiology & Risk Factors
SVS is a rare syndrome seen in 3-5% of patients with a VP shunt . The most common cause of ventricular shunt placement in an infant is hydrocephalus. Causes of hydrocephalus include aqueduct stenosis, intraventricular hemorrhage, cerebral infections (e.g., cysticercosis), and communicating hydrocephalus . Symptoms of SVS usually appear 2-5 years after shunt placement. The average time between diagnosis of hydrocephalus and SVS is 4.3 years . Peak incidence is between ages 4-6 . Risk factors include small head circumference under 25th percentile . SVS can be seen concurrently with secondary craniosynostosis due to skull growth derangements . SVS can also occur in adults after VPS.
Under normal conditions, the law of Laplace states that tension (T) equals the product of pressure (P) and radius, R. Unfortunately, with multiple cycles of ventricular enlargement and decompression with a VPS, the ventricle might lose compliance and fail to enlarge with increased ICP. In these cases, the amount of pressure (P) necessary to increase the radius, (R) is higher because the ventricle is collapsed. Thus, the law of Laplace suggests that patients with SVS may have shunt overdrainage or underdrainage.
Pathogenesis of SVS is incompletely understood. Several theories have been mentioned in literature .
- Overdrainage of CSF can cause collapse of ventricles, causing intermittent shunt obstruction and cerebral hypotension. On computerized tomography (CT) imaging, slit-like ventricles can be seen.
- Chronic CSF shunting can cause reactionary subependymal gliosis, creating proximal obstruction of the ventricles and subsequent collapse.
- Craniocephalic disproportion occurs when the brain grows out of proportion to intracranial volume. This is due to early shunting through the VP shunt and overriding of the cranial sutures. In this pathogenesis, shunt function is maintained.
- Patients with SVS may develop intracranial hypertension or hypotension in the setting of an under-functioning shunt or overdrainage from an overfunctioning shunt.
Hallmark SVS symptoms include headache, nausea, vomiting, and altered mental status however some patients may be asymptomatic. The clinical picture may be consistent with shunt dysfunction or shunt overdrainage. Headaches are typically episodic and last 10-15 minutes with associated vomiting or hyperventilation. Vital sign changes of bradycardia and hypertension may be present . Headaches in SVS patients can be associated with low or high intracranial pressure (ICP). Headaches in the setting of low ICP are exacerbated when the patient is upright and alleviated when lying down. These headaches are similar in character to post-lumbar puncture headaches and may be associated with valve failure . Headaches in the setting of high ICP tend to be worse on reclining, occur in the mornings, and can progress to all day headaches. These headaches are associated with shunt failure without ventricular enlargement or craniocephalic disproportion in the setting of a working shunt.
Complications of SVS include subdural hematoma, hypotension, intracerebral hemorrhage, and inversion of the cortical mantle . Patients with shunt malfunction may have visual loss (due to papilledema) or diplopia (secondary to non-localizing sixth nerve palsy in the setting of increased ICP). Patients with hydrocephalus may develop symptoms and signs of the dorsal midbrain syndrome from increased ICP at the level of the cerebral aqueduct.
Ocular manifestations of SVS are congruent with ocular manifestations of hydrocephalus, which can include visual loss, ocular motility abnormalities, pupil abnormalities, optic atrophy, papilledema, and cortical blindness. Although not a sensitive sign, papilledema is one of the cardinal ophthalmologic signs of shunt failure . Optic disk pallor can be a sign of chronically elevated intracranial pressure with post papilledema optic atrophy. Once a patient has optic disc pallor (a sign of optic atrophy), the disc is unable to swell, and papilledema may not manifest thereafter even with elevated intracranial pressure. Therefore, a flat optic disk does not rule out shunt malfunction.
There have been reported instances in which SVS presents solely with visual symptoms. In case reports, visual symptoms included decreased visual acuity, color vision, and visual field . The condition can progress with intermittent horizontal diplopia, atrophic papilledema, and optic atrophy. Systemic symptoms of increased ICP may not be present or may be very minimal (for example, manifesting as headache but no nausea, vomiting, lethargy, or altered mental status). Because magnetic resonance imaging of the brain did not show ventriculomegaly in SVS, ophthalmic findings were initially overlooked or misinterpreted. The lack of systemic symptoms of elevated ICP can cause the diagnosis of shunt failure to be considerably delayed. This delay in intervention can cause permanent vision loss . Thus, ophthalmologists and neurologists should be aware that visual loss may be the initial and sole presenting sign of ventricular shunt failure. For shunt- dependent patients, consideration should be given for periodic ophthalmologic examinations for signs of shunt failure.
During a symptomatic episode an immediate CT of the brain and shunt series should be performed. CT may show small ‘slit’ ventricles and an intact shunt system. If the shunt contains a pumping device, the valve will stay compressed on assessment of the shunt. Diagnosis is usually made based on clinical picture and imaging findings. Lumbar puncture can also be performed to assess the intracranial pressure. A functional shunt study (e.g., technetium scan) or a contrast injection into the cerebrospinal space can monitoring flow through the CSF system, and can identify the location of shunt blockage if present. A blocked shunt will have no contrast entering the ventricular system; or if injected into the shunt, the contrast will not appear in the peritoneal cavity . Patients may also demonstrate partial shunt obstruction, slow flow, or postural dependent flow.
Medical therapy is targeted at treating symptoms of SVS, though even with treatment, symptoms may be refractory. In cases of overdrainage and SVS, a programmable valve VPS can undergo reprogramming to reduce flow. In patients with shunt malfunction and SVS, the VPS may need to be replaced or revised.
Treatment is difficult because attacks can be severe but spontaneously subside and can be self limiting in some patients. In patients with subacute or chronic headaches and intact shunt function, antimigraine medications can decrease the frequency and intensity of headaches. Such medications include cyproheptadine or beta blockers. These medications target intracranial vessels to stabilize and decrease vasodilation . Corticosteroids can be considered for symptomatic SVS episodes to decrease intracranial pressure . Acetazolamide can be used as a temporizing measure to reduce CSF production.
Surgical treatment alternatives target increasing CSF drainage or increasing volume of the calvarium. SVS was traditionally managed with subtemporal decompression, however recurrence rate post procedure was high . Treatment of SVS with endoscopic third ventriculostomy (ETV) +/- shunt removal is considered more effective and decreases future complications. ETV should be considered in patients with non-communicating (obstructive) hydrocephalus, blocked shunt, or SVS. In this procedure, a small diameter fiberscope with a working channel is used to pass along the ventricular catheter into the slit ventricle. An opening in the floor of the third ventricle is created to allow CSF to escape . In patients with slit-like ventricles and over drainage, ventricular dilation should be considered prior to ETV .
Prognosis & Outcomes
One study reported an 82.7% success rate of endoscopic surgery in SVS . In multiple studies, a majority of patients became symptom free following surgical intervention, denying headache, nausea, vomiting, or lethargy . Surgical complications include VP shunt infection and interventricular hemorrhage. Prognostic factors in SVS patients include patient age at shunt placement (younger patients are at higher risk), cause of hydrocephalus, and mental status .
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