Pearls
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Hydrocephalus is a common brain disease characterized by a buildup of cerebrospinal fluid within the brain, causing elevated intracranial pressure. Untreated hydrocephalus can be a medical emergency, requiring prompt neurosurgical intervention.
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Arachnoid cysts are typically benign congenital malformations that are frequently identified incidentally. However, they may occasionally hemorrhage or cause symptomatic mass effect on the brain. They are more likely to grow and require treatment in the very young.
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Chiari malformations come in multiple types, the most common of which are type I and type II. Chiari I patients may experience headaches or develop a spinal cord syrinx that can cause neurologic dysfunction, both of which may resolve with surgery. Chiari II malformations occur in patients with myelomeningocele and may be associated with brainstem dysfunction.
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Dandy-Walker malformation is characterized by a posterior fossa cyst and cerebellar hypoplasia. It can be associated with hydrocephalus and a number of systemic abnormalities.
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Meningoceles and encephaloceles are characterized by herniation of meninges or the brain and meninges through a defect in the skull, which can be surgically repaired but often with significant long-term effects on development.
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Myelomeningocele is the most severe form of spinal dysraphism, in which failure of primary neurulation leads to herniation of the spinal cord through defects in the vertebrae, soft tissue, and skin. Exposure of neural tissue to amniotic fluid and air causes neurologic dysfunction; many patients with myelomeningocele also have some combination of hydrocephalus, spinal cord tethering, and Chiari II malformation.
Hydrocephalus
Background
Hydrocephalus is defined by a pathologic accumulation of cerebrospinal fluid (CSF) that leads to enlargement of the cerebral ventricles, mechanical deformation of brain structures, and increased intracranial pressure (ICP). The Monro-Kellie doctrine describes the relationship among the intracranial contents (brain, CSF, blood, and supportive tissues) in the fixed compartment of the skull (see also Chapter 63 ). A buildup of CSF puts pressure on the brain and impairs cerebral blood flow, resulting in brain injury. These changes can render the patient critically ill. A fundamental understanding of hydrocephalus is important for the clinician caring for these children in the intensive care setting.
Among children, the incidence of hydrocephalus is approximately 1:1000. In the United States, it accounts for approximately 40,000 hospital admissions and $2 billion in healthcare expenditures annually. Children with hydrocephalus often have comorbidities; the number of comorbidities has risen as improved care for affected children has improved their survival. These patients frequently require admission to an intensive care unit (ICU) for interventions related to their hydrocephalus and comorbid conditions.
Pathophysiology
CSF physiology and the pathophysiology of hydrocephalus are incompletely understood and remain an area of ongoing research. The prevailing hypothesis describes a bulk flow model, in which the CSF produced by the choroid plexus epithelium and the ependymal lining of the ventricles passes through the ventricular system, down the aqueduct of Sylvius, out the foramina of Luschka and Magendie, and into the spinal subarachnoid space. From there, the fluid continues through the subarachnoid spaces of the basal cisterns and the cerebral convexities until it reaches the arachnoid granulations, where it is reabsorbed into the venous system ( Fig. 59.1 ). According to this theory, obstruction at any point in this pathway leads to an accumulation of CSF proximal to the blockage. The term noncommunicating is used when there is an obstruction within the ventricular system; communicating hydrocephalus refers to obstruction at the level of the subarachnoid spaces, arachnoid granulations, or venous system. ,
The accumulation of CSF within the intracranial compartment is at first accommodated by compression of the venous structures and, to a lesser extent, the brain parenchyma. However, these compensatory mechanisms are quickly exhausted in the setting of progressive hydrocephalus, and ICP begins to rise. Although enlargement of the ventricles secondary to the accumulation of CSF does cause mechanical deformation of the adjacent brain, the principal concern is control of ICP. Numerous studies have shown a strong association between persistently elevated ICP and poor outcome in the setting of trauma. Intuitively, the same association holds true in the case of hydrocephalus. Therefore, prompt identification and treatment of intracranial hypertension is imperative.
Clinical presentation
The clinical manifestations of untreated hydrocephalus depend on the age of the patient. Early in the course of hydrocephalus, older children and adults will most commonly experience headaches. Nausea, vomiting, and somnolence are also common with progressive pressure elevation. Some patients develop an ataxic gait and visual symptoms, such as blurry vision. Parinaud syndrome or diplopia due to cranial nerve VI palsies may also be seen. Younger children and nonverbal adults may become unusually irritable or intolerant of feeding. Infants with open cranial sutures will demonstrate fullness of the fontanelle and splaying of the suture lines, potentially with apneic or bradycardic events that suggest more severe pressure changes.
Left untreated, these symptoms typically progress to frank lethargy, obtundation, and coma. Late in the course of the disease, bradycardia, hypertension, and abnormal breathing patterns may be seen. This triad, frequently referred to as the Cushing reflex, is a sign of dangerously elevated ICP and frequently heralds impending herniation. These signs are not seen universally, however; thus, their absence should not necessarily be reassuring in comatose patients. Conversely, patients with bradycardia who are awake and talking are likely not on the verge of herniation, even if they have a history of hydrocephalus.
Causes
In North America, the most commonly identified causes of hydrocephalus are intraventricular hemorrhage (IVH) of prematurity (25%), brain tumors (18%), and myelomeningocele (15%); idiopathic cases account for approximately one-quarter of the total. Postinfectious hydrocephalus is relatively rare in North America (under 4%) but accounts for as many as 60% of cases in sub-Saharan Africa. ,
Diagnosis
Clinical history and physical examination are helpful, but imaging studies are essential to establish the diagnosis. Newborns with IVH of prematurity, myelomeningocele, and other risk factors for hydrocephalus are frequently screened with cranial ultrasounds for ventricular enlargement. In borderline cases not requiring immediate intervention, serial imaging studies and head circumference measurements can be helpful in confirming the need for treatment. The presence of papilledema, a bulging fontanelle, or widely splayed sutures can help to corroborate imaging findings or symptoms concerning for intracranial hypertension and guide treatment decisions.
Management
The principal treatment for hydrocephalus is diversion of CSF. This goal can be achieved temporarily with an external ventricular drain (EVD) tunneled out through the scalp or permanently with an internalized CSF shunt. In selected patients, endoscopic third ventriculostomy (ETV) with or without choroid plexus cauterization may also be an alternative.
Children with acute hydrocephalus can be critically ill and may require emergent EVD placement, a procedure that can be rapidly performed at the ICU bedside. If the child is deteriorating and an EVD cannot be placed immediately, standard methods for controlling ICP should be employed: elevation of the head of the bed and neutral neck positioning to facilitate venous drainage, administration of analgesics and sedatives, mild hyperventilation with a target partial pressure of arterial carbon dioxide of 30 to 35 mm Hg, and hyperosmolar therapy with mannitol or hypertonic saline. Hypotension should be avoided to maintain cerebral perfusion. Because patients with acute hydrocephalus frequently have altered mental status, intubation may be required for airway protection.
To place an EVD, a burr hole is created through a small skin incision behind the hairline, and a catheter is then passed through the brain parenchyma into the lateral ventricle. The other end of the catheter is then tunneled out through the scalp and connected to a bedside collection system, which can be raised and lowered to control the amount of CSF drained. The typical reference point is the patient’s external acoustic meatus. These drains require close monitoring and frequent adjustments to maintain the proper height with changes in patient position; if they are not leveled appropriately, over- or underdrainage of CSF may result, with potentially serious consequences. When it is thought that the drain is no longer needed, many neurosurgeons “wean” the drain by raising the exit port progressively and ultimately clamping it. If this process is well tolerated without symptoms or imaging evidence of hydrocephalus, the drain is removed; if it is not, permanent CSF diversion may be required.
Permanent CSF diversion typically comes in one of two forms: a CSF shunt from the ventricular compartment to another body cavity where it can be reabsorbed or an ETV. The choice of intervention is dependent on a number of factors, most notably anatomy and etiology of the hydrocephalus.
CSF shunting systems are, in many cases, the standard treatment for patients requiring permanent CSF diversion. A shunt typically includes a ventricular catheter connected in series to a valve and a distal catheter ( Fig. 59.2 ). The distal catheter most commonly is tunneled subcutaneously to the peritoneum due to its high absorptive capacity. When the peritoneal cavity is not an option, most often due to related or unrelated abdominal pathology, some children require placement in the right cardiac atrium or pleural space. Shunted CSF is then reabsorbed within the distal compartment. Although shunt placement is lifesaving, these devices have the potential to fail, most commonly by occlusion or fracture. Patients with occluded shunts usually present with the typical symptoms of hydrocephalus as CSF accumulates, and imaging studies usually demonstrate enlargement of the ventricles ( Fig. 59.3 ). A subset of patients—usually those with complex neurosurgical histories—may have atypical presentations or ventricles that do not enlarge.
A failing shunt is a medical emergency requiring immediate neurosurgical intervention to avoid or resolve ICP crisis. Infection of the shunt hardware can also occur, most commonly due to seeding at the time of surgery. Patients generally present with fever and meningeal symptoms; shunt failure may also be present, and drainage of infectious material into the abdomen may lead to the formation of a pseudocyst or frank peritonitis. If untreated, the infection may progress to ventriculitis, which carries a poor prognosis. Treatment requires surgical removal of the shunt hardware and placement of a temporary EVD until the infection is cleared. After a prolonged course of antibiotics (up to 2 weeks for some organisms), the shunt may be replaced if CSF cultures remain negative. On average, children with a history of shunt infection have a significantly lower IQ than controls without infection. Fortunately, application of a standardized protocol has been shown to substantially reduce the rate of shunt infection to approximately 5%.
Certain types of obstructive hydrocephalus may be amenable to endoscopic intervention, essentially creating a bypass for CSF around the obstruction. Loculated hydrocephalus, in which one ventricle is enlarged, may be amenable to treatment by endoscopic fenestration of the septum pellucidum to reestablish flow. Obstructive hydrocephalus at the level of the cerebral aqueduct or fourth ventricle, as in aqueductal stenosis or posterior fossa tumors, may be treated by ETV, in which a perforation is made in the floor of the third ventricle. Recent clinical studies on the efficacy and indications for ETV have resulted in the establishment of a metric called the ETV success score, which helps guide the clinician in selection of CSF diversion and counseling patients on the likelihood of success with ETV.
In addition to ETV, a more recent addition to the arsenal of treatments for hydrocephalus is ETV with choroid plexus cauterization (ETV-CPC). This approach employs an ETV performed in the standard fashion, coupled with extensive cauterization of the choroid plexus in the bilateral lateral ventricles. This technique is targeted at the infantile hydrocephalus population, most commonly posthemorrhagic hydrocephalus. It has been employed in several studies with the stated goal of reducing CSF production. Although the success rate in most studies hovers around 40%, it provides an opportunity for shunt freedom for a subset of patients if the risks are deemed tolerable by the provider and family. The long-term efficacy and complication rates related to ETV-CPC remain to be determined, as studies are ongoing.
Arachnoid cysts
Background
Arachnoid cysts are congenital anomalies filled with fluid similar or identical to CSF and lined by thin membranes of arachnoid ( Fig. 59.4 ). The incidence of arachnoid cysts is 0.5% to 2.5% and is more common in males. Although arachnoid cysts can occur anywhere along the neuraxis, they are most commonly found in association with an arachnoid cistern, and approximately half are located in the Sylvian fissure. ,
It was previously thought that arachnoid cysts were related to agenesis of the adjacent brain region; however, modern radiographic techniques have clarified that there is no difference in brain volume between the side with an arachnoid cyst and the side without. Instead, the brain is displaced and, in some cases, the overlying skull is expanded. ,
Clinical presentation
Generally asymptomatic, most arachnoid cysts are found incidentally and do not change in size. However, a small subset can progressively enlarge and have a mass effect on adjacent structures. In these cases, the effects are generally referable to the location of the lesion, and symptoms of intracranial hypertension may be seen. If the CSF pathway is involved, hydrocephalus can result. In the special case of suprasellar arachnoid cysts, endocrinopathy, visual impairment, and hydrocephalus may result. , Growth of arachnoid cysts is typically seen only in the very young; in one series of more than 300 cases, no patient over the age of 4 years experienced cyst enlargement or required surgical intervention. ,
Rarely, children may present with hemorrhage, either into the cyst itself or into the overlying subdural space. As with any intracranial hemorrhage, these cases represent a medical emergency and require prompt neurosurgical attention. Surgery may be required to evacuate the hematoma and decompress the brain.
Diagnosis
When a suspected arachnoid cyst is identified, definitive imaging with contrast-enhanced magnetic resonance imaging (MRI) should be performed to rule out other pathology, such as a tumor with a cystic component. If the diagnosis of arachnoid cyst is confirmed, close clinical and radiographic follow-up is warranted for young children because of the increased risk of enlargement among this age group. As the patient ages, the need for serial imaging studies decreases. When arachnoid cysts are identified in teenagers and young adults, a single stable follow-up image is often sufficient for asymptomatic lesions.
Management
Surgical intervention for arachnoid cysts is reserved for cases with progressive growth or concerning mass effect. Asymptomatic small lesions that do not enlarge are generally left untreated, regardless of their location and size. When an operation is indicated, fenestration into an adjacent cistern is often performed, followed by placement of a cyst-to-peritoneal shunt if this is unsuccessful.
The development of minimally invasive endoscopic techniques for use in the brain has made treatment of arachnoid cysts less invasive and has raised some controversy over optimal approaches for different lesions. For example, sellar/suprasellar arachnoid cysts in the past have required an open craniotomy for fenestration. Today, a transventricular or transnasal endoscopic approach can be employed. Numerous retrospective reports on the subject have shown variability in safety and efficacy of each approach. At the present time, the optimal treatment approach is surgeon-dependent.
Chiari malformations
Chiari malformation is defined by a spectrum of brain compression at the level of the posterior fossa and foramen magnum. Many types of Chiari malformations have been described by anatomists and clinicians, but two are most commonly seen in the pediatric critical care environment.
Chiari I malformation
Chiari I malformation is defined by the presence of cerebellar tonsillar ectopia with the inferior tip of the tonsils at least 5 mm below the level of the foramen magnum ( Fig. 59.5 ). The low-lying tonsils can interfere with the normal flow of CSF across the foramen magnum or cause neurologic compression of the cervicomedullary junction. The severity of the malformation dictates the symptomatology, ranging from suboccipital headaches (often induced by coughing or Valsalva maneuver) to symptoms of brainstem compression, including central sleep apnea. In particularly severe cases, compression of the brainstem by the Chiari I malformation can cause lower cranial nerve dysfunction. These patients may experience dysphagia, dysarthria, or tinnitus. Headaches that are not posterior and not elicited by provocative maneuvers are unlikely to be due to the Chiari malformation.
