Neurosurgical Conditions

Hydrocephalus

The mention of pediatric neurosurgery to most health care professionals in a children’s hospital setting conjures up images of infants and children with hydrocephalus and shunts. At any one time, the inpatient census of a pediatric neurosurgical practice is likely to have a substantial proportion of children with hydrocephalus. For this simple reason, the topic merits discussion. Hydrocephalus also serves as a nice platform for the discussion of neuroanatomy and intracranial pressure (ICP) as foundation information for other topics later in this chapter.

Cerebrospinal fluid (CSF) is produced constantly. Although the volume produced is likely proportional to the size of the brain and the child and hence less in infants compared with older children, the volume is substantial regardless of age or size. By the age of 5 years, the brain has already achieved 90% of the adult size. In the full-sized or near full-sized brain, CSF production averages approximately 20 mL/h or 480 mL/day. A very few specific disease states can increase or decrease the amount of CSF produced. Under normal circumstances, the majority of CSF is produced by the choroid plexus within the ventricular system. The brain, however, can still produce CSF via bulk flow. This bulk flow is thought to be extracellular fluid within the brain parenchyma that moves centripetally toward the ventricular system. Hence, removing the choroid plexus cannot eliminate or even reduce CSF production. An adult human with normal neuroanatomy harbors approximately 150 mL of CSF within the ventricles and intraspinal and intracranial subarachnoid spaces. Hence, daily production is more than three times the static volume of CSF. The absorption of CSF is a pressure-dependent process as the CSF crosses from the subarachnoid space into the venous system via the arachnoid granulations. The failure to circulate or absorb even a small percentage of the CSF produced can cause problems of ventricular distention and/or raised ICP. An obstruction within the ventricular system, its outlets, or the subarachnoid cisterns or a failure of absorption at normal CSF pressures into the venous system can lead to ventricular distention and/or enlarged subarachnoid spaces. Likewise, elevated central venous pressures that can be seen in some congenital heart conditions, as well as jugular venous obstruction, can lead to a compensatory rise in ICPs.

Causes of hydrocephalus can be divided into congenital and acquired. Common congenital causes of hydrocephalus are congenital aqueductal stenosis and in association with myelomeningoceles. Acquired causes are intraventricular hemorrhage most often in association with prematurity, post-meningitis or post-traumatic status, and from brain tumors.

An infant with raised ICP may present with a history of irritability, emesis, poor feeding, and lethargy. In addition to these the child may present with complaints of headache, visual disturbance, loss of developmental milestones, impaired academic performance, and clumsiness. Hydrocephalus is rarely a cause of seizures, and a seizure in a child with raised ICP may actually be posturing (flexor or extensor) from impending herniation and not an electrical seizure. Posturing from raised ICP is an urgent matter. Although seizure disorders commonly coexist in the child with hydrocephalus, they are generally regarded as not being caused by hydrocephalus. Hence, it is important to get a very clear history as to what was being called a “seizure” in these children.

On examination of the infant, the occipital-frontal circumference (OFC), character of the fontanelle, separation of the cranial sutures, scalp vein distention, eye position, heart rate, blood pressure, and respiratory rate are important factors to assess. As a rule of thumb in the neonatal setting, OFC growth greater than a centimeter per week serves as a possible indication of raised ICP in the neonate. Plotting where the child’s OFC is on a growth chart and corrected for prematurity as necessary can be helpful. In the infant with hydrocephalus, the fontanelle may not be “tight” or “bulging” as the compliant skull readily gives way to elevated ICP. That is why palpation of the cranial sutures, particularly the coronal suture, is important to assess for separation of the frontal and parietal bones as a sign of raised ICP. Prominence of scalp veins can serve as a possible indication of elevated ICP. “Sun-setting” eyes with lid retraction is a downward deviation of the eyes with impaired upgaze ( Fig. 19.1 ). The upper sclera becomes unnaturally visible and is a sign of elevated ICP or midbrain abnormality. Esotropia, medial deviation of one or both eyes because of weakness in the abductor muscle supplied by the sixth cranial nerve, also can be a sign of raised ICP. Elevated ICP can cause vital sign changes referred to as Cushing’s triad of bradycardia, hypertension, and irregular respirations. In the monitored setting, trends of any one vital sign such as a slowing heart rate, an increasing blood pressure, or a reduced respiratory rate can signal a possible elevation in ICP. The whole triad does not need to be present when ICP is elevated.

A more in-depth examination is possible in an older child. With the exception of the fontanelle and cranial suture examination, many of the same signs are looked for when examining a child beyond infancy. In addition, a funduscopic examination may be possible to look for papilledema or optic pallor. If the child is cooperative, tests of memory, coordination, balance, and gait are helpful in trying to detect impaired brain function from elevated ICP.

Cranial imaging of the child with possible hydrocephalus can involve plain radiographs, ultrasound (US), computerized tomography (CT), and magnetic resonance imaging (MRI) ( Fig. 19.2A ). Plain radiographs are not recommended but may show the effects of chronically elevated ICP on the inner table of the skull causing a “copper beaten” appearance. An open fontanel is required for cranial US. The US can nicely show the lateral ventricles, but extra-axial spaces and the posterior fossa contents as well as fourth ventricle are usually incompletely visualized at best. However, the US can be performed at the bedside and without the need for transport or sedation. A cranial CT gives more information with very good visualization of all ventricles, the extra-axial spaces, and skull anatomy. Although a CT is fast and rarely requires sedation of the child, radiation is a necessary component of the examination. MRI provides much more exquisite detail of the brain anatomy but shows the skull anatomy and shunt components less well than CT. MRI sequences that take only a few minutes have been developed to simply assess the ventricular anatomy in the setting of known hydrocephalus to avoid or minimize sedation ; however, there remains a higher likelihood of the need for sedation and reduced patient access when they are in the bore of the magnet compared with CT.

The endoscopic approach to the third ventricle with or without choroid plexus coagulation has been used with greater regularity in recent years and serves a role in very select patient populations, though the mainstay of treatment for hydrocephalus remains the ventriculoperitoneal shunt (VPS) (see Fig. 19.2B ). It is not within the scope of this chapter to discuss the indications for endoscopic techniques versus shunt placement. However, there are shunt types other than the VPS. A CSF shunt is a catheter placed in an intracranial or intraspinal CSF space that allows the CSF to be shunted outside the central nervous system to be absorbed elsewhere. These terminal locations include the peritoneum, pleural space, venous system (right atrium–caval junction), gallbladder, iliac crest, and subgaleal space. The expertise of a pediatric general surgeon may be necessary or preferred for access of some of these terminal locations for the shunt catheter. Placement of an atrial shunt deserves mention in that pediatric general surgeons are now quite commonly involved in the placement of the distal end. The desired terminus should reside at the superior vena caval–atrial junction. Hence, some sort of intraoperative guidance is helpful. On an anteroposterior view of the chest, this target location is near the T4 vertebral body level as an estimate. Other techniques involve injection of contrast under fluoroscopy and visualization via transesophageal echocardiogram.

A common clinical scenario is the timing of gastrostomy and VPS placement in the premature intraventricular hemorrhage neonatal intensive care unit population as well as some children with malignant tumors in whom either the neurologic sequela or the chemotherapy impede adequate nutritional intake. Published studies have been inconsistent at defining the risks of either gastrostomy causing a shunt infection or the increased risk of shunt infection in the setting of gastrostomy. In practice, though, it is a not an uncommon experience in which an abdominal procedure is related to infecting an existing VPS. For that reason, thoughtful effort and planning should be instituted to separate VP shunt and gastrostomy interventions in time and space as much as possible.

Although most neurosurgeons place the peritoneal end without general surgical assistance by open, percutaneous, or even laparoscopic techniques, general surgical expertise may be requested for peritoneal access or assistance in certain situations. These situations include extreme obesity, revision surgery in which discerning bowel from thickened and scarred peritoneum is difficult or intraperitoneal adhesions prevent an adequate space for distal catheter placement, suspected bowel injury, or removal of intraperitoneal catheters that are either fractured and free floating or intensely scarred to intraperitoneal structures often in the setting of infection. Migration of distal catheters through the bowel wall has almost disappeared as a complication since abandoning use of catheters reinforced with a spiral wire that made them particularly stiff. In this setting, the catheter is usually easily removed and no direct bowel wall repair is necessary.

Neurosurgical Devices

The previous topic touched on VPS, which is the most frequently implanted permanent device by pediatric neurosurgeons; however, other common devices include intrathecal baclofen pumps, vagal nerve stimulators (VNS), and deep brain stimulators (DBS). Because all of these devices reside or course across the neck, chest wall, or abdominal wall in children who often have medically complex problems, an understanding of their components is important to avoid device damage and adverse clinical events.

The intrathecal baclofen pump is a programmable pump that delivers baclofen, a γ-aminobutyric acid (GABA) agonist, directly into the CSF. This is used in children primarily with quadriparetic spasticity with the goal of occasionally increasing function, but more often to make the care of the child easier and reduce discomfort. The pump is about the size of a hockey puck and is most commonly implanted either in the subcutaneous or subfascial compartment of the anterior abdominal wall ( Fig. 19.3 ). The incisions used vary by neurosurgeon. Most of these children are malnourished, and the sizeable device is easily palpable and often visibly protuberant. The catheter then runs laterally around the flank to enter the spinal subarachnoid space, which is accessed through a posterior midline incision. Although it is often intuitive to know which side the tubing courses around the flank from where the pump is placed, some neurosurgeons place these devices in the midline of the abdominal wall. Therefore, reviewing previous radiographs before any planned abdominal, retroperitoneal, or spinal intervention is important to avoid catheter damage. Baclofen withdrawal is a life-threatening condition marked by hyperpyrexia, seizures, cardiovascular collapse, and coma.

A VNS consists of the generator and electrodes. These devices are used in patients with medically refractory epilepsy in whom a surgical resection is not thought to be helpful. The electrodes are always placed on the left vagus nerve via a dissection of the carotid sheath within the anterior neck ( Fig. 19.4 ). Placement of these electrodes on the right vagus nerve is associated with cardiac side effects related to vagal stimulation and is not done. The generator often resides in the subcutaneous space below the ipsilateral clavicle. Rarely some conditions may warrant placement of the generator elsewhere. The electrodes and the insulation are quite fragile. In these children, central venous access should always be right sided if needed, contralateral to the generator and the electrodes to avoid damage or infection around the device.

The DBS is similar to the VNS except that the electrodes travel retroauricular to enter the brain via a frontal approach. These devices can be placed unilaterally, on either side, or bilaterally. The purpose of these devices is to treat severe movement disorders, most often dystonia. Since the generators often reside in the subcutaneous subclavicular space with electrodes coursing superiorly to the cranium, any interventions in the subclavicular or anterior neck should be performed on the contralateral side if possible. If this is not possible, radiographs should be obtained to be certain of the electrode course and all efforts made to avoid damage or contamination.

Skull Masses

It is not uncommon for a pediatric surgeon to be asked to excise lumps or bumps on the skull of a young child. Most of these masses will be dermoid cysts. A general rule is not to touch anything in the midline without intracranial imaging or neurosurgical input. Dermoid cysts can extend intracranially and intradurally. They are nontender, firm, rubbery lesions to palpation that inevitably enlarge. Rupture and drainage through a sinus tract are possible in all locations ( Fig. 19.5 ). Often diagnosed in the infant, they also can manifest later in life. Midline lesions are associated with the higher likelihood of intracranial extension, particularly in the occipital region. Anterior fontanel dermoids can be adherent to the dura of the sagittal sinus, and respect for this anatomy is paramount when resecting these lesions.

Langerhans cell histiocytosis (LCH) is second in frequency. Solitary LCH is usually present after infancy, and the lesions are characteristically tender and mushy to palpation. They are associated with bony destruction. They can have a far-ranging natural history from spontaneous regression of a single skull lesion to being associated with infantile disseminated progressive multiorgan disease (Letterer-Siwe disease). For isolated skull lesions, curettage is often curative. Larger lesions can be associated with dural erosion and significant vascularity. Involvement of a pediatric oncologist is important with LCH to help exclude the more severe varieties of multisite disease and to guide adjuvant therapy.

Atretic encephaloceles can be found in the midline in the region of the vertex. They are often associated with differences in hair density and quality of the involved skin and tend to remain small. A skull defect, though often mechanically inconsequential, is always present. Associated intracranial venous anomalies also can exist. Bothersome lesions that are either tender or bulky can be excised. The importance is not to mistake a dermoid with intracranial extension for an atretic encephalocele. An incompletely excised dermoid cyst will recur. All of these are elective conditions that are best handled by a pediatric neurosurgeon.

Neural Tube Defects

Defects in closure of the neural tube during development can be divided into two broad categories: myelomeningoceles and encephaloceles. Meningoceles differ from myelomeningoceles in that the spinal cord is not involved in a meningocele ( Fig. 19.6 ). Meningoceles are not commonly encountered. Although myelomeningoceles, also commonly referred to as spina bifida, can involve any level of the spinal cord, the lumbar region is the most common. Encephaloceles are commonly categorized as either anterior or posterior ( Fig. 19.7 ).

Myelomeningoceles represent a failure of primary neurulation. Neurulation is the process of the two-dimensional embryonic nervous system rolling or folding into a three-dimensional tube or cylindrical structure. The neural tube usually completes closure at approximately day 28 of gestation. When the tube fails to completely close at a spinal level, the anatomic result can be a myelomeningocele ( Fig. 19.8 ). The causes of myelomeningoceles are poorly understood, but folic acid in the diet of the mother prior to conception reduces their incidence. Genetic and teratogenic influences likely play roles as well.

With the exception of cervical myelomeningoceles, the site of the defect dictates the level of neurologic function. With cervical myelomeningoceles, there remains spinal cord function caudal to the lesion. On the other hand, a baby with an upper lumbar myelomeningocele (at approximately L2) may be expected to have hip flexor function (iliopsoas, L1-2) and impaired bowel and bladder function (S2-4) but no knee extension (quadriceps, L3-4) or motion at the ankle or toes (L5–S2).

The defect in the spine leads to more than just functional problems. The entire neural axis is affected both during and after development. The egress of CSF into the spinal defect allows mechanical changes to occur more rostrally, resulting in descent of the posterior fossa structures (both brain stem and cerebellum) into the cervical canal, which is known as the Chiari type II malformation. The anatomic abnormalities that lead to the Chiari II malformation occur almost exclusively in children with a myelomeningocele. (The Chiari I malformation will be discussed later.) Lower cranial nerve dysfunction can be life threatening early in the life of these children and can occur due to maldevelopment of the brain stem or compression of the medulla as hydrocephalus transmits further pressure into the Chiari II anomaly. Also, the supratentorial brain is not immune from injury, and hydrocephalus, polymicrogyria, enlarged thalamic adhesion, beaked midbrain tectum, and interdigitated falx are often seen. Approximately one-third of these children have below-normal intelligence.

The neonatal management is straightforward. The defect should be kept covered with a sterile, moist dressing. Prophylactic antibiotics are often administered following birth and continued through the postoperative period. Surgical closure is not an emergency but should be performed in the first 72 hours of life to reduce the incidence of infection. The treatment of any hydrocephalus in regard to timing and technique varies greatly by surgeon and institution. Relative indications for hydrocephalus treatment are progressive macrocephaly, an occipital frontal circumference growing greater than 1 cm per week, a bulging fontanel, split cranial sutures, downward gaze deviation, lower cranial nerve dysfunction, or CSF leak from the closure site. Ventricular size by itself is not a good determinant of the need for a VPS in this population because infants with small ventricles may require VPS and infants with large ventricles can sometimes be safely observed.

Prenatal closure has resulted in a significant reduction in the development of the Chiari II malformation and hydrocephalus. However, it should be noted that approximately half of these infants still have hydrocephalus that requires treatment. Because of the tremendous amount of effort to which the mother subjects herself in undergoing a prenatal closure, there is often unreasonable, though not unexpected, resistance to agreeing to postnatal surgical treatment of the hydrocephalus. Placing a VPS or performing an endoscopic procedure unfortunately is interpreted as a “failure” of the prenatal intervention.

Because of this patient population’s almost universal problem with bowel and bladder control secondary to the caudal spinal cord dysfunction, urologic procedures are commonplace. When entering the abdominal cavity of a child with spina bifida, whether to deal with the peritoneal end of a VPS or treat a primary intra-abdominal process, the surgeon should remember the possibility of preexisting conduits such as a Mitrofanoff stoma that may be hidden within the umbilicus.

Encephaloceles represent problems of neural tube closure at the more rostral end of the neuroaxis. Anterior encephaloceles are more common on the Asian and African continents. Posterior encephaloceles are more common in the Western world (see Fig. 19.7 ). Most encephaloceles are dramatically obvious on clinical examination; however, some anterior encephaloceles involving the skull base are difficult to see. Encephaloceles that herniate through the anterior skull base into the nasopharynx can manifest with obligate mouth breathing. There is often hypertelorism, though it may be subtle. The critical point is not to biopsy a nasopharyngeal mass without prior imaging. A simple transnasal biopsy of a skull base encephalocele can cause a CSF fistula into the nasopharynx followed by bacterial meningitis. Moreover, the more common posterior encephaloceles have a very high incidence of brain developmental problems and hydrocephalus. The child’s neurodevelopmental outcome tends to correlate with the amount of brain tissue in the encephalocele. The treating neurosurgeon also needs to be aware of critical vascular structures that may be present in the encephalocele sac before undertaking repair.

Craniosynostosis

Skull shape abnormalities or perceived abnormalities in the neonate or infant are very common reasons for pediatric neurosurgical consultation. Only in the setting of multiple fused sutures and life-threatening increased ICP is there ever an urgency for treatment. However, early diagnosis is important because the treatment options diminish with age. Beyond a certain age, the quality of outcome diminishes depending on the type of craniosynostosis.

Skull shape abnormalities also can be acquired in the infant. The most common acquired skull shape abnormality is labeled as “positional molding,” “positional plagiocephaly,” or “posterior plagiocephaly.” This acquired deformation is best understood by considering the neonatal skull as a parallelogram with all corners being hinges. Compression and flattening of one posterior side leads to advancement or protrusion of the ipsilateral anterior side of the cranial vault and base.

Whether the skull shape abnormality is truly craniosynostosis or acquired, the diagnosis and differentiation is best done by a physician expert in pediatric craniofacial abnormalities. Because some forms of craniosynostosis are best treated early in infancy, immediate referral to a specialist is important when craniosynostosis is suspected. The need for radiographic confirmation or refutation of a diagnosis is rarely necessary. In fact, imaging studies can sometimes introduce more confusion than clarity in cases of posterior flattening. Imaging studies are reserved for operative planning and assessment of intracranial abnormalities such as hydrocephalus, signs of raised ICP, and Chiari malformations.

Although somewhat of a simplification, a way to understand head shape abnormalities with craniosynostosis is based on principles put forth by Virchow. This principle is that bone growth occurs perpendicular to the normal cranial suture as the child’s head enlarges during the period of rapid brain growth in infancy. When a suture is “fused,” bone growth cannot occur efficiently perpendicular to the fused suture. Therefore, compensatory growth occurs perpendicular to the sutures remaining open. For example, when fused, the midline sagittal suture that separates the two parietal bones does not allow for normal biparietal widening. The open coronal and lambdoid sutures are sites of bone growth perpendicular to those respective sutures. Hence, the outcome is a head shape narrowed in the biparietal diameter, but elongated in the anteroposterior dimension. This head shape is referred to as scaphocephaly caused by sagittal synostosis ( Fig. 19.9 ). This entity is best evaluated earlier in infancy so that all surgical options can be considered because the less invasive techniques are not as effective later in infancy.

The other midline suture is the metopic suture that separates the two frontal bones. It runs from the anterior fontanel to the nasofrontal suture (between the eyebrows). This is the first suture to close normally. Ridging of the suture in infants is quite common because of this physiologic early closure. This ridging can be both palpable and visible. Simple ridging of the metopic suture can be considered a normal variant with no need for any workup or intervention. However, when the suture fuses too early (prenatally), the resulting deformity can lead to trigonocephaly. Trigonocephaly is a triangulated head when viewed from above. The forehead is pointed along with hypotelorism, and there is compensatory biparietal bossing. This type of craniosynostosis can cosmetically improve with time, unlike other forms of craniosynostosis. When it is severe, however, operative intervention is required to correct the deformity.

Either one or both coronal sutures can prematurely fuse. In the setting of unilateral coronal synostosis, the deformity creates significant asymmetry to the skull, orbits, and face. The fused coronal suture prevents advancement of the forehead on the involved side and elevates the sphenoid wing of the skull base. The forehead appears swept back and elevated ( Fig. 19.10 ). This leads to the ipsilateral eye appearing more open and bigger because the orbital volume is shallower. In contrast, the contralateral side has compensatory frontal bossing making the contralateral eye look as though there is ptosis of the lid. It is not uncommon for these children to be referred to an ophthalmologist first before it is recognized that the true pathologic process lies with the skull.

Bilateral coronal synostosis, though a symmetric deformity, can lead to more critical problems. The orbital volumes may be so shallow as to not allow full eye closure when the child sleeps, which risks exposure injuries of the cornea. In addition, there can be growth restriction of the brain as the ability of the other sutures to compensate for more than one fused suture may not be present. This process can lead to an elevated ICP as the brain continues to grow but is not afforded enough room for growth. In syndromic cases such as Crouzon’s syndrome, in which the lambdoid sutures can be involved as well, there is a high incidence of Chiari I malformations (to be discussed later). Barring any of these more critical complications of multisutural synostosis, operative correction is often delayed until after 6 months of age when the bone is thicker and allows for a more durable surgical reconstruction of the frontal bones and orbits.

Isolated lambdoidal synostosis is exceedingly rare. When suspected in a child, that child should be evaluated by a specialist in craniosynostosis. Unlike the parallelogram effect of positional molding, lambdoidal synostosis creates more of a trapezoidal head shape when viewed from above as growth is restricted on the involved side and contralateral compensatory parietal bulging develops. The ear on the involved side is commonly displaced slightly inferior and posterior relative to the contralateral ear. The degree of true cosmetic deformity with this type of synostosis is mild compared with the other forms, but it can at times require surgical intervention.

Salient points regarding head shape abnormalities in infants are (1) the diagnosis can almost always be made clinically without the need for imaging studies, (2) not all head shape abnormalities require therapy, and (3) early referral to a specialist is important to avoid unnecessary testing and ensure any necessary intervention is implemented at the appropriate time.