Chapter 8 – Congenital Neurosurgical Lesions

Chapter 8 Congenital Neurosurgical Lesions

Cynthia Tung , Vutskits , Lazslo and Sulpicio G. Soriano

Introduction

The development of the central nervous system (CNS) occurs early in gestation and is orchestrated by a combination of transcriptional and mechanical factors.¹ A basic understanding of normal and abnormal development of the CNS is valuable for understanding the perioperative management of congenital lesions of the CNS.²

The primitive CNS originates from the neural plate, which folds and fuses dorsally. Primary neurulation occurs when the neural plate folds to form the neural tube. The walls of the neural tube give rise to the brain and spinal cord, while the canal develops into the ventricles and central canal of the brain and spinal cord, respectively. Fusion of the cranial neural folds and closure of the cranial neuropore, which give rise to the forebrain, midbrain, and hindbrain, arise from these structures. Closure starts near the cervical spine region and extends cephalad and caudad. Closure of the neural tube begins at gestational age 22–23 days, with complete closure around days 26–27. Failure of the anterior neuropore to close by day 24 results in anencephaly, while posterior defects lead to encephaloceles and myelomeningoceles. Therefore, most commonly defects occur along the thoracic or lumbosacral region but can occur along the cervical region. Secondary neurulation ensues when the neuroepithelium caudal to the posterior neuropore closes. Derangements in this progression can lead to spinal dysraphism (spinal bifida, myelomeningocele, and tethered cord). If left untreated, these congenital lesions can lead to chronic conditions such as hydrocephalus and caudal neurological deficits. The incidence of neural tube defects is approximately 2–5/1,000 live births. Outcome studies reveal increased perioperative morbidity, mortality, and cognitive deficits in neonates with congenital neurological lesions.³^–⁶

Congenital Anomalies

Congenital CNS anomalies typically occur as midline defects. These neural tube defects may arise anywhere along the neural axis from the head (cranial dysraphism) to the spine (spinal dysraphism). They may be relatively minor and affect only superficial bony and membranous structures, or may include a large segment of malformed neural tissue. These lesions are associated with Arnold-Chiari malformations, hydrocephalus, and neurologic deficits. Cervical cord or brainstem compression are possible in patients with concomitant Arnold-Chiari malformations. After birth, the defects are usually covered with sterile, saline-soaked gauze in order to keep the lesion moist and clear. After birth, the patient is typically positioned prone to avoid direct pressure on the defect.

Cranial Dysraphism

Cranial dysraphisms or encephaloceles are characterized by a sac-like calvarial defect that arises anywhere from the nose to the occiput. The former can manifest as nasal polyps that protrude through the cribriform plate. Cranial meningoceles contain cerebrospinal fluid (CSF) and meninges, and the presence of neural elements classifies this cystic lesion as meningoencephalocele. Encephaloceles are classified by their location on the cranium, with sincipital lesions in the frontal calvarium and occipital encephaloceles sited posteriorly. Primary encephaloceles are often diagnosed in utero by fetal ultrasonography, with large encephaloceles delivered by elective cesarean section. Most small encephaloceles have minimal neurological deficits, whereas those with large lesions may present with cranial nerve abnormalities and subsequent developmental and growth delay, poor feeding, blindness, and seizures.

Preoperative diagnostic imaging is essential in delineating the content and margins of the lesion and require sedation or general anesthesia for computed tomography (CT) and magnetic resonance imaging (MRI) scans. Encephaloceles can be associated with hydrocephalus and other craniofacial and brain abnormalities such as anencephaly, microcephaly, ataxia, Meckel’s, and amniotic band syndrome.

If the neonate is stable, encephaloceles will continue to enlarge and surgery can be delayed and performed in stages if the overlying skin is intact. Innovations in neonatal care and surgical techniques, including image guidance and multidisciplinary reconstruction techniques, have improved the outcome for these patients.

Sincipital encephaloceles usually contain fibrous tissue, which can be safely transected at the level of the skull and the defect closed primarily. Nasal or sphenoethmoidal encephaloceles are rare and characterized by a skull base defect around the sella turcica. Large lesions may obstruct the airway and compromise pituitary function. However, smaller lesions may be undetected during infancy. Other midline nasal masses including nasal polyps, dermoid sinus cyst, and tumors should be considered in the differential diagnosis of these lesions. Image guidance based on three-dimensional image reconstructions and radionuclide ventriculography are useful. The resection and closure can be difficult during transpalatal surgical approaches due to exposure and inadequate soft tissue for closure. Other surgical procedures include transcranial, subfrontal, and endoscopic transnasal approaches. The postoperative course may be complicated by CSF leaks, meningitis, visual impairments, and endocrine derangements.

Occipital encephaloceles may contain functional brain tissue that needs to be preserved. Most encephaloceles with substantial neural tissue herniating through large cranial defect may require an expansion cranioplasty and a plastic surgeon to create split thickness calvarial grafts. When primary closure is not possible, a staged secondary repair is an option. Large occipital encephaloceles may be associated with twisting of the brainstem, lobar herniation, and hydrocephalus .

Spinal Dysraphisms

Spinal dysraphisms are lesions where the dorsal midline structures fail to fuse during embryogenesis and are categorized into spina bifida aperta and spina bifida occulta. Spina bifida aperta is easily identifiable by the sac-like lesion containing meninges (meningocele) or neural tissue and meninges (myelomeningocele), while spina bifida occulta has no superficial cutaneous manifestations.

These spinal defects can occur anywhere along the vertebral column, although lumbar and low thoracic defects are most common. Rachischisis is the most severe form where the posterior neuropore fails to fuse. A protruding membranous sac containing meninges, CSF, nerve roots, and a dysplastic spinal cord often protrudes through the defect in meningocele or myelomeningocele.

Prenatal ultrasonography affords early diagnosis and elective cesarean delivery and closure of meningomyeloceles. These lesions may be repaired in utero at specialized fetal surgery centers (Chapter 10). In order to minimize the risk of infection, meningomyeloceles undergo primary closure of the defect within the first 24 hours of life. These lesions are often associated with a type II Chiari malformation where both the cerebellum and brainstem tissue protrude into the foramen magnum.

Since type II Chiari (Arnold-Chiari) malformations predispose these patients to hydrocephalus, insertion of a ventriculoperitoneal shunt may be combined with the initial surgery. Alternatively, a ventriculoperitoneal shunt may be inserted a few days later or deferred if there is no evidence of hydrocephalus at birth. Patients with thoracic lesions may have poor autonomic control below level of the defect.

Patients with myelodysplasia are chronically at high risk for latex sensitivity and anaphylaxis, due to repeated exposure to latex products encountered during frequent bladder catheterizations and multiple surgical procedures. These children should be managed in a latex-free environment from birth to minimize the chances of sensitization.⁷ Latex allergy should be suspected if signs and symptoms of anaphylaxis develop during surgery. Suspected anaphylaxis should be treated with intravenous epinephrine in a dose of 1 to 10 µg/kg, as required.

Anesthetic Management

Preanesthetic Evaluation

Cranial and spinal dysraphisms are heterogeneous lesions and mandate an individualized approach based on the severity as well as its location. Therefore, a thorough review of the antenatal history, birth history, prematurity, other comorbidities, and other congenital anomalies should be completed prior to surgery (Chapter 1). Some patients with encephaloceles may have tenuous respiratory function due to direct airway obstruction or impairment of the pontomedullary respiratory control center. Depending on the size of the lesion and extent of the surgical procedure, significant blood loss should be anticipated during both the intra- and postoperative periods.

Intraoperative Management

Positioning the neonate for induction of anesthesia can be challenging. Anesthesia can be induced with a propofol, but hypotension and possible cerebral ischemia might ensue⁸ due to the lack of surgical stimulation. In most cases, tracheal intubation can be performed with the neonate in the supine position and the patient’s back or head supported with foam or gel head rings so there is no direct pressure on the lesion (Figure 8.1). Manipulation of these lesions should be limited because of the risk of rupturing the thin membranes. For very large defects, it may be necessary to place the infant in the left lateral decubitus position for induction of anesthesia and tracheal intubation.

Figure 8.1. The large encephalocele is cradled in sponge head rings, while the rest of the body is elevated with soft towels. This positioning provides optimal conditions for managing the airway and tracheal intubation.

Encephaloceles are associated with compromised airways by varying degrees. Effective mask ventilation may be impaired by protruding lesions of sincipital encephalocele and may hinder effective mask ventilation. In these patients, difficult airway precautions and techniques should be applied.⁹ Gigantic encephaloceles may prohibit proper positioning of the patient for intubation of the trachea. Some fluid-filled encephaloceles can be decompressed by aspirating CSF with a sterile needle and syringe under ultrasound guidance. Alternatively, giant occipital encephaloceles can be suspended through a pediatric horseshoe headrest. Mask ventilation may be difficult in this position, so an assistant can support the head as the anesthesiologist applies a mask seal. Intubation may also be more difficult in the lateral position and may require the use of a flexible fiberoptic bronchoscope or video laryngoscope.

The surgical repair is performed in a prone position so the patient’s face should be well supported by a padded foam on a horseshoe headrest to prevent direct pressure on the eyes and mouth (Figure 8.2). In this position, caution should be taken so that the endotracheal tube (ETT) is well secured to the patient. Since the airway will be inaccessible during repair of encephaloceles, a nasotracheal tube should be secured to minimize dislodgement. Prone positioning for the surgery also requires careful padding to prevent increased abdominal pressure.

Figure 8.2. The patient is placed in the prone position with a soft sponge padding the head on the Mayfield headrest. The eyes, nose, and mouth are suspended thorough an opening in the sponge to avoid direct pressure. Note that the thorax and abdomen are hovering on lateral silicone rolls to prevent thoracoabdominal compression.

Ensuring adequate oxygenation and perfusion of the developing brain is the cornerstone of neonatal anesthesia and critical care.¹⁰^, ¹¹ Typically, blood loss during the procedure is not significant enough to necessitate blood transfusions. However, the risk of bleeding and venous air embolization is greater in patients with larger cranial defects. Therefore, multiple intravenous and an arterial catheter should be inserted for these large lesions where large blood loss may be anticipated. Occasionally, rotational or myocutaneous flaps may be required for closure of large defects. Respiratory parameters and oxygenation should be carefully monitored during primary closure of large defects because tight skin closure may compromise tidal volume and reduce venous return. Significant hypotension is typically due to blood and CSF losses, but also can be a manifestation of hypothyroidism, adrenocortical deficiency, or diabetes insipidus. Patients who develop diabetes insipidus should be treated with a vasopressin infusion and urinary output replaced with crystalloid. Bladder catheter and arterial lines should be considered for any complex lesions. If risk for postoperative CSF leak is significant, a ventricular drain may be inserted.

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