While the MAP remains within a specific range, cerebral perfusion is under autoregulatory control. Autoregulation of cerebral blood flow is the intrinsic ability of the cerebral vasculature to maintain constant cerebral blood flow (CBF) despite changes in cerebral perfusion pressure (CPP). In the stressed neonate or preterm infant, loss of autoregulation or perturbations in CBF has been associated with periventricular hemorrhage, hence the importance of understanding cerebral autoregulation and its variables. Myogenic, neurogenic and metabolic factors have been hypothesized as mechanisms responsible for the control of this intrinsic function. If excessive hypotension, hypertension, hypoxia, hypercapnia or cerebral ischemia occurs, these mechanisms may begin to fail, rendering CBF pressure passive. In addition to physiologic variables, pharmacologic therapeutics such as inhalational anesthetic agents may blunt autoregulation via a dose-dependent cerebral vasodilatation (see above).
CBF is autoregulated to maintain oxygen delivery over a wide range of perfusion pressures. Three factors affect CBF: MAP, CO2, and O2. CBF remains constant between MAPs of 50 and 150 mmHg in older children. However, thresholds for autoregulation in infants have not been fully elucidated. Recent concepts in the autoregulation of CBF in stressed neonates have shifted from the notion of “loss of autoregulation” during stress (e.g., hypoxia) to a flattening of the autoregulation curve [5]. In addition, evidence has shown that cerebral autoregulation waxes and wanes with time [6].
Studies in neonatal animals suggest that the limits for autoregulation in this age group lie between 25 and 75 mmHg MAP (Fig. 11.1). Once the limits of autoregulation have been exceeded, CBF changes more passively with MAP [7]. Neonates in severe physiological distress may have blunted autoregulation, demonstrated by a minimal change in CBF in response to varying PaCO2 levels or a direct pressure-passive CBF [8]. The nadir of autoregulation in awake preterm infants 24–34 weeks gestation is 23 mmHg [9, 10]. Studies using more recent technology such as near-infrared spectroscopy have demonstrated that autoregulation is a dynamic phenomenon. For example, CBF is pressure passive 20–50 % of the time in sick preterm infants <1,500 g during the first 5 days after birth [6]. Cerebral autoregulation was maintained despite fluctuations in MAP in normotensive preterm infants weighing ~1,000 g whose lungs were ventilated, although their response to CO2 was blunted. In contrast, cerebral autoregulation to MAP was blunted in hypotensive preterm infants, and their response to CO2 was markedly attenuated or absent [11]. Clinical evidence suggests that fluctuating CBF as in preterm infants with impaired or abolished autoregulation may be at increased risk for IVH [12, 13].
Fig. 11.1
Cerebral blood flow and cerebral perfusion pressure in a neonate and older child. Note that the relationship between flow and pressure is poorly defined in the neonate especially at higher pressures
Change in PaCO2 exerts a profound effect on cerebral perfusion. Hypercapnia impairs cerebrovascular autoregulation, while hypocapnia increases vascular tone and hence cerebrovascular resistance, thus decreasing CBF. Changes in ventilation and intrathoracic pressure rather than PaCO2 may be the primary cause of the observed changes in cerebrovascular autoregulation [14]. These observations are in conflict with an understanding of interaction between PaCO2 and cerebrovascular autoregulation but may result from the technique by which cerebral autoregulation was quantified. In adults, CBF decreases 3 % with every 0.75 mmHg (0.1 kPa) decrease in PaCO2 between 20 mmHg (2.7) and 80 mmHg (10.7 kPa). In contrast, the immature brain is relatively unresponsive to small changes in PaCO2 [15]. In infants and children, cerebral vasoconstriction occurs at a much reduced PaCO2 compared with that in adults [16]. Although hyperventilation minimally increases cerebral vasculature resistance in neonates, a sudden increase in PaCO2 after chronic hyperventilation of more than 24 h may cause cerebral vasodilatation and increased ICP [17]. Mild hypothermia decreases, whereas hyperthermia increases dynamic cerebrovascular autoregulation, although these effects are small. Hemodilution reduces blood viscosity and vascular resistance, thereby increasing CBF [18]. This decrease in vascular resistance can decrease autoregulatory capacity, and the lower limit of autoregulation has been shown to increase with anemia [19].
Spinal Cord Blood Flow
Animal data suggest that spinal cord blood flow is influenced by the same factors that influence CBF, although the flow rate is less, reflecting the reduced metabolic rate of the spinal cord. Perfusion of the spinal cord is determined by an equation similar to that for CBF:
This highlights the importance of extrinsic pressure on the integrity of the spinal cord, an effect that occurs in the presence of tumors, hematomas, or spinal venous congestion.
CSF and ICP
Between 50 and 80 % of the cerebrospinal fluid (CSF) surrounding the brain and spinal cord is produced by the choroid plexus, which lines the floor of the lateral ventricles and the roof of the third and fourth ventricles. Up to 30 % of the CSF can be formed in other sites such as ependyma, the brain parenchyma and endothelium of cerebral capillaries. The CSF produced by the choroid plexus flows from the lateral ventricles, through the interventricular foramen of Monro into the third ventricle, then into the cerebral aqueduct of Sylvius to the fourth ventricle. It emerges from the interior of the brain through the two lateral foramina of Luschka and the single medial foramen of Magendie to enter the subarachnoid space. The CSF is absorbed by the arachnoid villi. The mean production rate of CSF in children is about 0.35 mL/min. CBF and CBV are much more important determinants of ICP than is the volume of the CSF.
Controlling ICP is critically important in maintaining CPP. Normal ICP is 8–18 mmHg in adults and 2–4 mmHg in children. The ICP in neonates is positive on the day of birth but then becomes negative, probably because of salt and water loss. Intracerebral volume also acutely and briefly decreases and is matched by a reduction in head size.
Neonates can compensate for slow increases in ICP because their fontanelles and suture lines are open. However, acute changes in intracranial volume are not tolerated because the fibrous connective tissue connecting the fontanelles and sutures is relatively difficult to separate. A small change in volume may result in an insignificant increase in ICP in a neonate with a normal baseline ICP, but once the noncompliant point on the intracranial compliance curve is reached, a small increase in volume causes a greater increase in ICP. Ultimately, the brainstem and cerebellar peduncles may herniate through the foramen magnum (coning), causing coma and death (Fig. 11.2).
Fig. 11.2
Idealized intracranial compliance curve in a neonate
Blood-Brain Barrier
The blood-brain barrier (BBB) is a lipid membrane interface between the endothelial cells of the brain blood vessels and the ECF of the brain. It acts as a barrier to water-soluble drugs. Research from animals suggests that the BBB in the neonate has restrictive properties that are similar to those in the adult. At birth, saturable, carrier-mediated transport mechanisms are present, regulating the entry of glucose, amino acids, organic acids, purines, nucleosides, and choline. No difference in brain uptake of glucose has been observed between adults and neonates in a rabbit model. In contrast to the early suggestion that the BBB in young animals is an immature barrier, studies indicate that the BBB at birth is sophisticated and selective, with carrier systems that have a vital function in regulating the concentrations of metabolites within the neonatal brain [20].
Embryology and the Pathology of Neural Tube Defects
The CNS is the first organ system to develop in the fetus. Development of the CNS involves the 3 major phases of neurulation, canalization and retrogressive differentiation. Neurulation is the process by which the neural plate, derived from the neuroectodermal layer, folds upon itself to make a groove and then fuses to form the neural tube. Differentiation of the neural tube occurs within the first 60 days after fertilization of the ovum. The nervous system appears in the second week of gestation. Differentiation of the cortex takes place during the third trimester.
Canalization is the formation of the caudal neural tube including the development of the lower lumbar, sacral and coccygeal segments. Within the neural tube, groups of cells with their corresponding vertebrae proliferate and produce an excess number of segments. These excess segments degenerate in a process called retrogressive differentiation, and the filum terminale and cauda equina remain. The growth of the spinal column brings the conus medullaris to its adult level. Neural tube defects occur during neurulation. Failure of neurulation in early development results in total dysraphism within the brain and spinal cord. Anencephaly occurs only if the brain fails to close. Abnormal neuronal migration results in cortical malformations. Failure of canalization results in spina bifida: a myelocele exposes only neural tissue, a myelomeningocele exposes meninges in addition to neural tissue and a meningocele contains only meninges.
General Principles of Neuroanesthesia in the Neonate
There are general principles and issues that apply to anesthesia in neonates for any surgical procedure. These have been discussed in other chapters. There are also general principles that apply to anesthesia for all neurosurgical procedures, regardless of age. What follows is a discussion of the latter principles as they apply to the neonate.
Access and Positioning
Access to the patient is limited during most types of neonatal surgery. Neurosurgery is no exception and indeed access to the airway may be particularly fraught. Great care must be taken that the tracheal tube is well secured and ideally located in the mid-trachea to avoid extubation or migration into a main bronchus. Nasal tubes are easier to firmly secure than oral tubes, particularly for posterior fossa surgery. The anesthesiologist must remain vigilant for the occurrence of dislodged or kinked tracheal tubes throughout the surgery.
Intravenous and arterial lines should also be placed before surgery begins and carefully secured. Attempting to establish new intravenous access during surgery is very difficult in the neonate, so if there is any question regarding the patency of the vascular access, then access must be established before surgery commences. For major neurosurgery, central venous access should be considered in order to provide a secure intravenous line, a line for vasopressor support and a means to measure central venous pressure and hence ensure optimal filling pressures. However, femoral or subclavian access is preferred over jugular venous access to preclude jugular venous obstruction.
ECG and noninvasive blood pressure and temperature monitoring should also be secured before the baby is covered. The eyes should be taped closed to prevent injury. Some neurosurgeons use paraffin and other elaborate means to protect the eyes from pressure and alcoholic skin preparation. Before the drapes cover the neonate, there should be a final check to ensure that pressure areas are well padded, that the tracheal tube is not at risk of being kinked and that all intravenous lines are free of excess tension.
Temperature
Cooling is neuroprotective and may reduce brain injury, whereas hyperthermia can exacerbate brain injury. Although mild hypothermia may be beneficial in terms of brain protection, it is also associated with cardiovascular instability, apnea, coagulopathy, and reduced immune resistance. If hypothermia is used to reduce potential brain injury, it must be undertaken cautiously with adequate cardiovascular and ventilatory support. Neonates can both lose heat and become overheated very quickly. Hyperthermia must be strictly avoided.
Neurosurgical procedures may involve exposure of relatively large areas as the head is relatively larger in neonates compared with that in adults. Particular care must be taken that heat loss is not excessive during antiseptic skin preparation. A forced air warmer should be used whenever possible and may be complemented with an overhead heating device to maintain thermoneutrality. All neonates need careful temperature monitoring during neurosurgery. Monitoring the temperature via the esophagus or pharynx is preferable to the skin and rectal sites.
Blood Pressure
Blood pressure control is crucial to maintain an adequate CPP during neurosurgery. With CPP depending on the venous pressure and ICP, the head must be carefully positioned to preclude any obstruction to venous drainage from the brain. Hypotension may lead to underperfusion and ischemia, whereas hypertension may lead to an increased capillary flow, transudation of fluid, and interstitial edema. Evidence suggests that hypotension may impair autoregulation as well as the reactivity to changes in PaCO2. Such edema increases the oxygen gradient between capillaries and neurons, thereby increasing the risk of hypoxic injury. In normal brain and under normal conditions, autoregulation ensures flow matches demand over a range of blood pressures. It reduces the risk of hypertension leading to edema or hypotension leading to ischemia. However, in the injured brain, this autoregulation may be impaired, and thus the range of blood pressures that avoids either excess or insufficient perfusion is much narrower.
Hypotension is poorly defined in both awake and anesthetized neonates and varies with specialists. In neonates born 26–30 weeks gestation, a MAP <30 mmHg has been associated with IVH. This led to the general principle that the MAP should be ≥gestational age in weeks [21]. Some advocate the use of vasopressors to maintain an adequate MAP, although these have been associated with sequelae. In a survey, the majority of pediatric anesthesiologists defined hypotension in neonates during general anesthesia as a MAP >20–30 % below than awake values [22].
The range of CPP in which autoregulation can be maintained is poorly defined in neonates. In the absence of better data, good practice is to maintain a normal blood pressure for a neonate—though even this is unclear. Certainly excessive hypotension and hypertension must be avoided. To maintain adequate blood pressure, the neonate should be volume replete first, but there should also be a low threshold for vasopressor support. This is best given as an infusion rather than by intermittent boluses as the latter may lead to large fluctuations in blood pressure. To ensure prompt and appropriate responses, blood pressure should be accurately monitored. For critical cases, intra-arterial blood pressure monitoring is ideal.
Many anesthetic drugs also reduce the capacity of autoregulation or lead to a mismatch of flow and demand. All inhalational anesthetics impair autoregulation in a dose-dependent manner, although this effect is limited at concentrations <1 MAC. Nitrous oxide tends to increase flow. Propofol affects autoregulation the least of all drugs, although there are few data describing total intravenous anesthesia dosing regimens for propofol in neonates. Opioids can be used to provide stable hemodynamics and to supplement anesthesia, with limited impact on the autoregulation of CBF. Fentanyl at 5–10 mcg/kg/min may be used, provided there is capacity to continue to provide ventilatory support postoperatively. There is limited experience with remifentanil in neurosurgery in neonates, but it may have a very promising role, given its rapid metabolism and fast recovery. (See section on neuropharmacology below.)
Ventilation
The primary aim of ventilation is to prevent hypoxia and strive to maintain normocapnia. Hypercapnia increases ICP, possibly causing cerebral edema and making operating conditions difficult. Hypocapnia may lead to hypoperfusion and ischemia. Similarly, hypoxia may exacerbate brain injury. In theory, excessive PEEP increases ICP, although if such a PEEP level optimizes oxygenation and ventilation, then this must take priority. Muscle relaxation may be used to prevent straining and the resultant increase in ICP.
Fluids, Glucose, and Electrolytes
Extreme fluctuations in blood glucose concentration may exacerbate brain injury. Hypoglycemia may itself lead to brain injury. Hyperglycemia may worsen existing brain injury, although in neonates, there is evidence that this risk is not as great as in older children and adults [23, 24]. Intravenous dextrose should be continued during major neurosurgery in neonates and the blood concentration of glucose checked regularly for extreme shifts in the concentration.
Hyponatremia reduces plasma tonicity, thereby exacerbating cerebral edema. During neurosurgery, isotonic fluids should be administered and the plasma sodium concentration measured regularly. Excessive amounts of sodium chloride may lead to hyperchloremic metabolic acidosis as the immature kidney is unable to excrete the excess chloride. If there is a risk of diabetes insipidus or increased ADH secretion, then the plasma concentration of electrolytes must be checked frequently.
With relatively large heads that receive a disproportionate fraction of the cardiac output, blood loss during neurosurgery in a neonate may be more significant than in older children. A coagulation profile and full blood count should be obtained before all major neurosurgeries. Packed red blood cells should be available for any neurosurgical procedure that may result in bleeding. In neonates, fresh blood is preferable to aged stored blood since rapid transfusion of the latter may lead to acidosis and hyperkalemia, for both of which the neonate has limited buffering capacity. The ideal hematocrit to trigger transfusion in the setting of acute blood loss in neonates is unknown. Not only is it very easy to underestimate ongoing blood loss, but acute losses can occur very rapidly and lead to hemodynamic instability. Therefore, if significant blood loss is occurring, red cells should be given earlier rather than later. During major neurosurgery, platelets and fresh frozen plasma should be available and given early before a coagulopathy develops.
Analgesia Postoperatively
Neonates have well-developed systems of nociception; they feel and respond to pain as do older children. Furthermore, failure to alleviate pain in neonates can lead to changes in spinal cord morphology and increase postoperative complications. Analgesia should be provided after neurosurgery. Minor procedures may require only simple analgesics such as paracetamol, whereas more involved procedures will require parenteral opioids.
Neuropharmacology
The effects of anesthetic agents on cerebrovascular autoregulation vary. Inhaled anesthetics exert a dose-dependent uncoupling effect on autoregulation. Sevoflurane, however, impairs cerebrovascular autoregulation the least of all of the inhaled anesthetics. In healthy children, cerebral pressure autoregulation is preserved up to 1.5 MAC of sevoflurane, a finding similar to that in adults [25]. Although regional CBF varies with inhaled anesthetics, global CBF remains unaffected [26]. CBF increases to the greatest extent during halothane anesthesia, followed by enflurane, isoflurane and then desflurane.
Hypercapnia impairs cerebrovascular autoregulation. Furthermore, this effect of hypercapnia compounds the effects of inhaled anesthetics on the loss of autoregulation. Propofol preserves cerebrovascular autoregulation at PaCO2 values as great as 56 mmHg (7.5 kPa), whereas a similar level of hypercapnia abolishes cerebrovascular autoregulation during sevoflurane anesthesia. Importantly, hypocapnia reverses isoflurane-induced impairment of cerebrovascular autoregulation.
The effects of the non-inhaled anesthetics on cerebral autoregulation vary. Nitrous oxide increases CBF alone and when given with other anesthetic agents. Propofol preserves cerebral autoregulation at both large and small doses in healthy adults. When remifentanil is combined with propofol, it induces a dose-dependent metabolism-coupled reduction in CBF with preserved cerebrovascular autoregulation in adults. Comparable data in neonates are lacking. Opioids have little or no effect on CBF and ICP and cerebral autoregulation remains intact. Benzodiazepines and barbiturates decrease CBF and thus ICP. Barbiturates cause vasoconstriction in the cerebral vasculature and preserve autoregulation. Ketamine may increase CBF by up to 60 % under normocapnia and is therefore contraindicated in patients with increased ICP.
Neurosurgical Conditions
Neural Tube Defects
Neural tube defects (NTDs) (also known as spinal dysraphism) include all congenital anomalies that involve failure of the neural tube to close during the fourth week of embryogenesis and can occur anywhere along the formation of the spinal cord, from the brain to the sacrum. The two most common forms of NTDs are spina bifida and anencephaly.
Spina bifida is almost always compatible with survival, although severe physical and cognitive impairments are common. Spina bifida lesions are classified depending on whether or not neural tissue is exposed. Myelomeningocele is the most common and most severe form of spinal dysraphism. It is an open spina bifida lesion in which the spinal cord and meninges protrude through a defect in the vertebral arches and are either uncovered or covered with only a thin membrane. The defect can occur anywhere along the spinal column but is most common in the lumbar region. It can result in marked neurological deficit caudal to the level of the protruding sac.
In contrast to spina bifida, anencephaly is almost always fatal before or soon after birth. In this defect, there is partial or complete absence of the skull bones with only a minimal remnant of a brain.
Occult spinal dysraphism refers to a spina bifida lesion with intact skin covering. This form of dysraphism includes lipomeningocele, meningocele (when the sac contains meninges and cerebrospinal fluid but the spinal cord and spinal root are in their normal position), myelocystocele, dermal sinus, tight filum terminale, and diastematomyelia. With encephalocele, the brain and its covering membranes with CSF protrude through the skull, most often in the occipital region (Fig. 11.3). Spina bifida occulta is a benign and common abnormality in which the spinous processes of the lower lumbar or sacral spine fail to fuse. With these lesions, individuals are asymptomatic. The diagnosis is usually an incidental finding on plain radiographs of the spine/abdomen.
Fig. 11.3
Sagittal T1 MRI scan of a child with severe cranial dysraphism, with a pronounced brain malformation and encephalocele
The birth frequency of NTDs varies among countries and regions within countries but, in general, is approximately 1 in 1,000. In the UK, the prevalence is around 3 per 1,000 live births compared with 1 in 10,000 in sub-Saharan Africa. Worldwide, the prevalence of NTDs at birth appears to be decreasing. The prevalence in the UK was about 4 per 1,000 in the 1970s and has decreased to 3 per 1,000 live births. There is a significant genetic component to the development of NTDs. If either parent has had an affected child or is affected by the condition, the risk of further offspring having an NTD is approximately 10 %. If 2 affected pregnancies occur, the risk to a further pregnancy is increased by about 20-fold. Nevertheless, at least 90 % of NTDs occur to women without a family history. Since the 1970s, maternal nutrition, particularly regarding folate, has been linked to the occurrence of NTDs. In 1991, a large randomized trial determined that the recurrence risk of NTDs in mothers who consumed folic acid before conception was reduced by 72 % [27]. Two other controlled trials also showed similar reductions in the recurrence risk, with a pooled reduction in risk among those who were compliant of 87 % [28]. A definitive randomized controlled trial that compared the frequency of NTD in a multivitamin-supplemented group (containing 0.8 mg of folic acid) with a non-supplemented group showed no occurrences in the supplemented group (n = 2,104 pregnancies) and six in the non-supplemented group (n = 2,052). It is now recommended that a daily dietary supplement of 5 mg of folic acid before conception will prevent a recurrence of NTD. All women should be advised to take 400 mg of folic acid daily prior to conception to prevent a first occurrence of NTD as well as increasing consumption of folic-rich foods, such as green vegetables and fortified breakfast cereals, until the 12th week of pregnancy. However, because the compliance in taking these supplements is poor, the USA and several other countries instituted folic acid fortification of foods. In 1998, a US policy was drafted to require that enriched grain products be fortified with folic acid. Since this policy was adopted, the frequency of NTDs in the USA has been reduced to about 31 % for spina bifida and 16 % for anencephaly [29]. In addition, infants with spina bifida who were born after the fortification policy had a significantly better first-year survival rate than did those who were born before the policy [30]. However, many countries have not been as willing to embrace fortification. For example, in Finland, mandatory fortification is not permitted over concerns of possibly masking megaloblastic anemia caused by vitamin B12 deficiency in women older than 65 years, a possible small increase in the risk of wheeze and respiratory tract infections in offspring whose mothers took folic acid supplements during pregnancy and a possible increased incidence of lung cancer. However, there is no clear evidence confirming these concerns. The American College of Medical Genetics recommends that all women who are capable of becoming pregnant should strive for an intake of 0.4 mg of folic acid daily, and women who have had a previous NTD-affected pregnancy, who are themselves affected or have a first- or second- degree relative with an NTD, should ingest 4 mg of folic acid, commencing 3 months before conception and continuing throughout the first trimester.
Myelomeningocele
This is the most common and severe form of spinal dysraphism resulting from a failure of closure of the neural tube around day 21 of development (Fig. 11.4).
Fig. 11.4
Sagittal T1 MRI scan of a child with a severe Chiari II malformation and hydrocephalus
Open myelomeningocele is immediately apparent at birth as a defect on the back with a neural placode, which is the open spinal cord. Abnormal nerve roots emerge from it ventrally. It is surrounded by arachnoid adhesions, an incomplete dura and associated paravertebral soft tissues. Antenatal diagnosis is usual. Maternal serum alpha-fetal protein is used to establish the diagnosis, and ultrasound will demonstrate the contents of the lesion as well as other congenital abnormalities. The defect may be covered by a thin epithelial or arachnoid layer, but in some cases, this may have ruptured and CSF can be seen leaking from the defect. Most infants will develop hydrocephalus and a Chiari II malformation (disorganization of brainstem topography, a small posterior fossa, and herniation of the cerebellum through the foramen magnum) is very common. Abnormalities of cerebral gyration, the posterior fossa contents and agenesis of the corpus callosum as well as vertebral anomalies may also be present.
The infant with an open myelomeningocele is preferably delivered by Cesarean section to avoid acquiring a CNS infection during passage through the birth canal. The baby should be nursed prone or in the lateral decubitus with a sterile moist dressing covering the defect. Surgery cannot restore neurological function but will protect existing neural structures and prevent infection. Neural tube lesions are investigated with ultrasound, CT and/or MRI scanning. The open myelomeningocele should be closed within 48 h of birth since the risk of infection increases with the more time the defect is exposed.
Two less common forms of dysraphism occur in relation to skull defects. Cephaloceles involve herniation of either meninges (cranial meningocele) or brain and meninges (encephalocele) into the cele. These neural tube defects occur in ~1–3:10,000 live births [31]. Occipital encephaloceles occur two to three times more commonly than nasal (or anterior), parietal, and temporal encephaloceles, with a geographic distribution in which occipital cephaloceles are more common in Western countries, whereas anterior cephaloceles are more common in Asian countries [32, 33]. Occipital cephaloceles more commonly include brain tissue in addition to CSF (which may have to be excised) and are more commonly complicated by hydrocephalus and seizures and thus carry a poorer prognosis than anterior celes [32]. Neurological defects most commonly associated with dysraphism include brainstem hypoplasia, cerebellar dysplasia, Arnold-Chiari defect, Dandy-Walker syndrome, and lissencephaly (smooth gyri and sulci) [31, 32]. Cephaloceles may also be associated with other congenital defects including cleft lip and palate, syndactyly, ocular defects, and congenital heart defects [31, 34]. Although surgery is usually undertaken in older infants and children, 20–30 % of neonates with cephaloceles undergo surgery in the neonatal period [32, 33].
Anesthetic Considerations for Neonates with Neural Tube Defects
During the preoperative anesthetic assessment, the presence of any associated congenital anomalies should be excluded and the degree of the neurological deficit confirmed. With cervical encephalocele, the neck is often short and rigid which can make tracheal intubation difficult. These neonates may have feeding difficulties due to the neurological deficit and be volume depleted secondary to evaporative and third space losses from the exposed area. Consequently, preoperative intravenous fluids may be required. Induction of anesthesia may be accomplished using either an inhalational or intravenous induction. In the past, tracheal intubation has been recommended in the left lateral position, but this is not necessary if the spinal defect is first padded to prevent pressure from being applied to the layers covering the spinal cord. The head and body can be raised and supported using foam pads. Surgery is usually performed with the neonate in the prone position. Tracheal intubation is usually accomplished with a reinforced orotracheal tube. However, some anesthesiologists prefer nasotracheal intubation as it may better secure the tube fixation in the prone position because these tubes are less likely to become dislodged by secretions and loose tape than orotracheal tubes. In the case of cephaloceles, tracheal intubation may be difficult. For occipital celes, the airway is often secured with the neonate in the left lateral decubitus position as it is difficult to stabilize the large CSF-filled cephalocele with the neonate supine. With anterior cephaloceles, the child may be positioned supine with the airway secured using orotracheal intubation.
Large-bore intravenous access should be secured, and for cephaloceles that require a craniotomy, an arterial line should also be secured. Blood loss is usually small unless a large skin flap or a craniotomy (in the case of cephaloceles) is planned [32, 34]. However, if brisk blood loss is anticipated (as in the case of cephaloceles), packed red blood cells should be in the operating room at the time of skin incision. Additional monitoring includes core temperature and urine output. In the prone position, particular attention should be paid to the eyes, which should be padded and protected from pressure. Pressure on the abdomen should also be avoided to prevent compression of the IVC and engorgement of the paraspinal vessels as well as to allow free abdominal movement with respiration. This can be achieved by placing “jellies” under the chest and hips. Large lesions may require rotational flaps or tissue expanders to allow skin closure. Analgesia should be multimodal with small doses of intraoperative opioids, intravenous paracetamol, and wound infiltration with local anesthetics. Postoperative analgesia should be paracetamol for smaller defects. Nurse-controlled analgesia using morphine may be required when larger defects are closed. In this case, a high-dependency postoperative bed is needed for the monitoring of respiration and oxygen saturation. Preterm infants are at increased risk of postoperative apnea, especially after neurosurgery [35].
Hydrocephalus and Shunts
Hydrocephalus occurs as a result of impaired circulation or absorption of CSF. In addition, hydrocephalus can occur as a result of the overproduction of CSF, e.g., in association with choroid plexus papillomas, although these tumors are rare.
Between 70 and 80 % of CSF is produced from the choroid plexus and the remainder of the ependyma and brain parenchyma. Production occurs by filtration across the capillary endothelium and active secretion of sodium by the choroidal epithelium. CSF production is mainly independent of ICP, although production is reduced somewhat in the presence of increased ICP and reduced CPP. CSF absorption is linearly related to ICP. Most CSF is absorbed at the arachnoid villi, which are herniations of arachnoidal tissue into the dural venous sinuses. The precise mechanism of CSF absorption remains unclear.
In adults, CSF is produced at a rate of about 550 mL/day. The total volume of CSF is 100–150 mL, of which 15–25 mL is contained within the ventricular system.
Etiology and Pathophysiology of Hydrocephalus
An obstruction at any point along the CSF pathway can result in hydrocephalus. Obstructive or noncommunicating hydrocephalus is an obstruction within the ventricular system or at the fourth ventricular outflow. Communicating hydrocephalus is impaired circulation through the subarachnoid spaces or impaired absorption into the venous system.
Posthemorrhagic Hydrocephalus
Intraventricular hemorrhage is detected in 40–45 % of preterm neonates with a birth weight less than 1,500 g. In this weight range, the hemorrhages often occur in the germinal matrix because the blood vessels there are irregular, have immature connective tissue architecture and lack the ability to autoregulate [12]. About 20 % of neonates who suffer an IVH, which usually occurs within the first few days after birth, require a shunt because of increased ICP. In addition to prematurity, vigorous resuscitation, respiratory distress syndrome, pneumothorax and seizures are associated with an increased risk of IVH [36].
IVHs can be identified using cerebral ultrasound and graded according to the location of the hematoma and its effect on the size of the ventricles. The presence of blood and its breakdown products may lead to hydrocephalus by obstructing the subarachnoid space and arachnoid villi. In addition, it leads to an ependymal reaction with blockage at the aqueduct or outlet foramina of the fourth ventricle. Increasing head circumference and progressive ventricular enlargement require intervention. Preterm, low-birth-weight neonates are at increased risk of shunt infection, and the presence of heavy blood staining or excessive cellular debris in the CSF precludes shunt insertion due to an increased risk of blockage. A shunt can be inserted once the CSF is clear of blood products.
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