Книги по МРТ КТ на английском языке / Neurosurgery Fundamentals Agarval 1 ed 2019

Evaluation of Head Shape

Craniosynostosis is the premature fusion of a cranial suture, with an incidence rate of approximately 4 in 10,000 live births.12 While a number of genetic abnormalities may predispose to various abnormalities in cranial shape, craniosynostosis is most commonly a sporadic occurrence affecting a single suture. The most common is sagittal craniosynostosis.3,​13,​14 The presence of craniosynostosis impacts skull growth perpendicular to the fused suture, and creates an abnormal shape to the head, with classic morphologies that are often seen with various fused sutures ( Fig. 14.1). Examination of the head shape from all angles may help to

differentiate craniosynostosis (a relatively rare condition) from positional plagiocephaly which develops secondary to positional molding of the skull and has an incidence of as high as 46.6%.15 In addition, premature infants may demonstrate disproportional fron- to-occipital growth, leading to a slightly scaphocephalic shape and which may also have a dramatic effect on overall head circumference.3 Examination of a newborn and young infant may include palpation of sutures for mobility in any child with an abnormality of head shape. Infants with craniosynostosis benefit from early identification as intervention includes options for minimally invasive treatment or open repair. The exact incidence of elevated ICP

varies widely in the literature (between 4.5 and 24% and as high as 44% in one study) and exact attributable sequelae of nontreatment is controversial. Nevertheless, correction is typically recommended to improve head shape as well as prevent future sequelae of craniocephalic disproportion which may lead to elevated ICP, among other multifactorial concerns.16

14.1.3 Evaluation of Alertness

The evaluation of alertness in an infant is largely dependent on time of day, time since feeding, external stimuli at the time of examination, gestational age, and behavioral development ( Table 14.2).18,​19,​20,​21

from the patient. Table 14.3 summarizes the most common techniques used to assess the cranial nerves in infants and young children as compared to those used in adults.3

14.1.5 Motor Evaluation

The major components of the infant’s neuromuscular examination serve to assess tone, strength, and reflex responses. Throughout gestation and in the postnatal months, these measures of neuromuscular function fluctuate and are an indication of the infant’s development­ and maturity.3 In addition to the primitive reflexes previously ­discussed ( Table14.1),deeptendon reflexes are also an important component of the infant neurologic examination and are assessed in examination.

14.1.4 Cranial Nerve Evaluation

During the cranial nerve evaluation of infants, the physician must be more creative than during an adult examination due to the lack of verbal communication

The observance of clonus upon eliciting the Achilles tendon reflex is a normal response in infants up to 3 months of age but should be considered abnormal after this period.3

14.1.6 Sensory Evaluation

Beyond the scope of sensory stimulation used to test reflexes, the sensory evaluation is rarely performed in infants without

considerable suspicion of sensory deficit.23 Pinprick tests of sensation will elicit withdrawal and crying in an infant greater than 28 weeks of gestation, if necessary to test for subtler sensory deficits.3,​24

14.1.7 Examination of Toddlers and Older Children

While the neonatal examination requires a number of modifications due to lack of communication abilities, and physical development, the examination of older pediatric patients resembles that of an adult. In the neurologic examination in particular, it is important to follow head circumference and shape closely until preadolescence. Verbal children older than 2 years are typically capable of following simple commands and providing responses to examination questions (e.g., “follow my finger with your eyes,” “shrug your shoulders and don’t let me push them down”).

14.2  Developmental Anomalies

14.2.1 Arachnoid Cysts

Arachnoid cysts (ACs) are congenital abnormalities that arise from a separation of the layers in the arachnoid mater, typically filled with a fluid that is similar or identical in composition to CSF. The most common locations for ACs to develop are near CSF cisterns in the middle fossa, cerebellopontine angle, suprasellar region, posterior fossa, and in the spinal canal. Spinal ACs are rare in the pediatric population.25

Diagnosis

Cysts may be an incidental finding on brain imaging and focal deficits may appear disproportionately mild relative to the size of the fluid collection. Symptomatic ACs almost always present in early childhood with variant symptoms relative to location of the cyst or elevation of ICP.26,​27,​28

Treatment

Neurosurgical treatment, when indicated for symptomatic ACs, involves two primary techniques: fenestration and shunting. Fenestration is a treatment method by which an opening in the cyst membrane is created either by way of an open craniotomy or endoscopically through a burr hole.

Currently, endoscopic fenestration is the preferred method of treatment in pediatric patients due to the decreased invasiveness compared to craniotomy and the avoidance of shunt dependence and complications.29

In a cohort of pediatric patients, fenestration has showed further improved revision-free survival if accompanied by a simultaneous ventriculocystocisternotomy, a procedure that entails creating further fenestrations in the basal surface of the collapsed cyst.30 Shunting has largely become an outdated practice due to the complications of long-term shunting and improvements in endoscopic technique.31

14.2.2 Chiari Malformations Naming and Subtypes

Though seemingly nuanced, “Chiari malformation,” in reference to Hans Chiari, generally refers to the type I malformation, while the term “Arnold–Chiari malformation,” named for Julius Arnold, is in reference to the type II malformation. Though there are four total pathologies under the umbrella of “Chiari malformations,” types I and II are predominate as occurring most frequently. Type I Chiari malformation may be diagnosed during childhood or adulthood. However, given its relationship to myelomeningocele, type II Arnold–Chiari malformation is classically diagnosed in infancy.

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General

The hallmark of Chiari I malformation is the impairment of CSF flow through the foramen magnum with an otherwise heterogeneous morphology of the posterior fossa obstruction. Distinction of the Chiari II (previously known as the Arnold–Chiari malformation) from the Chiari type I include the caudal displacement of the medulla and vermis (rather than the cerebellar tonsils which are displaced in the Chiari I malformation) as well as the presence of a myelomeningocele.32 In both conditions, the downward displacement of the cerebellum poses some obstruction to the flow of CSF through the foramen magnum. A Chiari malformation may be primary when it is not associated with other intracranial abnormalities, or secondary as a result of

hydrocephalus or a mass lesion. Table 14.4 compares the subtypes of Chiari malformation and their associated symptoms.

Diagnosis

The best diagnostic tool for a Chiari malformation is magnetic resonance imaging (MRI) showing tonsillar herniation and caudal displacement of the cervicomedullary junction ( Fig. 14.2).

Treatment

Chiari I Malformations

Treatment for Chiari I malformations in children is a decompressive procedure of the posterior fossa to allow release of

pressure on the brainstem and improved CSF flow at the foramen magnum. A suboccipital craniectomy and C1 laminectomy is the general standard of care, additional laminectomy levels may be necessary depending on the extent of tonsillar descent.34 Current controversy exists as to whether a bone-only decompression or decompression with duraplasty yields superior outcomes.

Posterior fossa decompression classically involves suboccipital craniectomy, cervical laminectomy to the level of the tonsillar herniation, and a Y-shaped dural opening from the tonsillar herniation to the foramen magnum.35,​36 Recently, it has been suggested that dural openings and suboccipital craniectomies may not be obligatory in pediatric patients, who often have more pliable meninges than adult patients and show no clinical benefit to more extensive bony decompressions.37,​38 Chiari I malformations in children are treated with a similar approach, but may also require the placement of a shunt or drain for the management of symptomatic syringomyelia if the syrinx does not

resolve and/or remains symptomatic after decompression.39,​40

Chiari II Malformation (Arnold–Chiari)

Chiari II malformations are present in patients with myelomeningocele. Conservative observational treatment is preferred in almost all cases of Chiari II malformation. Later in life, symptomatic Chiari II malformations may be indicative of shunt malfunction or tethering of the spinal cord, where treatment of the Chiari II malformation is accomplished by a shunt revision or detethering.41 On the rare occasion that surgical decompression is indicated, surgical technique is identical to that of a Chiari I malformation.

14.2.3 Neural Tube Defects

Neural tube defects (NTDs) refer to abnormalities that occur during the closure of the embryonic elements of the central nervous system (CNS), disrupting

formation and function of the CNS. Early detection along with a comprehensive understanding of the genetic and environmental factors that cause NTDs are crucial for identifying their heterogeneous presentations and neurosurgical management.

Prenatal Diagnostics

Maternal Serum Alpha Fetoprotein Test

The maternal serum alpha fetoprotein (MSAFP) test is frequently elevated in NTDs, but is not specific.42 Of note, when the level of MSAFP is low during gestation, the risk of the fetus having trisomy 21 is elevated.

Ultrasound (US)

Prenatal US is a more definitive way to detect the presence of an NTD than MSAFP and is done later in pregnancy.

Amniocentesis

Amniocentesis in the second trimester is used to determine the presence of a myelomeningocele (MMC) or other open NTD. AFP levels are elevated in the presence of an open NTD and may be indicated with a suspicious US despite a risk of pregnancy loss from the procedure.11

Fetal MRI

MRI has historically been unfavorable due to the sedation required to avoid motion artifact from the fetus. In recent years, imaging technique has improved and offered more comprehensive fetal neuroimaging for complicated posterior fossa and spinal defects. This provides a more detailed view of the brain, hindbrain, and neural tube defect anatomy.43,​44

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Prevention

Recurrence rates in subsequent pregnancies following a diagnosis of an NTD is between 2 and 4% with one affected sibling, and approximately 10% with two affected siblings.11,​45,​46,​47 Therefore, it is an important consideration when counseling women with potential future pregnancies on the ways in which this risk can be ameliorated.

NTDs are believed to be linked to a deficiency of folic acid in the early stages of pregnancy.48,​49,​50,​51 For this reason the Food and Drug Administration (FDA) approved fortification of many foods in the United States, however, these fortifications have not been definitively shown to be sufficient to prevent NTDs.52 The Centers for Disease Control and Prevention (CDC) advises that pregnant women should take folic acid 400 to 800 μg/d, and those with a strong family history or prior pregnancy with NTD should take 4 mg/d.

Open Neural Tube Defects

Anencephaly

A lethal condition in which the cranial neural tube fails to close between days 25 and 27 postconception, causing a lack of skin and bone over the cranial neural tissue.53

The neural tissue may be underdeveloped or destroyed due to the lack of protective covering.54 While often these pregnancies spontaneously terminate in utero, occasionally the fetus is born alive and may live for hours or rarely, days to weeks. The incidence of anencephaly in the United States was reported to be approximately 9.4/100,000 live births in 2001.55 Diagnosis: MSAFP and ultrasonography (see above).

Encephalocele

Protrusion of the meninges and cranial tissue through an opening in the skull, usually, occurs in the occipital region. Though less common than MMC, encephalocele may occur in up to 20% of cases.13 In some cases referred to as basal encephalocele, the defect may present as a craniofacial defect such as a nasal polyp causing recurrent meningitis or CSF leak.

Diagnosis: MSAFP, ultrasonography, postnatal evaluation ± MRI/computed tomography (CT) imaging.

Treatment: In both cases of a basal and occipital encephalocele, surgical excision, when possible, of the protruding mass and dural closure is the definitive treatment. Occipital encephaloceles often extensively involve the posterior cranial vasculature, and extreme caution must be taken when excising the mass. If extracranial neural tissue is present, it should be preserved.

Management of basal encephaloceles usually requires combined intracranial and transnasal approaches to avoid hemorrhage, CSF leak, and/or infection. The prognosis is largely based on the size of the sac, type of extracranial tissue, hydrocephalus, and accompanying pathologies.56

Myelomeningocele/Meningocele

Within the category of spina bifida aperta, a further division exists to describe the degree to which the defect has remained “open” with meningocele referring to a defect in the bony spine not affecting the underlying neural tissue and myelomeningocele referring to a defect that affects both the bony spine and underlying spinal cord.

Risk factors: All open NTDs have been linked to a low maternal folic acid level early in pregnancy.48,​49,​50,​51 The caudal neuropore closes at gestational day 28 (4 weeks), so it is crucial that folic acid

supplementation is begun at a very early stage of pregnancy. For this reason, pregnant women or women wishing to become pregnant in the near future are advised to take folic acid supplements to lower the risk of NTD formation prior to maternal knowledge that she is pregnant. Though no clear pattern of inheritance has been identified, the increased risk of myelomeningocele, within families with a history of NTDs suggests that multifocal genetic factors may be at play.45,​46,​47

Diagnosis: Frequently, the diagnosis of NTD is made prenatally. Elevated MSAFP between weeks 15 and 20 is highly suggestive of an open NTD. Ultrasound may also be helpful in the prenatal detection of spina bifida when the defect is large enough to been seen with sonographic imaging. Lastly, open NTDs cause an elevation of AFP in amniotic fluid. Amniocentesis between gestational weeks 13 and 15 is recommended for all women who have a family history or prior gestation resulting in an open NTD.11 Exceptions to these diagnostic methods include spina bifida occulta, which can be difficult to detect prenatally. Occasionally, neonates with closed spinal dysraphism will have a small tuft of hair in the lumbosacral region, or the diagnosis will be made later when accompanying conditions become symptomatic.

Treatment:

Meningocele and MMC must be closed immediately due to risk of infection in the neonatal period.

While immediate closure is preferred, it is unlikely that time to surgical correction has an effect on the neurologic sequelae of the NTD. In the case of MMC, as many as 65 to 85% of children will develop hydrocephalus requiring eventual placement of a shunting device. If present at birth, hydrocephalus may be corrected simultaneously with the

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NTD repair.57 Recently, a prospective study on fetal surgery for the correction of open NTDs has shown to decrease the rates of CSF leakage, Chiari II malformation, and hydrocephalus in fetuses known to have meningocele or MMC.58

Closed Neural Tube Defects

Tethered Cord Syndrome

The term “tethered cord syndrome” (TCS) is typically used to describe a set of symptoms that describe a tethered cord. These symptoms may occur as a result of several etiologies including but not limited to a fatty filum, filar lipoma, diastematomyelia, lipomyelomeningocele, or a secondary tethered cord from scarring after repair of any of the preceding or an MMC.59,​60 Clinical symptoms tend to fall into three categories: neurologic, urologic, and orthopedic. Neurologic symptoms may include pain in the back or lower extremities, lower extremity weakness, sensory deficits, and muscle wasting. Urologic symptoms include bladder dysfunction and frequent urinary tract infections. Orthopaedic symptoms include gait instability, limb-length discrepancy, and scoliosis. There are known higher rates of occurrence in patients with spina bifida, particularly with MMC.61,​62

Fatty Filum

A fatty filum is typically described as a low-lying conus medullaris as a result of a short and thickened filum terminale, resulting in many of the aforementioned constellation of symptoms, which taken together are called TCS.

Diagnosis: MRI imaging showing a low-ly- ing conus medullaris and thickened filum.59 Treatment: Lumbosacral laminectomy with division of the filum terminale is the definitive neurosurgical treatment for TCS when evidence exists that a shortened and thickened cord is the inciting factor.63

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It is particularly important to correctly distinguish the filum terminale from nerve roots of the cauda equina. This is accomplished using intraoperative electrical stimulation of thelowerextremitiesandanalsphincter.

Monitoring the onset or progression of symptoms is also an option in asymptomatic patients.64

Lipomyelomeningocele

Subcutaneous lipoma that passes through all of the layers of the lumbar dorsum and enters the dura through a defect and attaches to the spinal cord. This is the most clinically relevant of the subtypes of lipomyeloschisis due to the incidence of TCS or spinal compression. Half of patients with lipomyelomeningocele present with no neurologic signs or symptoms and only complain of a back mass. In the other 50% the symptoms resemble TCS such as bladder difficulties, leg pain, gait abnormalities, and paralysis.65 Diagnosis: MRI imaging.

Treatment: Treatment can involve correction of two possible etiologies: (1) TCS (2) spinal cord impingement due to increased volume of the intradural fat. Surgical intervention is frequently performed in infancy to prevent development of neurologic defects and timing is weighed against the risks of anesthesia in young infants. The goal of surgery is primarily prevention of worsening symptoms, but many patients do not experience clinically significant improvement if they are symptomatic at the time of presentation. Complete isolation of the lipoma via resection and invagination of the pial surfaces surrounding the lesion is utterly critical. Partial resection has a low rate of progression-free survival and does worse than historical cohorts of patients that were observed without surgery.

Spina Bifida Occulta

Often, spina bifida occulta is an incidental finding that is otherwise observed unless