SPINE AND SPINAL CORD: DEVELOPMENTAL DISORDERS

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CHAPTER 38 SPINE AND SPINAL CORD: DEVELOPMENTAL DISORDERS

Developmental disorders of the spine and spinal cord occur because of defects in the development and maturation of the nervous system. A basic understanding of embryology is therefore important in appreciating the nature and scope of these disorders. There are various ways of approaching and thinking about the developmental disorders:

As knowledge and understanding of these disorders continue to expand, the most reasonable approach is to integrate advances in genetic and etiological models within the traditional phenotypic clinicoanatomical framework. Developmental disorders can be classified anatomically, depending on whether they affect primarily the spinal cord and neural tissues or the spine and supporting structures (Table 38-1).

TABLE 38-1 Developmental Disorders That Can Affect Vertebrospinal Structures

Disorders Affecting the Spine Disorders Affecting the Spinal Cord and Neural Tissues Vascular Malformations
A variety of vascular malformations can affect the spine and spinal cord
Scoliosis Developmental cysts

There is a wide spectrum of developmental disorders affecting the spine and spinal cord, ranging in age at presentation, in clinical manifestations, and in severity. Although more likely to manifest in the pediatric population, some milder forms may manifest later in life as well. Neurologists for both pediatric and adult populations should have an awareness and appreciation of the range and manifestations of these disorders. The aim of this chapter is to provide a broad overview and appreciation of the developmental disorders affecting the spine and spinal cord, with a few select disorders covered in greater depth. A host of vascular malformations and metabolic conditions may affect the spine and spinal cord; they remain, however, beyond the scope of this chapter and are not discussed.

NORMAL DEVELOPMENT OF THE NERVOUS SYSTEM

The human embryonic period spans the first 8 postfertilization weeks and is classified on the basis of a morphological system. There are 23 distinct Carnegie stages, each stage covering a period of 2 to 3 days. This staging system facilitates an understanding of the timing and sequence of embryonic development. The fetal period spans the 9th week after fertilization to birth. Fetal age is thought of mainly as measurements, inasmuch as a similar morphological staging system is not available. The great majority of congenital malformations begin during the embryonic period.13

The neural tube is the embryonic structure that develops into the brain and spinal cord. Primary neurulation refers to the development of the neural tube from the neural plate. Complete development and closure of the neural tube occurs from days 17 to 30 after conception and spans Carnegie stages 8 to 12 in the embryonic period (Table 38-2). The caudal eminence is the continuation of the primitive streak and develops into the terminal portions of the notochord, somites, vertebrae, and hindgut. The neural cord arises from the caudal eminence and forms the caudal portion of the spinal cord. This process is secondary neurulation. The level of the junction between primary and secondary neurulation is at the level of the future second sacral vertebra. There is elongation of the previous neural tube together with formation of the lower sacral and coccygeal segments. This part of the development of the nervous system commences in stage 12 at 4 weeks and ends approximately at stage 20.26

TABLE 38-2 Important Stages in Primary Neurulation and the Formation and Closure of the Neural Tube

Development Stage No. Days in Embryonic Life Nervous System Development
8 17-19 Development of the neural plate
9 19-21 Development of the neural folds
10 22-23 First steps in the fusion of the neural folds: There are two initial sites of fusion: fusion proceeds bidirectionally from the lower medullary rhombencephalic site and in a caudal direction from the higher prosencephalic site. The fusions terminate in two neuropores: the rostral and caudal neuropores.
11 23-26 Closure of the rostral neuropore
12 26-30 Closure of the caudal neuropore

Based on a table from Lemire RJ: Neural tube defects. JAMA 1988; 259:558-562 Copyright © 1988 American Medical Association. All rights reserved; and on information from O’Rahilly R, Muller F: The two sites of fusion of the neural folds and the two neuropores in the human embryo. Teratology 2002; 65:162-170.

The vertebral bodies of the spine are generated from the mesenchymal cells. The process of gastrulation occurs at day 14 in embryonic life to generate the mesenchymal cells that will in turn develop into the head, cardiac, and the paraxial and lateral mesoderm. At 20 to 30 days in embryonic life, the paraxial mesoderm, in a process of somatogenesis, subdivides into spherical somite segments on each side of the spinal cord in a rostral-to-caudal direction. The somites first appear in stage 9; differentiation commences at stage 10. The somites mature and further subdivide into the sclerotome to form the vertebral bodies, the myotome to form the musculature, and the dermatome to form the dermis. The sclerotome undergoes a resegmentation process in which the caudal end of one somite links with the rostral end of the next somite to form a vertebral body.79

DEVELOPMENTAL DISORDERS AFFECTING THE SPINE

The developmental disorders affecting the spine that are covered in this chapter include scoliosis and a cluster of disorders originating primarily at the craniocervical junction. Some of the other generalized spinal disorders, including metabolic disorders, are covered in other chapters.

Klippel-Feil Anomaly

Clinical Features

Individuals with Klippel-Feil anomaly may be asymptomatic. Clinical manifestations can range from pain, through orthopedic or neurological manifestations, to a more widespread malformation disorder. The initial description was of a classical triad of shortening of the neck, limited range of neck movement, and a low posterior hairline. Affected individuals may complain of cervical pain and present with cosmetic deformities. The Klippel-Feil anomaly may be associated with spinal instability. This occurs if there are unstable fusion patterns or if stenosis and arthritis develop at the interspaces between the fused joints. The associated axial and spine anomalies include cervical or fused ribs, cleft vertebrae or hemivertebrae, and kyphoscoliosis.8

Neurological impairment depends on the presence and degree of neurocompression. For instance, a cervical rib can cause neurocompression with upper limb numbness and pain. Cervical cord compression can lead to a myelopathic picture. A traditional observation is the presence of mirror movements. This can be demonstrated by asking the patient to supernate/pronate a single arm. Mirror movements are those in which the other limb supernates/pronates at the same time.

The Klippel-Feil anomaly may be associated with other spinal developmental disorders, including syrinx, tethered cord, and split cord malformations (SCMs). Alternatively, the Klippel-Feil anomaly could be part of a widespread multisystem developmental disorder as well, in which other features include hearing deficits, congenital heart disease, and genitourinary manifestations.8

Basilar Impression

Definition

Basilar impression (or basilar invagination) occurs when there is an abnormal upward displacement of the basilar and condylar portions of the occipital bone. This leads to invagination of the foramen magnum into the posterior cranial fossa with associated translocation of the upper cervical vertebrae into this depression. We believe the terms basilar impression and basilar invagination are interchangeable, but some authors differentiate basilar impression from basilar invagination on the basis of differences in anatomical definitions and causation (e.g., acquired or primary).10,11 Some texts include platybasia in the spectrum of basilar impression. Platybasia is an abnormal flattening of the base of the skull with an abnormally obtuse angle between the plane of the clivus and the plane of the anterior fossa. This may occur together with basilar impression but is of no real clinical significance.

Basilar impression has traditionally been defined on the basis of a deviation from set anatomical and radiological parameters:

For instance, if Chamberlain’s line is the defining parameter, basilar impression is present when more than one third of the odontoid process lies above this line.10,12

Atlantoaxial Dislocation

Scoliosis

Management

It is important to treat or rule out an underlying spinal cord or brainstem disorder in a young patient with scoliosis. Evaluation includes the use of radiographs (Fig. 38-2) to chart the pattern of scoliosis and MRI to look for associated anomalies in the neural elements. Posterior fossa decompression for the Chiari-syringomyelia malformation can lead to an improvement in the scoliosis, especially if done early in the patient’s life. Initial treatment may involve use of a spinal orthotic brace as well. Spinal fusion surgery may ultimately be required for surgical correction of the scoliosis.18,19

DEVELOPMENTAL DISORDERS AFFECTING THE SPINAL CORD AND NEURAL TISSUES

The developmental disorders affecting the spinal cord and neural tissues broadly includes the craniospinal anomalies, including the Chiari malformation and syringomyelia, and a diverse range of spinal cord abnormalities, including NTDs and the various causes of spinal dysraphism.

Chiari Malformation

Clinical Features

As with the other developmental disorders, the Chiari malformation may be asymptomatic or, alternatively, may be symptomatic with a wide spectrum of clinical manifestations. A very common manifestation is headaches with or without cervical pain. The headache is typically a protracted occipital-suboccipital headache and is exacerbated by the Valsalva maneuver, postural changes, and coughing or straining. Another common complaint is of weakness and altered sensation, including paresthesia and dysesthesias. A myelopathy may manifest with the sensorimotor or sphincter disturbances. Individuals with Chiari malformations may present with ataxia and other cerebellar signs. Downward-beating nystagmus and other oculomotor disturbances may occur. Other features of brainstem dysfunction may result, including cranial neuropathies, neuro-otological symptoms, sleep apnea, and dysphagia.2124

Syringomyelia occurs in 32% to 74% of individuals with Chiari type I malformations. A cervical syrinx may manifest with upper limb neurological deficits, whereas a thoracic syrinx may lead to scoliosis. In addition, osseous anomalies of the base of skull, including basilar impression, may be seen with Chiari type I malformations. Chiari type II malformations are often associated with an NTD, other brainstem deformities, and hydrocephalus. Features of raised intracranial pressure are commonly the clinical manifestations of the associated hydrocephalus. The Chiari type I malformation is usually asymptomatic in childhood and tends to manifest in the second to third decade of adulthood. The Chiari type II malformation, however, is usually evident in childhood.21,23,24

Syrinx

Definition

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