Musculoskeletal Disorders in Neonates



Musculoskeletal Disorders in Neonates


Raymond W. Liu and George H. Thompson


Musculoskeletal abnormalities of the extremities, spine, and pelvis are common in the neonate. Some are pathologic and others physiologic in origin from normal in utero positioning. The congenital absence of all or part of a limb, deformities of the feet or hands, and abnormalities of the spine are rarely diagnostic problems, whereas others, such as developmental dysplasia of the hip (DDH), may escape diagnosis even after repeated screenings by experienced examiners. Bone and joint infections in the neonatal period produce few of the diagnostic signs and symptoms present in the older child and require a high index of suspicion and careful diagnostic evaluation. When an infection is diagnosed early and prompt treatment rendered, the growth potential of the neonate yields an excellent prognosis for normal development and function.



Normal Embryology


Because many neonatal musculoskeletal disorders are congenital in origin, it is important to understand the basic aspects of musculoskeletal embryology. Prenatal development is divided into two major stages: the embryonic period, consisting of the first trimester, and the fetal period, consisting of the middle and last trimesters of pregnancy. The components of the musculoskeletal system differentiate during the first trimester; the second and third trimesters are periods of further growth and development.8,10,16,21 Abnormalities during the embryonic period produce congenital malformations, whereas the fetal period produces deformations and alterations in the configuration of essentially normal parts.



Embryonic Development


The early embryonic development of the musculoskeletal system is oriented around the notochord, a tubular column of cells running cranially and caudally along the long axis of the embryo. During the third week of gestation, the neural crests develop dorsally and on either side of the notochord. These crests fold over and are joined dorsally to produce the neural tube, from which the spinal cord and associated spinal nerves develop. At the same time, paraxial collections of mesodermal tissue develop on either side of the notochord and segment cranially and caudally into 44 distinct condensations called somites. From the primitive mesodermal tissue comprising the somites, the skeletal tissues, muscle, and dermal elements of the body develop.



Extremities


The upper and lower extremities develop from the limb buds. They become recognizable during the fourth week of gestation. These buds grow and differentiate rapidly in a proximal to distal sequence during the next 4 weeks. The cells differentiate into three segments: the dermatomes, which become skin; the myotomes, which become muscle; and the sclerotomes, which become cartilage and bone. By the fifth week, the hand plate forms, and mesenchymal condensations occur in the limbs. By the sixth week, the digits become evident, and chondrification of the mesenchymal condensations occurs. Notches appear between the digit rays during the seventh week. The failure of the rays to separate at this time results in syndactyly. Also during the seventh week, the upper and lower limbs rotate in opposite directions. The lower limbs rotate internally to bring the toes to the midline, whereas the upper limbs rotate 90 degrees externally to the position of the thumb on the lateral side of the limb.



Spine


The differentiation of the spinal column begins during the fourth week of the embryonic period. The somites first appear in the occipital region of the embryo; further development occurs simultaneously in a cranial-to-caudal direction. Dorsal and anterolateral migrations of the mesodermal tissue derived from the somites give rise to the connective tissue elements of the trunk and limbs. Anteromedial extensions of the somatic mesoderm migrate to surround the notochord, separating it from the neural tube and forming the primitive anlage of the vertebral bodies. The differentiation of the neural and vascular elements of the spinal column occurs simultaneously to somite development.


Definitive formation of the spinal column occurs from the fourth through the sixth week of gestation. The somatic mesodermal tissue surrounding the notochord differentiates into a less cellular and dense upper portion and a more dense and cellular lower portion. The somites cleave together, the lower portion of the superior somite joining with the upper portion of the inferior somite. The intervertebral disc develops at the site of the cleavage. The notochord, which is contained within the newly joined primitive vertebral bodies, degenerates, and those portions at the site of cleavage become the nucleus pulposus of the intervertebral disc. The neural arches and ribs develop from the dense portions of the somite, and the vertebral body develops from the less dense portions.


Chondrification begins in the primitive mesodermal vertebrae during the sixth week of pregnancy. It progresses rapidly to form cartilaginous models of the vertebral body by the end of the first trimester. Ossification of the cartilaginous models begins during the second trimester. The ossification of each side of the neural arch and of the body at each level proceeds separately. In the neonate, the ossified vertebral body and neural arches at each level are clearly visible radiographically, separated as they are by the nonossified synchondritic junctions. The ossification centers of the neural arches and body coalesce during the first 3 years of postnatal development.


The first and second cervical segments are embryologically and anatomically distinct from the remainder of the spinal column. The first cervical vertebra—the atlas—lacks the physical form characteristic of other vertebrae, having instead only a narrow anterior arch. This arch is not ossified at birth or during the neonatal period but is most often visible by 1 year of age.


The most striking feature of the second cervical vertebra—the axis—is the prominent odontoid process, derived from the caudal portion of the first cervical somite. The odontoid process joins with the remainder of the C2 body through synchondritic links with each neural arch and the vertebral body. The synchondrosis with the centrum lies below the level of the neural arches and may be radiographically confused with a fracture line; it closes by 3 years of age. The vertical synchondroses separating the centrum from the neural arches of C2 close by 7 years of age.


The radiographic evaluation of the neonatal spine requires experience. In the lateral view, the vertebral bodies are often notched at their waists and are trapezoidal rather than rectangular. In the anteroposterior view, the synchondritic links between the neural arches and vertebral bodies are not ossified and may give the false impression of a fracture. In the cervical region, the odontoid process may be confusing to those not familiar with the normal developmental anatomy of the region. Fortunately, fractures of the cervical spine are uncommon in the neonatal period and, when present, are usually associated with a suggestive history and other signs on physical examination.



Fetal Development


The appendicular and axial skeletons are pre-formed in cartilage. By the end of the first trimester, primary ossification centers are present in the long bones of the extremities. Further increases in length occur through endochondral growth at the ends of the long bones. This growth continues in the postnatal period until the end of adolescence, when growth plates close and the epiphyseal regions fuse with the remainder of the long bone.





Genetic Disorders


Genetic disorders that affect the musculoskeletal system may be divided into three categories: Mendelian, chromosomal, and multifactorial.






Congenital Limb Malformations


Congenital limb malformations are relatively common among neonates. Some are grossly obvious, such as a congenital amputation, whereas others are subtle and perhaps unrecognizable for years, such as a congenital proximal radioulnar synostosis (fusion). Congenital limb malformations are classified according to the parts that have been primarily affected by embryologic failure. Swanson and colleagues developed a seven-group classification system.25



These various malformations are caused by alterations in the organization of the limb mesenchyme; the time of the insult, the sequential development of the part, and the location of the destructive process determine the type of ensuing deformity. The presentation, diagnosis, and treatment of the more common congenital limb malformations are discussed in later sections.



Failure of Formation of Parts


This group of failures of part formation is subdivided into transverse and longitudinal deficiencies. A transverse deficiency is manifested as an amputation type of stump that is classified by naming the level at which the remaining limb terminates. All elements distal to the level are absent. Longitudinal deficiencies represent all other skeletal limb deformities. In identifying longitudinal deficiencies, all completely or partially absent bones are named. Bones not named are considered to be present. These deficiencies are separated into preaxial and postaxial divisions of the limb and include longitudinal failure of the formation of an entire limb segment, such as phocomelia, or of the preaxial (i.e., radius, tibia), central, or postaxial (i.e., ulna, fibula) components of the limb. The more common deformities include phocomelia, an absent radius (i.e., radial clubhand), and an absent fibula (i.e., fibular hemimelia).3,9,11









Spinal Defects


Errors in the embryologic derivation of the spine cause a number of congenital defects of the spine and spinal cord. These errors range in severity from isolated hemivertebrae to the complex errors of vertebral formation and segmentation associated with massive defects of the neural tube or spinal cord. When such errors result in asymmetric vertebral formation or produce asymmetric vertebral growth potential, structural spinal curves such as congenital scoliosis or kyphosis develop. Partial or complete failure of the formation of a vertebra (i.e., hemivertebra) or errors in the normal pattern of vertebral segmentation and recombination (i.e., unsegmented bars and trapezoidal, butterfly, and block vertebrae) may occur as single anomalies or be associated with other malformations of the osseous, neural, and organ systems. When vertebral defects are unbalanced and have growth potential, striking spinal deformities may occur, such as with a unilateral unsegmented bar with a contralateral hemivertebrae. The nature of the deformity depends on the growth potential of the abnormal segments and their positions in the spinal column. For example, lateral abnormalities produce congenital scoliosis, and anterior or posterior midline defects result in congenital kyphotic or lordotic deformities.


Malformations of the head and neck, especially the internal and external auditory apparatuses, maxillae, and mandibles, occur frequently in patients with high thoracic and cervical curves. The association of a short neck, low posterior hairline, and restriction in neck motion caused by the congenital fusion of cervical vertebrae represents Klippel-Feil syndrome. Renal anomalies, the congenital elevation of the scapulae (i.e., Sprengel deformity), impaired hearing, and congenital heart disease are common associated anomalies in affected patients. The VACTERL syndrome (vertebral defects, imperforate anus, cardiac anomalies, tracheoesophageal fistula, renal dysplasia, and limb deficiencies often in the radius), in association with limb defects, underscores the complex nature of the relationships between the rapidly evolving organ systems and the musculoskeletal system during the first trimester of pregnancy. Any child with suspected VACTERL should be screened for cardiac and renal defects.


The association of vertebral anomalies with neural tube or spinal cord defects is to be expected, given the intimate relationships of their embryonic development. Spina bifida occulta is the most common and least serious and myelomeningocele the most severe of such anomalies. Not all anomalies are easily identified on physical or radiographic examination. McMaster reported a 20% incidence of occult intraspinal abnormality in patients with congenital scoliosis.19 Possible anomalies included intradural and extradural lipoma, cysts, teratoma, spinal cord tethers, diplomyelia, and meningocele. In some instances, the spinal cord may be split by a bony, fibrous, or cartilaginous bar extending from the posterior aspect of the vertebral body to the vertebral arches. This condition, known as diastematomyelia, commonly occurs in association with defects at the thoracolumbar junction. Unless the spurs are ossified, they are not easily visualized by conventional radiographic techniques. The physical signs of underlying spinal dysraphism include hairy patches, midline dimpling, nevi, inequality in the length of the lower extremities or circumferential asymmetry, and asymmetry in foot size. Screening MRI of the spine is considered when there is a significant or draining defect, or if there are progressive changes in the neurologic exam.

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Jun 6, 2017 | Posted by in PEDIATRICS | Comments Off on Musculoskeletal Disorders in Neonates

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