Other Disorders of the Skull and Spine Dwarfism

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Although dwarfism is primarily a defect of the musculoskeletal system, central and peripheral nervous system manifestations are encountered in a variety of these disorders. The most frequent neurological manifestations are a result of impingement of the nervous system as a result of narrowing of bony foramina. For example, in some dwarfism syndromes, such as achondroplasia, narrowing of the foramen magnum can obstruct CSF flow and result in hydrocephalus. In achondroplasia, the development of clinical symptoms usually requires a narrowing of the foramen magnum to under -3 to 4 SD (standard deviations are available in radiology texts). Thanatophoric dysplasia, which is due to mutations in the same genes as achondroplasia (see earlier), has a unique neuropathology. y The temporal lobes are distorted by deep radial fissures arranged as spokes of a wheel (^Fig. 28-11 ). Microscopically, the temporal lobes, particularly medially, are disorganized, often showing polymicrogyria. Polymicrogyria may also occur outside the temporal lobes.

The diagnosis of thanatophoric dysplasia is important for management decisions and for the purpose of genetic

Figure 28-11 Thanatophoric dysplasia. A ventral view of the brain from a 20-week-old fetus with thanatophoric dysplasia. Radially oriented, deep fissures in the medial temporal lobe replace the normal smooth surface at this age and are pathognomonic for this disorder.

counseling. Clinically, there are short limbs, a narrow thorax with short ribs, a short spine with flat vertebral bodies, and a large-appearing cranium with a flat nasal bridge. There are two subtypes: the more common variety (type I) with bowed bones but rarely a cloverleaf skull, and a second form (type 2) with straight bones but with a complex craniosynostosis giving rise to the cloverleaf skull. Hydrocephalus is common with the second type of craniosynostosis. Both forms are associated with mutations in FGFR3.

Achondroplasia can be associated with a number of neurological complications. y Stenosis at the level of the foramen magnum can cause a high cervical or lower medullary myelopathy. In case of lower medullary myelopathy, apneic spells or respiratory insufficiency can result; high cervical myelopathy can give rise to long tract symptoms, including frank quadriparesis. Stenosis of the spinal canal with myelopathy or radiculopathy can occur at any level. Careful assessment for lower extremity hyperreflexia, clonus, and Babinski's sign is important for the early recognition of a myelopathy. Surgical decompression may become necessary in these cases. The development of hydrocephalus in achondroplasia is possible due to stenosis at the level of the foramen magnum, although this occurrence is not very common. A milder degree of hydrocephalus has been postulated to be due to impaired CSF resorption because of diminished venous return from the cranial compartment. A degree of macrocephaly, however, is normal in achondroplasia, and head circumference measurements should be compared with head circumference charts specifically corrected for achondroplasia. The large number of skeletal dysplasias, many of which have secondary neurological complications, are beyond the scope of this chapter. The reader is referred to reference 90 for a complete discussion.

SKULL BASE AND NECK

Malformations and deformities of the skull base can be part of a number of conditions, including the craniosynostosis syndromes (short and steep cranial base), neurofibromatosis I (aplasia of the sphenoid wing), Chiari types I and II (platybasia, flat cranial base), osteogenesis imperfecta (basilar impression, invaginated cranial base), and Hajdu-Cheney syndrome (basilar impression), as well as other disorders. They can also occur in isolation or in a familial context. Other conditions such as Paget's disease, rickets, rheumatoid arthritis, and hypoparathyroidism have to be considered in the differential diagnosis. Nuchal pain and vertigo may be early and nonspecific complaints. If they are present, the primary position downbeating nystagmus of craniocervical junction origin may give a diagnostic lead in this situation. The neurological syndrome of basilar impression, including platybasia and the more extreme convexobasia, can begin insidiously with increasing spasticity in the lower extremities, ataxic gait, and loss of proprioception also in the upper extremities. In addition, lower cranial nerve function may become impaired late in the course, including dysphagia. y The neck may be short and cervical motility impaired; in conjunction with the nuchal pain, the presentation may be neck stiffness or torticollis. Symptoms and signs of associated conditions such as the Chiari malformations and syringomyelia can complicate the picture. Sagittal CT reconstruction plus sagittal MRI is the optimal imaging modality combination to confirm the diagnosis. Surgical decompression at the foramen magnum is the treatment of choice, occasionally requiring additional laminectomies of C2 and C3 with cervico-occipital fusion. Platybasia (defined as the angle between planes of the anterior cranial fossa and the clivus being greater than 140 degrees) in isolation is generally asymptomatic.

Congenital fusion of the cervical vertebrae is known as the Klippel-Feil anomaly. Presumably this reflects a failure of segmentation rather than a secondary fusion. This anomaly can be part of a number of syndromes (such as Turner's, Noonan's, and Wildervanck's syndromes) but can also occur sporadically or be inherited in isolation as an autosomal dominant or autosomal recessive trait. Vertebral fusions can extend beyond the cervical spine, but most commonly they are restricted to levels C2 and C3 or C5 and C6. Clinically, there is a short neck, low posterior hairline, and limitation of neck movements, particularly rotation and bending to the sides. Atlanto-occipital anomalies are frequently associated, and one of the major reasons for associated morbidity. Kyphosis and scoliosis are often associated, as is spina bifida occulta in some cases. y Many patients also have a variety of genitourinary anomalies, including unilateral renal agenesis. '92' Hearing impairment can be noted in about 20 percent of patients. Other skeletal malformations such as Sprengel's deformity can be associated. The main neurological complications result from craniocervical instability with the possibility of spinal cord compromise. It has also been noted that patients with the Klippel- Feil anomaly may have mirror movements on neurological examination'93] ; the reason for this observation is not clear.

Occipitalization of the atlas or assimilation of the atlas refers to the fusion of the anterior arch and ring of C1 with the foramen magnum. A Chiari type I malformation, Klippel-Feil anomaly, achondroplasia, or syringomyelia may be associated. Atlantoaxial instability (atlas-dens interval greater than 3 mm) can occur in rheumatoid arthritis and also in a number of genetic conditions and syndromes,

most notably Down's syndrome. It is recommended to screen trisomy 21 patients at 5 and 10 years for atlantoaxial instability and, if the condition is present, restrict potentially harmful physical activities.

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