The Cervical Spine and Skeletal Dysplasia

William G. Mackenzie

Suken A. Shah

Masakazu Takemitsu

OVERVIEW

Cervical spine disorders in skeletal dysplasia patients are very common. The wide spectrum of pathology is not clearly described in the sparse literature on these disorders. Careful evaluation of the cervical spine in each individual, with attention to the most typical abnormality, should result in safe and effective care.

For example foramen magnum stenosis is present commonly in neonates with achondroplasia, but seen rarely in other disorders. The stenosis can be severe resulting in upper cervical cord compression with hypotonia, quadriparesis, and apnea. Other pathologic features in the development of the cranium, nasopharynx, and chest wall can aggravate the respiratory abnormalities and result in significant morbidity. Upper cervical spine instability is rare in achondroplasia but common in other forms of skeletal dysplasia such as spondyloepiphyseal dysplasia and its variants, pseudoachondroplasia, metatropic dysplasia, Morquio’s syndrome, and metaphyseal chondrodysplasias. Many of these disorders have osseous abnormalities such as odontoid hypoplasia or aplasia and os odontoideum as well as nonosseous abnormalities such as ligamentous hyperlaxity or soft tissue deposits. Upper cervical stenosis may be dynamic due to instability or stable due to a congenital abnormality or fixed translation. Rotatory fixation is uncommon. Children with metatropic dysplasia can develop severe cervical deformity after surgical decompression without fusion and inadequate immobilization.

Subaxial cervical spine disorders include kyphosis, stenosis, and instability. Kyphosis is seen in children with diastrophic dysplasia, camptomelic dysplasia, and Larsen’s syndrome. The kyphosis in diastrophic dysplasia can have associated spina bifida, an important factor when planning operative management. Kyphosis can develop after a laminectomy without fusion.

This chapter reviews cervical spine disorders associated with the various skeletal dysplasias. These are summarized in Table 31.1.

ACHONDROPLASIA

Craniocervical stenosis secondary to a narrow foramen magnum and upper cervical stenosis is seen in 60% or more of children with achondroplasia (Fig. 31.1) (1). The foramen magnum is smaller than normal at all ages, and this does not correlate with the circumference of the head (2). One-third of patients with craniocervical stenosis have symptoms of spinal cord compression. Symptoms including apnea, hypotonia, and quadriparesis can progress and be lethal (2). They, however, may be alleviated with suboccipital decompression. In a report by Wang et al. (3), patients who required suboccipital decompression had foramen magnum measurements of less than 20 mm in the sagittal diameter, 15 mm in the coronal diameter, and a cross-sectional area of 239 mm². The reported cross-sectional area is approximately one-half of the normal controls. The rate of foramen magnum growth is impaired during the first year of life (4). The incidence of sudden death in children with achondroplasia under 1 year of age is markedly higher than that in children in the general population (5). Assessment of the child should include a careful neurologic examination, a sleep study, CT, and magnetic resonance imaging (MRI) during infancy. Short-latency somatosensory evoked potentials are described as useful for neurologic evaluation (6).

Surgical decompression is effective for symptomatic patients (1,7,8). In addition to the bony decompression procedure, duraplasty is often required for adequate decompression in some patients who have fibrous constriction band. The patients have abnormal cerebrospinal fluid (CSF) dynamics and postoperative CSF leaks are common (8).

A few case reports of cervical instability in patients with achondroplasia have been published, but this is a rare finding (9,10).

Developmental subaxial cervical stenosis can occur in adults with achondroplasia (11). Myelopathy and/or radiculopathy may be presenting findings. Laminoplasty is often considered the treatment of choice for subaxial stenosis.

TABLE 31.1 Cervical Spine Disorders Associated with Various Skeletal Dysplasias

Diagnosis of Skeletal Dysplasia		Cervical Problems
Achondroplasia		Craniocervical stenosis (1, 2, 3, 4 and 5,7,8) Developmental cervical subaxial stenosis (11)
Pseudoachondroplasia		Atlantoaxial instability due to odontoid hypoplasia and ligamentous laxity (12)
Spondyloepiphyseal dysplasia		Atlantoaxial instability due to odontoid hypoplasia or os odontoideum and ligamentous laxity (13,14,15,16)
Spondylometaphyseal dysplasia		Atlantoaxial instability due to odontoid hypoplasia and ligamentous laxity (18)
Kniest’s dysplasia		Atlantoaxial or occipitoatlantal instability (17, 18, 19)
Metaphyseal chondrodysplasia		Atlantoaxial instability due to ligamentous laxity (21,22)
Metatropic dysplasia		Atlantoaxial instability due to odontoid hypoplasia or lateral mass defect of C2 (23,24)
Chondrodysplasia punctata		Atlantoaxial instability due to os odontoideum (Conradi-Hunermann type) (22) Subaxial canal stenosis (rhizomelic type) (25) Coronal clefts or hypoplasia of the vertebral bodies (26)
Diastrophic dysplasia		Kyphosis due to hypoplasia of vertebral bodies, hypotonia, and/or spina bifida (28,29) Atlantoaxial instability due to dysmorphism of the odontoid process(30)
Camptomelic dysplasia		Excessive lordosis and/or kyphosis (34,35)
Mucopolysaccharidosis
	Type 1 Hurler’s syndrome	Odontoid dysplasia (37) Abnormal soft tissue formation around the tip of the odontoid (37,38)
	Type 2 Hunter’s syndrome	Cervical canal stenosis with thickening of the soft tissue (dura) posterior to the odontoid (39,40)
	Type 4 Morquio’s syndrome	Atlantoaxial instability due to odontoid hypoplasia or os odontoideum with soft tissue thickening (41, 42, 43, 44, 45, 46, 47, 48 and 49)
Oculoauricular vertebral dysplasia		Atlantoaxial instability due to odontoid hypoplasia Occipital-C1 instability Occipitalization of C1 Failure of segmentation or formation of vertebrae (50)
Osteopathia striata		Kyphosis due to dysplasia and hypotonia (51)
Pyknodysostosis		Kyphosis due to C2 and/or C3 spondylolysis (52, 53)
Thanatophoric dysplasia		Platyspondyly Atlantoaxial instability due to odontoid hypoplasia (55)
Multiple epiphyseal dysplasia		Atlantoaxial instability due to os odontoideum (56)
Osteopetrosis		Spondylolysis (57) Foramen magnum narrowing (58)
Osteopoikilosis		Canal stenosis (59)
Cleidocranial dysostosis		Basilar impression with enlarged foramen magnum (60) Atlantoaxial instability (61)
Spondyloepimetaphyseal dysplasia		Atlantoaxial instability due to odontoid hypoplasia and ligamentous laxity (62)

PSEUDOACHONDROPLASIA

Odontoid hypoplasia or os odontoideum with ligamentous laxity is seen in patients with pseudoachondroplasia (12,13). Instability at the atlantoaxial level increases with age (12). The instability resulting in neurologic impairment requires operative intervention (Fig. 31.2).

SPONDYLOEPIPHYSEAL DYSPLASIA

Atlantoaxial instability due to ligamentous laxity, odontoid hypoplasia, or os odontoideum is commonly seen in patients with spondyloepiphyseal dysplasia (13,14). This instability may not be present at birth and usually presents later (Fig. 31.3). The incidence differs among the various types of spondyloepiphyseal dysplasia. The congenital form (spondyloepiphyseal dysplasia congenita [SEDC]) is associated with a very high incidence of instability (14, 15 and 16). Atlantoaxial instability is also commonly associated with spondylometaphyseal dysplasia.

KNIEST DYSPLASIA

Atlantoaxial and occipitoatlantal instability secondary to odontoid hypoplasia and lack of ligamentous integrity at the occipitoatlantal level are reported in Kniest dysplasia (17, 18, 19 and 20).

METAPHYSEAL CHONDRODYSPLASIA

Odontoid hypoplasia and ligamentous laxity with atlantoaxial instability is seen in the McKusick type of metaphyseal chondrodysplasia (21,22).

METATROPIC DYSPLASIA

Atlantoaxial subluxation due to odontoid hypoplasia is common in patients with metatropic dysplasia (23). Rotatory fixation can develop after decompression surgery at the C1-C2 articulation without fusion and inadequate
immobilization. C2-C3 instability secondary to defect of C2 lateral mass is reported (24). Cervical canal stenosis without instability is seen in these children.

Figure 31.1. Foramen magnum stenosis in a child with achondroplasia. The small sagittal diameter of the foramen magnum and spinal cord kinking are demonstrated on MRI. Hydrocephalus is highly associated with the foramen magnum stenosis.

CHONDRODYSPLASIA PUNCTATA

Atlantoaxial instability due to os odontoideum and/or cervical stenosis can result in cervical cord compression. Kyphosis with subluxation has also been reported (25). As the vertebrae are severely dysplastic (26), ventral fusion combined with posterior fusion is recommended for subaxial kyphosis.

DIASTROPHIC DYSPLASIA

Cervical kyphosis resulting from vertebral body wedging, ligamentous laxity, and/or spina bifida is seen in many children with diastrophic dysplasia (27, 28 and 29). Atlantoaxial instability is also reported (30). The kyphosis develops in the neonate, and spontaneous resolution of this malalignment is observed in the majority of these children before the age of 6 years if the kyphosis was less than 60 degrees at the first radiographic evaluation (29) (Fig. 31.4). The kyphosis can progress resulting in quadriplegia and death, but this is rare (31). Progressive kyphosis with a normal neurologic examination and no cord compression can be managed by posterior fusion surgery. Some correction of the kyphosis can be observed due to continued growth of the anterior column of the cervical vertebrae and tethering posteriorly by the fusion (Fig. 31.5 and 31.5). Severe kyphosis with spinal cord compression requires anterior decompression, strut grafting, and dorsal fusion. During exposure, care must be taken because spinal dysraphism is common.

Cervical spondylosis is seen in diastrophic dysplasia after the second decade (32). Long-term follow-up is essential.

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