Evaluation of Metabolic Bone Disease in Children

Examination

The clinical evaluation and follow-up of skeletal dysplasias are often complex due to multiple and sometimes competing clinical priorities. These conditions are commonly multisystem disorders and thus are best managed in the context of multidisciplinary teams.

Despite the large number of skeletal dysplasias, the neurosurgical evaluation can be performed according to certain basic principles.

• Altered facial features: Blue sclerae and abnormal dentogenesis are well-known features in patients with osteogenesis imperfecta, but they are not seen in all forms of the disease. Frontal bossing, a low nasal bridge, and mid-facial recession are typical of achondroplasia. In patients with mucopolysaccharidoses, the facial features are coarse, particularly in the supraorbital region and around the nostrils and lips.
• Strength: Muscle weakness may be part of the underlying condition, further complicating the clinical examination.
• Tone examination: While the motor examination may reveal increased tone and deep tendon reflexes suggesting upper motor neuron disturbance, some younger children with bone dysplasias may show hypotonia and areflexia (common in achondroplasia) even in the presence of significant spinal compression.
• Joint mobility: Most children with metabolic bone disorders have generalized musculoskeletal involvement. Ligamentous laxity, joint subluxations, and joint deformity are common.
• Altered motor development: A history from the parents or the child’s physical therapist detailing the evolution of motor milestones and functional abilities is frequently more helpful than eliciting specific neurological signs.
• Bone fractures: Bone fragility with a tendency to repeated fractures in the appendicular skeleton is characteristic of osteogenesis imperfecta.

Signs related to specific compression sites

• Basilar invagination: Basilar invagination rarely presents with symptoms or signs in the young child. Symptoms tend to evolve in adolescence and may include occipital headache and episodic facial numbness. Swallowing disturbance may evolve but is usually a later sign. Tongue wasting or fasciculation are not infrequently seen.
• Foramen magnum stenosis: Signs of hypotonia and slowly progressive motor weakness in both the upper and lower limbs are certainly recognized. Central sleep apnea is an important presentation and has been cited as a cause of sudden death in this group. Formal respiratory evaluation will be required to distinguish central from obstructive apnea. Stenosis of the foramen magnum is particularly a feature of achondroplasia.
• Atlantoaxial instability: This condition is frequently asymptomatic. Neck pain, limited neck movements, and a history of motor deterioration (either acute or chronic) or clinical features of myelopathy should alert the physician to the possibility of atlantoaxial instability.

Laboratory Tests

• Diagnostic–disease specific: A variety of hematological, urinary, and genetic tests are now available to diagnose many of the metabolic bone disorders.
• Preoperative–routine: There are no specific laboratory tests beyond the standard preoperative investigations that are required for these patients.

Radiologic Tests

Radiological examinations should be requested according to the information required. There are three aspects to the radiological evaluation of the spine in metabolic bone disease, namely, the assessment of stability, the assessment of bony anatomy, and the evaluation of its effect on the neuraxis. These questions can be addressed by the following investigations.

Plain x-rays

• Flexion-extension cervical spine x-rays: Cervical spine x-rays performed in flexion and extension are the simplest and most effective means to assess stability. In the very young child where ossification may be incomplete and also in the presence of coronal or rotatory deformit,y plain x-rays are of limited use.

Atlantoaxial subluxation in Down syndrome: In flexion there is an increase in the atlantodental interval.

Atlantoaxial subluxation in Down syndrome: In extension the subluxation is reduced. Dynamic cine imaging of swallowing: Cine imaging of swallowing may be indicated as part of an evaluation of lower brainstem function when compromise of brainstem function is suspected.

Dynamic cine imaging of swallowing: Cine imaging of swallowing may be indicated as part of an evaluation of lower brainstem function when compromise of brainstem function is suspected.

CT scan

• 3D for bone anatomy: High-resolution, thin-slice CT scanning with reformatting is used to delineate the bony anatomy of complex malformations, particularly where there maybe anomalous segmentation.
• High-resolution CT scan with bone windows: This imaging is important in assessing suitability for internal fixation.
• CT angiography: CT angiography is needed to evaluate the course of the vertebral artery in complex deformities and thus aid surgical planning (4).
• Flexion-extension if plain x-ray inadequate: A CT scan can be performed in flexion and extension if plain x-rays are inadequate. However, the range of excursion achieved in a CT scan is usually less than that with plain x-ray.
• Rotational CT: A CT scan in left and right rotation may be indicated to assess the reducibility of any rotatory deformity.

3D CT of the craniovertebral junction in achondroplasia: The deep midline keel of bone is demonstrated. Due to the narrow foramen magnum, the distance between the vertebral arteries at the level of the atlas and foramen magnum is reduced.

MRI

• Assessment of effect on CNS: MRI is indicated when there is concern about the effect of the bony malformation on the neuraxis. Signal change in the spinal cord on T2-weighted images, evidence of hemorrhagic contusion, or thinning of the spinal cord may all occur as a consequence of either intermittent or chronic compression. Compression of the brainstem due to basilar invagination is typical of osteogenesis imperfecta and Hajdu-Cheney syndrome. Compression of the cervical-medullary junction due to foramen magnum stenosis is common in achondroplasia.
• Assessment of ligaments: Additional information such as ligamentous thickening and its effect on the spinal cord can be best evaluated by MRI. Particular anatomical patterns tend to accompany the various disorders. Ligamentous thickening, atlantoaxial instability, and abnormal vertebral body morphology are typical of the mucopolysaccharidoses.

Sagittal T2-weighted MRI of the spine of a child with Morquio syndrome: The spinal cord is compressed at the craniovertebral junction due to foramen magnum stenosis.

DEXA (dual energy x-ray absorptiometry)

• Bone density evaluation: This radiological technique is used to assess bone density. While DEXA is most commonly used in the evaluation of osteoporosis, in metabolic bone diseases such as osteogenesis imperfecta and Hajdu-Cheney syndrome it is used to monitor the course of bone-strengthening treatments such as with bisphosphonates.
• Useful evaluation of fusions: Knowledge of bone density can be helpful in assessing suitability for internal fixation techniques.

Radiographic evaluation specific conditions

• Achondroplasia
• Mucopolysaccharidoses
• Osteogenesis Imperfecta
• Larsen Syndrome
• Spondyloepiphyseal Dysplasia

Nuclear Medicine Tests

• Not routine: No nuclear medicine tests are regularly used in the evaluation of metabolic bone disease.

Electrodiagnostic Tests

• Limited use to establish baseline: Electrodiagnostic tests are of limited value in the routine evaluation of children with metabolic bone disease. SSEPs and MEPs should be monitored during spinal surgery for these conditions. Baseline recordings prior to surgical interventions may be of use, particularly in the presence of severe myelopathy.
• Sleep studies: Symptoms suggesting bulbar compromise should be specifically asked about, particularly if craniovertebral involvement is known or suspected. A history of poor quality sleep, snoring, or sleep apnea is common in children with craniovertebral anomalies. If present, respiratory sleep studies may be indicated (6).

Neuropsychological Tests

• Not indicated

Correlation of Tests

The timing of spinal interventions in cases of metabolic bone disease can be difficult to determine and requires careful correlation of the clinical presentation with the radiographic findings. Not all patients require intervention, even in the presence of very abnormal imaging. By example, the foramen magnum appears stenotic in most children with achondroplasia, and in children with the mucopolysaccharidoses cervical stenosis and deformity are almost inevitable. The imaging findings need to be interpreted in the context of the clinical history of the child.

• Symptoms and radiographs: Symptoms may remain stable or progress extremely slowly, and therefore a policy of regular clinical and radiological follow-up is entirely appropriate for many patients.