Preparation for Surgery for Metabolic Bone Disease in Children

Indications for Surgery

Spinal abnormalities are present in almost all children with metabolic bone disease, although not all will require surgery. Patients with abnormalities of the craniocervical region will commonly present to the neurosurgeon. The indications for surgery may be clinical and/or radiological:

Clinical

• Myelopathy: Limb weakness, hyperreflexia, and clonus are cardinal features of myelopathy. However, these conditions can be difficult to evaluate in the presence of bone dysplasia due to coexisting disease in the major limb joints. In young children hypotonia rather than hypertonia may signify the onset of myelopathy. Perhaps more important than the evolution of clinical signs is a history of progressive functional loss or regression of milestones.
• Neck pain: Both instability and craniovertebral deformity may cause pain. Radiation to the occipital region (C2 neuralgia) is sometimes misinterpreted as headache or migraine. Young children who cannot vocalize the sensation of pain may simply develop stiffness of the neck, “guarding” against pain on movement.
• Torticollis: Abnormal neck posture commonly indicates rotatory deformity at the craniocervical region. This may be congenital due to bony asymmetry (e.g., unilateral atlantooccipital assimilation or cervical hemivertebra) or acquired, as in rotatory atlantoaxial fixation.

Radiological

• Progressive narrowing of the spinal canal: On serial MRI, progressive narrowing of the spinal canal with effacement of the CSF spaces around the brainstem or spinal cord is seen.
• Signal change on T2-weighted MRI: Signal change may evolve slowly and in the absence of clinical symptoms. This finding indicates severe compression and risk of acute neurological deterioration.
• Progressive spinal deformity: Spinal kyphosis in the cervical spine or thoracolumbar spine is a feature of many bone dysplasias, e.g., mucopolysaccharidoses. Deformity may cause pain and threaten mobility.
• Atlantoaxial instability: Atlantoaxial instability is associated with clinical or MRI evidence of spinal cord damage.

Preoperative Orders

Imaging Appropriate imaging modalities are performed and reviewed to establish clear surgical objectives. Intraoperative imaging should be considered if there is a possibility for it being needed, for example, to ascertain the correct operative levels and establish an adequate anatomical reduction. Image intensifier screening will also be needed to assist in screw placement where instrumented fixation is to be performed.

• Steroid therapy: Steroid therapy is not routinely used, although it should be considered in cases of acute presentation or when severe compression is revealed by MRI.
• Immobilization of unstable cervical spine prior surgery: In the presence of craniovertebral instability, particularly in association with spinal cord compression and myelopathy, there is a significant risk for additional spinal cord injury during intubation, at the time of positioning for surgery, and during operative manipulations such as the passage of sublaminar wires or placement of pedicle screws. For this reason, placing the child in a halo-body orthosis at a preliminary procedure is advocated. Intraoperative fluoroscopy is used to ensure optimal reduction and alignment.  The definitive procedure can then be carried out without need for further manipulation of the craniovertebral junction.
• Traction: A period of preoperative halo traction is advisable in cases of basilar invagination (e.g., osteogenesis imperfecta, Hajdu-Cheney syndrome). This may reduce cervicomedullary compression and avoid the need for ventral surgical decompression (29). 

Anesthetic Considerations

• Instability of spine: Severe spine instability and limited neck mobility can present specific anesthetic challenges, especially for intubation. The anesthesiologist needs to be made aware of any concerns along these lines. As part of the anesthetic assessment, the comfortable range of cervical spine movement that can be tolerated by the patient is determined. This determination is a guide to the limits that should not be exceeded during induction of anesthesia.
• Chest deformity and ventilation: Chest deformity can affect ventilation during surgery and also postoperative management. This issue needs to be discussed in advance of surgery.
• IOM: The anesthetic technique has to be modified to facilitate intraoperative neurophysiological monitoring.

Devices to Be Implanted

Rigid instrumented fixation

• Occipital plates: Plates with midline screws are preferred in pediatric practice to take advantage of the increased thickness of the bone in the midline of the occiput.
• Screws: A selection of lengths of screws suitable for pedicle and lateral mass fixation should be available.
• Sublaminar wires: Flexible cables are extremely useful in pediatric practice, as bone mass may be insufficient for screw placement in young children and those with bony deformity. These cables can be used to secure bone grafts or attach to rods.
• Autologous bone graft: Rib, iliac crest, and calvarium have all been used. In children with bone dysplasias the ribs are often thin and the iliac crest is underdeveloped; thus full thickness calvarium is commonly preferred.

Ancillary/Specialized Equipment

• IOM: Surgery should be performed with monitoring of SSEPs and MEPs.
• Image intensifier: X-ray screening is required to ascertain optimal craniovertebral alignment and to verify screw placement.
• Neuronavigation: Neuronavigation is beneficial in some circumstances such as in transoral approaches in the presence of significant deformity (35).
• Operating microscope: An operating microscope is required in cases of transoral approach and to improve visualization during bone decompressions in infants and young children.