Myelocele and Myelomeningocele

      A type of spina bifida aperta, myelocele and myelomeningocele result from localized failure of closure of the neural tube. The neural folds remain in continuity with the cutaneous ectoderm at the skin surface, forming the neural placode. The mesenchyme destined to form the posterior elements remains trapped laterally, causing a wide spina bifida.

      Anomalies of vertebral segmentation or hemivertebrae are commonly present, resulting in short radius kyphoscoliosis in approximately one-third. Another 65% develop kyphoscoliosis as a result of neuromuscular imbalance.

      The spinal cord is always tethered. Aside from fetal ultrasound, imaging studies usually are not performed because early surgical closure of the open spinal cord is most critical to avoid further damage to the neural elements. The patients typically have a stable neurologic defect unless other associated anomalies cause problems. If preoperative imaging is done, a number of features require definition, such as the location of the neural placode, fibrovascular tethering band(s), ventral and dorsal roots, dorsal root entry zones, and any nerve roots crossing in an aberrant fashion. It may be difficult to determine whether a low-lying placode in a dorsal meningocele is actually tethered or simply positioned in the meningocele as a result of more cephalad tethering by the fibrovascular band. Endnote Possible associated anomalies include syringohydromyelia (found in 40% of Arnold Chiari II patients), diastematomyelia (30%-45% of myelomeningoceles) lipoma, arachnoid cyst, dermoid, or epidermoid.

      Imaging of the postoperative myelomeningocele spine is usually performed because of deterioration of neurological function. When imaging these patients, one must look for postoperative hematoma or complicating infection, compression of cord by residual or recurrent tumor, cord ischemia or infarction, myelomalacia, arachnoid cyst, diastematomyelia, and re-expansion of a syringohydromyelia. T2-weighted coronal images are helpful to rule out occult diastematomyelia. Focal cord narrowing may occur if the dura was pulled too tight at surgery. Symptomatic re-tethering is a diagnosis of exclusion. Although retethering is a clinical diagnosis, MR signs of retethering by scar include angulation or kinking of the cord, a straightened or taut-appearing cord, or a direct cord interface with dural or epidural structures. Because of the spectrum of abnormalities associated with myelomeningoceles, imaging of at least the lumbar and thoracic (and possibly the entire) spine should be performed in the postoperative patient. If the spine is normal, imaging of the brain should be considered to rule out hydrocephalus or shunt malfunction. 

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