Definition and Embryologic Basis

Diastematomyelia is a variety of spinal dysraphism in which there is a congenital splitting of some part of the spinal cord or, rarely, of more than one part. The name is derived from Greek words meaning "separation" and "marrow" (i.e., spinal cord). The condition is sometimes called diplomyelia ("double spinal cord"), because the medial aspect of the gray matter of the two hemicords often shows small ventral and dorsal "horns," suggesting duplication. But double innervation of the lower extremities has not been observed in humans.

From the point of view of the neurosurgeon, the important variety of diastematomyelia is that in which a median bony, cartilaginous, or fibrous septum covered by dura intervenes between the two hemicords. These, in most cases, reunite immediately caudal to it, thus preventing normal upward movement of the cord during physical exercise, particularly spinal flexion, and rendering the cord liable to be traumatized by direct pressure, traction or vascular injury. Occasionally, however, the cord does not reunite caudal to the septum or there is a significant gap between the septum and the point of reunion. Although a septum was found in less than half of a series of cases of split cord studied by computed tomography (CT) with metrizamide enhancement, a low conus medullaris was almost always present. The management of low conus (tethered cord) is discussed elsewhere.

Diastematomyelia is a common feature of myelomeningocele. Rarely, one hemicord is enclosed by the appropriate mesodermal elements and covered by intact skin, while the other lies exposed on the body surface (hemimyelocele).

There are conflicting explanations of the nature of the embryologic defect in diastematomyelia: (1) the developing anterior fissure completely splits the neural tube into two halves at the time when it should close; (2) the neural tube closes but is ruptured secondarily in the midline either by abnormal hydrodynamic pressures within the central canal of the spinal cord or by the development of clefts in its wall; (3) the persistence either of an accessory neurenteric canal or of the posterior blastopore divides the developing neural ectoderm into two parts. The known association of neurenteric cysts with diastematomyelia favors the third explanation.

Clinical Features

Diastematomyelia with septum (DWS) is more common in female than in male patients by about 3.5 to 1. Cutaneous manifestations of spinal dysraphism, comprising, in descending order of frequency, localized hypertrichosis (40 percent), subcutaneous lipoma, nevoid skin, atretic meningocele, and dermal sinus, were seen in almost every case of one large series. Hypertrichosis is significantly more common in diastematomyelia than in other types of dysraphism.

Two distinct types of neurological disturbance may occur in relation to diastematomyelia. First, DWS is frequently associated with the neuro-orthopedic deformities characteristic of occult spinal dysraphism at lumbosacral level, although they may be found even when the septum itself is as high as the thoracolumbar junction. These usually come to attention in infancy or early childhood and comprise various combinations of the following: dwarfing of one lower limb with muscle weakness and wasting; absence of the ipsilateral ankle jerk (and sometimes of the knee jerk as well); impaired sensation on the foot; pes cavus and clawing of the toes. This may be accompanied by hypoplasia of the lateral mass of a lower lumbar or sacral vertebra on the same side. Second, DWS may present in children of any age, whether or not they also exhibit unilateral dwarfing of a lower limb, as a syndrome of pain in the back and legs, especially related to physical exercise, which is often accompanied by deterioration of sphincter control and by spastic paraparesis. These symptoms, unlike the neuro-orthopedic deformity, are due to the traumatization of the cord by the tethering effect of the septum.

DWS, with or without neurological deficits, may be associated with congenital scoliosis. Among 60 patients with DWS seen in one large center, scoliosis was present in 36 (60 percent) and required correction in 31 cases. There are reports of patients developing paraplegia following surgical correction of congenital scoliosis, in whom an unsuspected DWS was later found. Prior identification and removal of the septum prevents this tragedy.

Radiographic Findings

Numerous radiologic abnormalities have been described in DWS (Table-1). Of these, the combination of spina bifida with vertical fusion of laminae is particularly frequent. The level of the septum lies between LI and L4 in most cases. In about half the patients the septum is bony and is visible in plain x-ray films. In other cases, however, it is fibrous or cartilaginous, and more sophisticated imaging techniques are required to establish the diagnosis. Until recently, CT with intrathecal metrizamide enhancement was the investigation of choice, revealing much anatomic detail with only modest exposure to radiation. Magnetic resonance imaging (MRI) now presents an excellent alternative, giving a particularly clear picture of the neural elements, including the presence of hydromyelia. However, the configuration of the bony spur may be less well defined, especially if there is an accompanying scoliosis.

TABLE -1 Vertebral Anomalies Associated with Diastematomyelia with Septum

Central bony spur (involving 1 to 3 vertebrae)
Broadened spinal canal
Spina bifida (local and/or remote)
Vertical laminar fusion
Fused ("block") vertebral bodies
Split vertebral body (butterfly vertebra)
Hemivertebral hypoplasia
Fused or deformed spinous processes
Scoliosis
Kyphosis or lordosis

In about one-third of cases of DWS the two hemicords are of unequal size; when this is so, the smaller hemicord may give off fewer nerve roots than the larger one-hence the unilateral neurological deficit.

The absolute indications for surgery for DWS are ( 1) the existence of a progressive neurological deficit and (2) as a preliminary to the surgical correction of scoliosis. Other indications are relative. Treated expectantly, many children with DWS who initially are asymptomatic eventually require operation on account of backache, painful or spastic lower limbs, or sphincter troubles. Generally, a child a median septum, once discovered, should be removed. Sudden deterioration to the point of total paraplegia is rare, as is the development of symptoms in adult life, but both have been documented. An exception to the rule of advising prophylactic surgery in a child would be the absence of symptoms or signs combined with a demonstration by neuroimaging that the lower end of the split in the cord lies at least one vertebral level below the lower limit of the dural sheath of the septum. If a conservative policy is adopted, children should receive annual neurological and urologic evaluations, including studies of their lower limb or pudendal somatosensory evoked potentials. Asymptomatic adults may be treated expectantly but should be cautioned about the risk of physical activity involving flexion of the spine.