Delayed myeloradiculopathy produced by spinal X-irradiation in the rat.

Rats were subjected to 3,500 r of X-irradiation in a single dose while breathing oxygen at 1 ATM pressure. Comparison was made between the delayed effects of irradiating thoracic, lumbar, and the cauda equina fields. The lumbar field involved the alpha-motoneurons and spinal roots supplying the sciatic nerve, while the cauda equina field involved these spinal roots but spared the alpha-motoneurons in the spinal cord. Thoracic irradiation produced paraplegia after an interval of 127-150 days. In the irradiated zone, the spinal cord was severely damaged, but the thoracic spinal roots were spared. Lumbar irradiation produced paraplegia after an interval of 83-211 days. In the irradiated zone, the alpha-motoneurons were largely spared, the spinal cord showed mild to moderate white matter damage, but the most severe damage was of the lumbosacral spinal roots. The posterior roots were more affected than the anterior. In longer interval cases the degeneration of the roots appeared to be due to focal devitalization. Evidence is advanced that root degeneration had been progressing for at least 4 weeks before the onset of paraplegia. In the cauda equina series the lumbosacral spinal root changes were similar to those in the lumbar series. This study indicates that different levels of the neuraxis have different degrees of susceptibility to X-irradiation. The thoracic cord appears more susceptible than the lumbosacral; the lumbosacral roots appear more susceptible than the thoracic; the posterior roots are more susceptible than the anterior. These findings may have relevance to the study of radiation damage in man, even though the dose schedule used in this experimental study differs greatly from that used for radiotherapy.