Effects of x-radiation on the spinal cord: an experimental study of the morphological changes in central nerve fibres.

The morphological changes in central nerve fibres after irradiation have been studied in the spinal cord of young adult rats exposed to 100-6 000 rad doses of 250 kV x-rays using the technique of single fibre teasing as well as conventional light and electronmicroscopic examination of cord sections. Two groups of degenerative changes were found in myelinated fibres. The first consisted of breakdown of paranodal myelin and nodal widening. These changes were found as early as two weeks after exposure to 500-6 000 rad doses and increased in frequency with dose and time in the first two months after irradiation. Paranodal myelin breakdown was less frequent after two months but nodal widening was more prominent. Increasing numbers of thinly myelinated fibres were found after three months suggesting that paranodal demyelination was followed by remyelination. This early group of changes confined to the myelin sheath provides a possible pathological basis for the self-limited sensory syndrome which sometimes occurs within a few weeks or months of irradiation of the spinal cord in man. The second type of change which was also first detected as early as two weeks after irradiation and which appears to be unrelated to the first, consisted of random Wallerian-type degeneration of fibres of all calibres in the spinal white matter. The number of affected fibres was initially small but increased with time and there was no clear dose relationship. This random fibre degeneration is probably the forerunner of the later-occurring delayed radionecrosis of the spinal cord which other workers have found to occur with a latent period of up to twelve months after exposure to doses exceeding 1900 rads to the rat.