Myelin basic protein and magnetic resonance imaging for diagnosing radiation myelopathy.

The identification of radiation myelopathy using biochemical assays and imaging techniques has not previously been accomplished but has clear clinical application. Measurement of myelin basic protein (MBP) in the cerebrospinal fluid (CSF) and visualization of the spinal cord using magnetic resonance imaging (MRI) gives a potentially accurate diagnosis of radiation myelopathy. Female New Zealand white rabbits were irradiated to the thoracic spinal cord with single doses of 15-45 Gy. Animals receiving higher doses (greater than or equal to 22 Gy) generally demonstrated an early paresis (4-8 weeks) that temporarily improved, and then progressed to complete paralysis by 14-18 weeks. MBP levels in the CSF became strikingly elevated to 100-1000 times the normal value. Subsequent, experiments in which rabbits were serially assessed for MBP levels demonstrated a transient elevation, which corresponded to the transient paresis, followed by dramatic elevations concurrent with the onset of paralysis. Magnetic resonance imaging (MRI) of the irradiated spinal cord showed a geographically distinct region of abnormality that corresponded to the radiation field. Histopathology demonstrated demyelination, focal astrocytosis, erythrodiapedesis, and perineuronal edema in the irradiated sections. It appears that MBP levels in the CSF reflect not only radiation-induced myelopathy but also transient demyelination, and that MRI may have the potential to indicate the region of damage.