Latent X-ray damage in the rat sciatic nerve results in delay in functional recovery after a heat treatment

The influence of X-irradiation on the sensitivity of the rat sciatic nerve to local hyperthermia was investigated. A 10 or 20 mm long segment of the nerve was irradiated intraoperatively using 50 kV X-rays. Hyperthermia (30 min at 45 degrees C), was applied to the irradiated part (over a length of 5 mm) of the nerve using a brass thermode. Functional damage to the nerve was assessed using the toe-spreading test, which mainly assesses the motor function of the sciatic nerve. Radiation alone (doses up to 70 Gy) did not lead to detectable damage for at least 90 weeks. Hyperthermia alone (30 min at 45 degrees C) resulted in complete loss of motor function. This function loss was transient and complete recovery took place in about 4 weeks. Recovery time was scored as the number of days between hyperthermia and the day on which 50% of the motor function had returned. Irradiation (35 Gy) of a nerve segment, which included the heated part, resulted in a delayed recovery from the heat treatment compared to controls (heat only). The time interval and sequence between irradiation and hyperthermia hardly influenced the recovery delay. The size of the irradiated nerve segment did influence the recovery delay. Irradiation of a 20 mm nerve segment led to longer recovery delays than irradiation of a 10 mm segment (a delay of 5-10 days and 1-5 days respectively). A dose-response relation for the irradiation-induced delay in recovery was observed when a large segment (20 mm) of the nerve was irradiated immediately after heat with a dose ranging from 5 to 40 Gy. The delay in heat recovery was dose-dependent below 20 Gy, but after radiation doses above 20 Gy the recovery delay remained almost constant.