Effects of 434 MHz microwave hyperthermia applied to the rat in the region of the cervical spinal cord

Hyperthermia was applied in the region of the vertebral column between the cervical vertebrae 5 and thoracic 2, using a ring-shaped applicator operating at a microwave frequency of 434 MHz. This region was focally heated, including spinal cord, vertebrae, intervertebral discs and nerve roots. In all experiments temperature was measured at a 'reference' thermocouple probe which was placed against one of the cervical vertebrae 6, 7 or thoracic 1. Temperatures inside the vertebral canal were measured separately and proved to be below the 'reference' temperature: at 42 degrees, 43 degrees, 44 degrees and 45 degrees C (+/- 0.1 degree C) respectively the temperature in the canal was 41.2 degrees, 42.3 degrees, 42.9 degrees and 43.2 degrees C (+/- 0.4 degree C). Temperatures in tissues close to the vertebrae (e.g. within 2 mm lateral to the vertebrae, in the region of the brachial plexus) did not differ significantly from the temperature inside the canal. The temperature inside the intervertebral disc was as high as the 'reference' temperature. Temperatures measured at other sites, e.g. in the oesophagus, rectally and in the cervical muscles 5 or 10 mm lateral from the vertebral column showed that these sites were only slightly heated. The effects of hyperthermia at temperatures inside the spinal canal ranging from 41.2-43.2 degrees C for 30-120 min were investigated. One day after treatment at 41.2 degrees C for 120 min or 42.3 degrees C for 60 min neither neurological symptoms nor deaths were observed. Minor neurological symptoms were observed one day after 75 min at 42.3 degrees C. The incidence and severity of the neurological symptoms (ranging from unco-ordinated use of the forelegs to paralysis) increased with increasing temperature and duration of the hyperthermic treatment. Thermal damage even resulted in lethality: 74 per cent of the rats that died did so between 2 and 42 h after treatment. The LD50 value at 60 days at 43.2 degrees C was 30 min, at 42.9 degrees C, 41 min, and at 42.3 degrees C, 92 min. In most rats with neurological symptoms after treatment, recovery from motor dysfunctions took place within about two weeks. Even severe neurological symptoms which did not lead to lethality recovered completely. At day 60 no neurological symptoms were observed