Neurological Complications after 434 MHz Microwave Hyperthermia of the Rat Lumbar Region Including the Spinal Cord

The effects of prolonged endurance exercise on the neurological system in horses

Horses compete routinely in endurance-type activities. Many of the various pathophysiological mechanisms which arise during endurance exercise have implications for the health and function of the neurological system. The development of centrally-mediated fatigue is a normal homeostatic physiological event with several possible mechanisms. Development of pathophysiological phenomena such as cerebral oedema may be near-terminal events during or after endurance exhaustion. Cellular damage resulting in cytotoxic cerebral oedema may result from decreases in circulating blood volume (dehydration), blood pressure, oxygen, and glucose, or increases in brain temperature. Vasogenic cerebral oedema arises from changes in cerebral vascular perfusion, tone, and permeability. Increased vascular permeability results from increased brain temperature, poor vascular integrity due to severe dehydration, disseminated intravascular coagulation due to hemoconcentration or endotoxemia, and iatrogenic overhydration during therapy. Clinical signs of intracranial disease after endurance exercise include staggering, shaking, ataxia, paresis, poor tongue tone, facial twitching, collapse, recumbency, seizures, and death. Treatment should include active and aggressive cooling, intravenous polyionic fluids, acid-base imbalance correction, intravenous glucose and calcium supplementation, non-steroidal anti-inflammatory agents once the patient is better hydrated, intra-nasal oxygen therapy if practicable, and achievement of a non-dependent head posture to prevent jugular venous hypertension and further increases in intracranial pressure. The prognosis for central fatigue is good with appropriate supportive care, but the prognosis for successful treatment of cerebral oedema must be considered guarded at best. Prevention is critical and must be through incorporation of mandatory rest stops with sufficient length and veterinary monitoring to allow prevention and detection of exhaustion, excessive dehydration, and neurological signs. Management flexibility in shortening or postponing rides in hot and humid conditions, mandated use of aggressive cooling techniques, and more restrictive entry criteria for upper level Fédération Equestre Internationale races should all be considered as viable options for optimising the safety of endurance horses.

Heat distribution of polymerisation temperature of bone cement on the spinal canal during vertebroplasty

PURPOSE

In the last 15 years, vertebroplasty and kyphoplasty have become established operative procedures for treating osteoporotic vertebral-body fractures and vertebral bodies afflicted with metastases. These procedures are quickly performed with few personnel and material resources and have a low rate of complications. However, cases of neurological impairment are reported in the scientific literature. We analysed whether potentially harmful heat is radiated/conducted by the polymerisation temperature of polymethylmethacrylate (PMMA) bone cement in the spinal canal.

METHODS

We performed vertebroplasty on 25 vertebral bodies and measured the temperature distribution during polymerisation of bone cement within the spinal canal using heat probes placed in the respective areas. The vertebral bodies were located in a circulating water bath at 37°C.

RESULTS

During polymerisation of the bone cement, a temperature rise was measured. The peak temperature was reached after few minutes. Temperature curves differed; a maximum temperature of up to 43.16°C was detected for a few seconds only.

CONCLUSION

When vertebroplasty is performed correctly, there is no temperature development that could eventually damage the spinal cord or spinal nerves.

Effects of hyperthermia on the peripheral nervous system: a review

The present paper overviews the current knowledge about effects of hyperthermia at temperatures used in clinical oncology on the peripheral nervous system. From the experimental studies it may be concluded that the heat sensitivity of the nerve is determined by the sensitivity of the nerve vasculature. These studies show that in order to avoid induction of severe neuropathy, application of heat to the peripheral nerves should not be in excess of doses of 30 min at 44 degrees C or equivalent. Using modern equipment for application of loco-regional hyperthermia the incidence of even mild neurological complications is very low. In hyperthermic isolated limb perfusion (HILP) neurotoxicity is an often-mentioned side effect, this is in spite of the fact that in all studies a relatively mild hyperthermic temperature is used that, based on the experimental studies, should be well tolerated by the nerves and other normal tissues in the limbs. It seems that the neurotoxicity observed after HILP results from thermal enhancement of drug toxicity, very probably combined with effects of a high tourniquet pressure that is used to isolate the blood flow in the leg. Whole body hyperthermia (WBH), using anesthesia and appropriate monitoring to avoid cardiovascular stress is at present considered a safe procedure. Still in the recent past cases of neuropathy after treatment have been described. When chemotherapy, and notably cisplatin, is administered before or during hyperthermia there are several clinical and experimental observations that indicate a limited tolerance of the peripheral nervous tissue in such case. Also previous radiotherapy may limit the tolerance of nerves to hyperthermia, notably when radiation is applied with a large field size. Experimental studies show that combined treatment with radiation and heat leads to enhancement of effects of radiation (enhancement ratio approximately 1.5 at 60 min at 44 degrees C). A clear contraindication for the application of hyperthermia in patients is the presence of a neurodegenerative disease, such as multiple sclerosis. Vigilance is also required in the treatment of diabetic patients with hyperthermia, this based on experimental animal studies, but so far no clear clinical data are available.

Effects of hyperthermia on the central nervous system: What was learnt from animal studies?

Animal studies show that nervous tissue is sensitive to heat. Although inter-species variations may play a role, the data indicate that the maximum heat dose without obvious complications after localized hyperthermia in regions of the central nervous system (CNS) lies in the range of 40-60 min at 42-42.5 degrees C or 10-30 min at 43 degrees C. Expression of thermotolerance after a 'conditioning' heat dose was clearly observed in the spinal cord of rodents and the thermotolerance ratio's (ratio between heat doses with and without conditioning required to obtain a certain defined effect) were high, approximately 2. The thermotolerant state of CNS is shown to protect also against other types of injury as well: pre-treatment of rats with hyperthermia protected against spinal cord ischemic injury. During the rather long period required for temperature elevation which is inherent to WBH, some degree of thermotolerance may develop. The correlation between thermotolerance and hsp70 induction in CNS is obvious. Heat, at least if applied shortly after X-rays, enhances the response of nervous tissue to radiation. Data on the combined effects of X-ray irradiation and hyperthermia on rodent spinal cord clearly show that the radiation response can be enhanced with a factor of 1.1-1.3. There are no clear experimental data indicating an increase in adverse effects specific to the CNS after localized or whole body hyperthermia as a result of combined treatment with chemotherapy.

In vivo temperature profile of intervertebral discs and vertebral endplates during vertebroplasty: an experimental study in sheep

STUDY DESIGN

Experimental study of temperature profiles during vertebroplasty.

OBJECTIVES

To define accurate temperature profiles during vertebroplasty in an in vivo animal model.

SUMMARY OF BACKGROUND DATA

Controversy still exists whether vertebroplasty achieves its analgesic effect by stabilizing fracture fragments or due to thermal damage of sensory nerves inside the vertebral body. Furthermore, the energy release during cement polymerization may cause thermal injury to the spinal cord or intervertebral discs.

METHODS

A total of 4.0 mL PMMA was injected into three lumbar vertebrae of four skeletally mature mixed-bred ewes. Nine separate K-type thermocouples were placed into 12 vertebrae and their adjacent intervertebral discs. Temperature changes were recorded continuously before, during, and after the injection.

RESULTS

The highest peak temperatures were measured at the cranial endplate (mean 42.4 +/- 8.8 C) and in the center of the vertebral body (mean 43.3 +/- 6.9 C). Temperatures stayed above 40 C for approximately 2.5 minutes. Peak temperatures in the discs and at the dorsal wall (closest to spinal cord) did not exceed 41 C and 47 C and stayed above 40 C for approximately 1.5 minutes.

CONCLUSIONS

The intervertebral discs and the spinal cord do not seem to be in danger of thermal damage during vertebroplasty. The measured energy does not seem to be sufficient for the palliative effect achieved by vertebroplasty.

Neurological observations after local irradiation and hyperthermia of rat lumbosacral spinal cord

PURPOSE

Investigation of the effects of hyperthermia on the radiation response of rat lumbosacral spinal cord with respect to: (a) incidence of paralysis, (b) latency, (c) histopathology, and (d) tumor induction.

METHODS AND MATERIALS

Rat lumbosacral spinal cord with the cauda equina was single-dose irradiated with 15 to 32 Gy of x-rays. Hyperthermia for 30 min at a spinal cord temperature of 41.1, 42.3, and 42.6 +/- 0.4 degrees C was applied 5 to 10 min after irradiation by means of a 434 MHz microwave applicator. Animals were observed for 21 months while recording myelopathy and development of tumors.

RESULTS

The latent period for hind leg paralysis decreased with increasing radiation dose from 359 +/- 31 days (n = 9) after 20 Gy to 200 +/- 4 days (n = 5) after 32 Gy. Hyperthermia enhanced the radiation response of the lumbosacral spinal cord as evidenced by shortening of the latent period for paralysis and a decrease in the biological effective dose. After 20 Gy followed by 30 min 41.1 degrees C, latency was diminished to 214 +/- 16 days (n = 7, p < 0.001 vs. 20 Gy alone). The ED50 was 21.1 Gy, which was diminished to values between 16 and 17 Gy if radiation was followed by hyperthermia, giving a thermal enhancement ratio between 1.24 and 1.32. Histopathological examination of the spinal cord after combined treatment of x-rays and hyperthermia showed necrosis of nerve roots. Irradiation with 16, 20, 24, and 28 Gy (n = 77) alone led to tumor induction in 17 +/- 8% of the animals (pooled data). If followed by hyperthermia (n = 96), it was increased to 33 +/- 12% (p < 0.01). Most tumors induced by radiation and hyperthermia were sarcomas.

CONCLUSION

First, the radiation response of rat lumbosacral spinal cord was enhanced by heat. Second, latency for paralysis was shortened in the lower dose range. Third, no difference in pathology between x-rays alone or in combination with hyperthermia. Fourth, hyperthermia did increase radiation carcinogenesis.

Effect of hyperthermia on the central nervous system: a review

Experimental data show that nervous tissue is sensitive to heat. Animal data indicate that the maximum tolerated heat dose after local hyperthermia of the central nervous system (CNS) lies in the range of 40-60 min at 42-42 x 5 degrees C or 10-30 min at 43 degrees C. No conclusions concerning the heat sensitivity of nervous tissue can be derived from clinical studies using localized hyperthermia. The choice whether or not to exceed the critical heat dose, as derived from laboratory studies, in clinical practice is very much dependent on the clinical situation such as the anatomical site and volume of the tissue involved, and prior therapy. Data on clinical application of whole body hyperthermia (WBH) show that nervous tissue can withstand a slightly higher heat dose than after localized heating, which might be the result of developing thermal resistance during treatment. Expression of thermotolerance was observed in the spinal cord of laboratory animals. After WBH in man at a maximum between 40 and 43 degrees C for 6 h-30 min CNS complications were reported, but other complications seemed to be more life-threatening. Most studies indicate that impairment of the CNS after WBH was not due to direct heat injury to the brain or spinal cord, but was secondary as a result of physiological changes. Heat, at least if applied shortly after X-rays, enhances the response of nervous tissue to radiation. Neurotoxicity of chemotherapeutic drugs does not seem to be a limiting complication in hyperthermia if combined with chemotherapy, but only few data are available. The limited clinical experience shows that safe hyperthermic treatment of CNS malignancies or tumours located close to the CNS seems feasible under appropriate technical conditions with adequate thermometry and taking the sensitivity of the surrounding normal nervous tissue into account.