Effects of previous x-irradiation on the cellular response of nervous tissue to injury

Radiation-induced conduction block: Resolution following anticoagulant therapy

Neurophysiologic studies documented proximal conduction blocks in a patient harboring a delayed radiation-induced brachial plexopathy. Since anticoagulants have been reported to be beneficial in radiation-induced neuropathies, the patient was started on acenocumarol. After 3 months of treatment there was significant improvement of clinical deficits, which correlated with resolution of conduction blocks. This observation suggests that ischemic nerve injury leading to disruption of the conduction properties of motor axons contributes to the pathogenesis of delayed radiation-induced peripheral nerve injuries.

Effects of External Beam Radiation in the Rat Tibial Nerve after Crush, Transection and Repair, or Nerve Isograft Paradigms

INTRODUCTION

In head and neck surgery, radiation therapy is often administered to an injured nerve. Previous studies have examined the effects of either preoperative or postoperative radiation on nerve regeneration in rodents. In these studies, histomorphometric analysis was performed up to 8 month postoperatively. Given the exceptional neuroregenerative capacity of rodents, significant differences in nerve regeneration may go undetected if nerves are evaluated at such distant postoperative time points. This study is designed with a more appropriate model and investigates the effects of radiation after three common nerve injury paradigms.

METHODS

Sixty-four Lewis rates were randomized to 8 groups corresponding to uninjured, tibial nerve crush, transection and repair, or reconstruction with isografts. Half of the animals in each of these paradigms (n = 8 per group) were treated with 10 Gy of external beam radiation to the site of nerve injury at 7 days postoperatively. On postoperative day 28, functional recovery and histomorphometric assessment was performed.

RESULTS

For a given paradigm of nerve injury, no significant differences in nerve fiber number, neural density, neural debris, or fiber width were noted between the control and radiated groups, and radiation did not affect functional recovery.

CONCLUSION

Radiation had no discernible effect on nerve regeneration or functional recovery in the rodent nerve injury models studied. All assessments were made at time points suitable for detecting differences in nerve regeneration between groups. These findings suggest that administration of radiation to fields containing injured peripheral nerve is unlikely to adversely affect functional outcomes.

Long-term nervous system damage from radiation of the spinal cord: an electrophysiological study

A group of 13 patients suffering from Hodgkin's disease who had undergone chemotherapy and radiotherapy (above and below the diaphragm) approximately 10 years earlier was studied. The total chemotherapeutic dose was similar for all patients; the radiotherapy dose, however, was standard for 7 patients, while the other 6 received much higher dosages over limited regions of the spinal cord. Although most of these patients appeared normal both clinically and on magnetic resonance imaging, a neurophysiological study was performed to determine whether there was any involvement of the central or peripheral nervous system. Motor conduction velocity and sensory conduction velocity were measured in the lower limbs as well as spinal- and scalp-recorded somatosensory evoked potentials (SEPs) in response to stimulation of the posterior tibial and sural nerves at the ankle. In addition, motor evoked potentials were recorded from the upper and lower limbs during cortical stimulation. All neurophysiological data were normal in patients who had received a standard radiation dose, while most of those who had been exposed to higher doses showed altered cortical SEPs and a slowing of central conduction time (D10-P1). Thus even though they were asymptomatic, these patients appeared to have sustained CNS damage, mainly at the level of the spinal cord.

Effect of ionising radiation on the axon reaction of mouse anterior horn motor neurons. A histological and immunocytochemical study using a monoclonal antibody to neurofilament protein

The changes taking place in irradiated central nervous tissue prior to the onset of delayed radionecrosis are poorly understood, but functional abnormalities occurring during the latent interval after irradiation are likely to be of importance. In order to investigate functional disturbances in neurones during this period, unilateral sciatic nerve crush was performed in mice following sub-lethal X-irradiation of the lumbar spinal cord. Alterations in the axon reaction of anterior horn cells were studied using a monoclonal antibody to neurofilament protein. With irradiation immediately prior to crush, the normal, well-defined increase in perikaryal neurofilament protein was significantly diminished, although there was no concurrent radiation necrosis and no alterations were seen in contralateral neurones with intact distal axon processes. The effect was more marked in neurones irradiated one month prior to nerve crush, and in the non-irradiated nerve crush region regeneration was delayed, with diminished neurofilament protein in the regenerating axons. These observations indicate that ionising radiation can progressively impair the ability of neurones to synthesise neurofilament protein during distal axon regeneration. This may result from inadequate repair of radiation induced DNA strand-breaks, but may also follow more generalised damage to protein transcription enzymes and RNA metabolism.

Electrophysiologic evidence of subclinical injury to the posterior columns of the human spinal cord after therapeutic radiation

Spinal somatosensory conduction velocity (SSCV) was indirectly estimated from cerebral evoked potentials in 15 adults who had received therapeutic radiation (RT) (2000-4380 rad) to the thoracic spinal cord during treatment for lung cancer, and in 15 age-matched normal controls. Thirteen of the patients had also received 4400-5500 rad to the supraclavicular fossae. One-way impulse conduction time in the arm, estimated from F-wave latency, was prolonged in the patients as compared to controls (12.0 +/- 1.2 versus 10.4 +/- 1.0 msec; P less than 0.001) but conduction time in the leg was similar in the two groups (22.4 +/- 2.4 versus 22.0 +/- 2.5 msec; P less than 0.1). SSCV was significantly slower in the patient group (37.9 +/- 13.9 versus 54.5 +/- 12.9 m/sec; P less than 0.001) whereas supraspinal latency (cervical cord to cortex) was identical (5.5 +/- 0.9 versus 5.5 +/- 0.8 msec; P less than 0.1). SSCV in the patient group was not related to total RT dose (r = 0.15; P = 0.2), but was correlated with both treatment time and number of fractions (r = 0.49 and 0.43; P = 0.003 and 0.007, respectively). These findings suggest that RT may produce subclinical spinal cord dysfunction even at conventional dosage schedules, and that it may be possible physiologically to monitor the myelopathic effects of RT in individual patients.

Late radiation damage to the central nervous system: a radiobiological interpretation

The evidence in support of the two opposing views as to the pathogenesis of late radiation damage to the central nervous system after therapeutic doses of radiation are briefly reviewed. Analysis of the radiation dose levels used by various investigators holding opposing views as to pathogenesis of CNS lesions they reported, suggests that both groups of protagonists were in part correct. Both vascular and glial changes are important in the development of late radiation damage to the nervous system; the preponderance of one or the other type of cell damage depends upon the radiation dose. Vascular effects occur at lower dose levels but after a longer latent period than effects mediated through damage to neuroglia. Current radiobiological concepts and their possible importance in establishing the radiosensitivity of the central nervous system are discussed. The origins of both vascular and neuroglial cell mediated changes are discussed in terms of the reproductive loss of radiation damaged cells. This view is supported by some recent experimental findings. The radiobiological basis of some recently investigated treatment modalities aimed at improving the control of intracranial gliomata by radiotherapy are also considered.