Lhermitte's sign following head and neck radiotherapy.

Lhermitte's sign is an uncommon sequel of radiotherapy to the cervical spinal cord. Although the exact mechanism underlying its occurrence remains unclear; it is felt to be the result of a temporary interference with the turnover and synthesis of myelin, leading to focal demyelination. We have undertaken a detailed analysis of the radiation delivered to four patients who developed the sign after irradiation for malignancies of the head and neck. Our data support the view that radiation dose is crucial to its development, but calculations using the linear-quadratic radiobiological model raise interesting questions regarding the dose-response relationship. In particular, we find that calculations of biologically effective doses are predictive of a late rather than an early normal tissue response. The onset of symptoms after irradiation was apparent in all four patients within 4 months, with resolution in all being complete within a further 6 months. The recognition of this benign transient form of radiation-induced paraesthesia and its differentiation from the later onset, progressive and unremitting symptoms associated with radiation myelopathy is essential in reassuring patients undergoing head and neck irradiation.