Lhermitte's sign following head and neck radiotherapy

Lhermitte's sign is an uncommon sequel of radiotherapy to the cervical spin al cord. Although the exact mechanism underlying its occurrence remains unc lear; it is felt to be the result of a temporary interference with the turn over and synthesis of myelin, leading to focal demyelination. We have under taken a detailed analysis of the radiation delivered to four patients who d eveloped 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 i nteresting questions regarding the dose-response relationship. In particula r, we find that calculations of biologically effective doses are predictive of a late rather than an early normal tissue response. The onset of sympto ms after irradiation was apparent in all four patients within 4 months, wit h resolution in all being complete within a further 6 months. The recogniti on 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 un dergoing head and neck irradiation.