A positron emission tomography study of primary orthostatic tremor

NEUROLOGY 1996;46:747-752

Primary orthostatic tremor (OT), a rapid (14 to 16 Hz), regular lower limb tremor causing unsteadiness upon standing, ^[1,2] is synchronous between homologous muscles and may be associated with a rapid upper limb postural tremor. ^[3-5] There has been debate whether OT is a variant of essential tremor (ET) or a separate entity. ^[5-7] OT and ET share a number of characteristics such as variable age of onset, the absence of a resting component, and EMG patterns that show either alternating or synchronous cocontraction of antagonist muscles. However OT, unlike ET, shows a high degree of synchrony between different muscles and cannot be reset by a peripheral nerve stimulus. ^[5] OT rarely responds to therapeutic agents (such as propanolol and alcohol) that have proven benefit in ET, but can be dramatically eased by clonazepam. ^[5] In addition, a negative family history is usual in OT and there is a small female preponderance of cases. These factors all suggest that OT and ET have separate etiologies.

PET with H₂¹⁵ O is a minimally invasive technique that can be used to measure regional cerebral bloodflow (rCBF) repeatedly in subjects while they perform different tasks. Areas of normal and abnormal cerebral activation may be delineated. In previous studies, we have shown that ET is associated with abnormal bilateral overactivity of cerebellar connections whereas voluntary wrist oscillation only activates the ipsilateral cerebellum. ^[8]

In this study, we examined the abnormal patterns of cerebral activation induced by the postural upper limb tremor of OT and contrasted this with previous findings in other tremulous disorders. The 10.65-cm axial field of view of our camera included the whole brainstem to ascertain the role of inferior structures in the generation of tremor. The cerebral cortex was only partially visualized. PET images of individual subjects …