A POSITRON EMISSION TOMOGRAPHY STUDY OF PRIMARY ORTHOSTATIC TREMOR

Primary orthostatic tremor (OT), a clinical syndrome in which a rapid (14 to 16 Hz), regular lower limb tremor causes unsteadiness on standi ng, may be associated with a postural upper Limb tremor of similar fre quency. We used H-2 O-15 PET to analyze the abnormal pattern of cerebr al activation associated with the postural upper limb tremor in four p atients with primary OT. Patients had regional cerebral bloodflow (rCB F) measured during involuntary tremor while maintaining a posture with their outstretched right upper limb and again at rest. Tremor was ass ociated with abnormal bilateral cerebellar and contralateral lentiform and thalamic activation. These findings were evident on group analysi s of pooled PET data after transformation into standard stereotactic s pace and in single subjects when PET images were coregistered with str uctural MRI of the brain. At rest, cerebellar blood flow was significa ntly increased bilaterally in OT when compared with age- and sex-match ed controls. We have previously demonstrated similar abnormal bilatera l cerebellar activation in essential and writing tremors and conclude that abnormal bilateral overactivity of cerebellar connections is a co mmon feature of tremulous disorders.