SYMPTOMATIC AND ESSENTIAL PALATAL TREMOR .1. CLINICAL, PHYSIOLOGICAL AND MRI ANALYSIS

Palatal tremor (brief rhythmic involuntary movements of the soft palat e) apparently comprises two different nosological entities: essential palatal tremor (EPT) and symptomatic palatal tremor (SPT). The site of the abnormality in EPT is unknown, whereas SPT is believed to arise f rom a lesion of the brainstem or cerebellum (within the Guillain-Molla ret triangle). The clinical and physiological properties of these cond itions were studied in four patients with EPT and six patients with SP T Patients with EPT had normal cerebellar function, but those with SPT had clinical signs of cerebellar dysfunction. The palatal movements w ere consistent with activation of the tenser veli palatini muscle in E PT and of the levator veli palatini muscle in SPT. During sleep, EPT s topped, whereas SPT continued with only slight variations in the tremo r rate. The cycle of palatal tremor could not be reset by stimulation of trigeminal afferents in either EPT or SPT patients, and Valsalva's manoeuvre did not consistently affect the rhythm of the tremor in eith er group. The palatal tremor cycle exerted remote effects on the tonic electromyographic activity of the upper and lower extremities only in patients with SPT. These effects were present only on the side of the cerebellar signs (opposite the side with the enlarged inferior olive) in patients with a unilateral syndrome. Essential palatal tremor pati ents had only polysynaptic brain stem reflex abnormalities, whereas SP T patients had abnormalities of monosynaptic, oligosynaptic and polysy naptic brainstem reflexes. Magnetic resonance imaging showed no eviden ce of structural abnormalities in EPT patients, but SPT patients had a hyperdense signal of the ventral upper medulla (the region of the inf erior olive) on T-2-weighted images. These observations support the hy pothesis that EPT and SPT are two different diseases. In SPT cerebella r dysfunction ipsilateral to the palatal tremor may be due, in part, t o abnormal function of the contralateral hypertrophic inferior olive. The proposed basis of SPT is a disturbance of electrotonic coupling be tween the cells of the inferior olive induced by a lesion of the denta to-olivary pathway. Similar mechanisms could be responsible for postur al tremors in general. The pathophysiological basis of EPT remains unk nown.