How Salient is the Silent Period? The Role of the Silent Period in the Prognosis of Upper Extremity Motor Recovery After Severe Stroke

Abstract

Transcranial magnetic stimulation (TMS) has been successful in the prediction of motor recovery in acute stroke patients with initially severe paresis or paralysis of the upper extremity. Motor evoked potentials (MEP) appear to have a high specificity but a rather low sensitivity with regard to motor recovery. The silent period (SP) has been proposed as an additional factor to the MEP for predicting motor recovery that might optimize the sensitivity of TMS. The authors reviewed the literature and case series focusing on the additional value of the SP to the MEP for predicting poststroke hand motor recovery. Studies that have analyzed the SP for predicting poststroke motor recovery have rather inconsistent results and suffer from heterogeneity in technical methods, methodology, and patient characteristics. In most studies, prolonged SPs have been found immediately after stroke, whereas in the (sub)acute phase thereafter, different patterns of SP duration have been found. These differences are thought to be related to stroke localization, though contraction-induced reduction phenomena and recovery-related intracortical phenomena may also be responsible. Although the SP might be used to identify clinically silent or minor strokes, in acute stroke patients with initial severe paresis or paralysis, the SP seems to have no additional value to MEP for predicting poststroke motor recovery. Nevertheless, the SP (poststroke-reduced SPs and contraction-induced inhibitory phenomena) has been proposed as a prognostic factor for poststroke spasticity. This review emphasizes the significance of the SP in predicting poststroke motor recovery and spasticity. Although the relation among the SP, recovery-related intracortical phenomena, and spasticity remains unclear, a neurophysiologic model underlying the SP is discussed. However, more research is needed on the value of the SP for predicting poststroke spasticity.