The auditory startle response in the Steele-Richardson-Olszewski syndrome and Parkinson's disease

The startle response to an unexpected auditory stimulus was studied in eight patients with a clinical diagnosis of the Steele-Richardson-Olszewski syndrome (SRO), 11 patients with idiopathic Parkinson's disease (PD) and 12 normal subjects. The patients with PD were studied 'on' at the time of maximal effect of their treatment; five of these patients were also studied in their 'off' state without treatment. The auditory startle response was absent in three patients with SRO: in the remaining five the latency to onset of earliest electromyography activity (EMG) of the auditory startle response was delayed and few muscles (orbicularis oculi, sternocleidomastoid and rectus abdominis) were recruited in the response. In PD the auditory startle response was similar to that recorded in normal subjects, both in terms of the pattern of muscles recruited and the amplitude of the EMG responses, but the latency of responses in orbicularis oculi and sternocleidomastoid muscles were significantly delayed. This result was not influenced by treatment with L-dopa. In patients with SRO the finding of an abnormal startle response is consistent with loss of neurons in the lower pontine reticular formation. This region is intimately involved in the startle response in animal studies. In patients with PD the late auditory startle response might be related to withdrawal of facilitatory input to brainstem centres and reticulospinal pathways from basal ganglia. The similarity of the responses in patients when 'on' and 'off' suggests these pathways are not under potent dopaminergic control.