Reaction Time in the Submental Muscles of Normal Older People

To the Editor: To determine the influence of normal aging on the swallowing reflex, we examined premotor time (PMT) in the submental muscles of 23 healthy volunteers (11 men, 12 women) aged 21 to 92 (mean ± standard deviation (SD) = 54.4 ± 22.8). Subjects were asked to sit on a wooden chair. After appropriate areas were rubbed vigorously with alcohol to minimize interelectrode resistance, a pair of surface silver/silver chloride electrodes was placed on the skin overlying the submental muscles of the neck, with the ground electrode sited over the external surface of the lateral epicondyle of the humerus. Each subject was instructed to hold 5 milliliters of water in the mouth and to swallow as rapidly as possible after a visual signal.1 The PMT value was measured as the time from the imperative stimulus until the first reliably detected change in the electromyographic (EMG) activity. The same examination was repeated after 24 hours. The reproducibilities of PMT and duration of EMG activity were evaluated in 23 consecutive subjects by comparing the difference between test and retest. The PMT of the older subjects was significantly slower than that of subjects under age 65 (P = .0009). There was a positive relationship between PMT and subject age (r = 0.642; P = .001). There was no significant relationship between duration and subject age (r = 0.267; P = .218). For reproducibility of PMT assessed by a univariate correlation, there were positive relationships between test and retest for PMT (r = 0.868; P < .0001) and duration (r = 0.437, P = .0372). The results of regression analysis for PMT showed a determination coefficient (R2) of 0.754, meaning that 75.4% of the total variance of the set of measurements on the test was validated by the retest measurements. Previous reports have shown that older subjects have prolonged reaction time (RT) and PMT. However, these RT and PMT values were measured in the muscles of the extremities.2, 3 The mean PMT of older subjects in the extensor digitorum communis was 153.0 ms, and that of subjects under 65 was 109.3 ms.2 The mean PMT of the biceps brachii muscle was 130 ± 18 ms in young subjects and 141 ± 21 ms in older subjects.3 In our study, the mean PMTs of the submental muscles were 227.9 ms in subjects under 65 years of age and 310.1 ms in older subjects. We think that our PMTs were longer than those of previous studies are because of the differences in measuring methods and the differences in the muscles measured. Most previous studies have shown that the PMTs of healthy subjects tended to be longer with age. The main finding in the present study was that the delayed response in the central nervous system might be a factor in the mild dysphagia that older people experience. Because differences in PMTs between young and older subjects were small, we concluded that most older people do not suffer from dysphagia. If older people suffer stroke, head trauma, or other neurological diseases, severe swallowing disturbances can occur. There are several possible explanations for the delay in PMT of the submental muscles in older peoples. The PMTs corresponded to central nervous system delays occurring before the onset of muscle action potentials, and the delay in PMTs was thought to reflect central nervous system delays. For example, the conduction velocity of the peripheral sensory system, such as vision organs and afferent pathways, may decrease with aging. The number of neurons in the brain may also decrease with age, and the synapse conduction in the central nervous system may be delayed. There was a positive relationship between test and retest measurements for PMT. Although the reproducibility of duration was low, that of PMT was high. Our results suggest that PMT of the submental muscles might be influenced and prolonged with normal aging on the swallowing reflex and that the study of PMT of the submental muscles may be a useful tool for a more specific assessment of the swallowing function.