Hyperactive tendon reflexes in spastic multiple sclerosis: Measures and mechanisms of action

Objective: To develop new measures of tendon reflexes and evaluate hyperact ive reflexes in patients with spastic multiple sclerosis (MS). Design: With the subject relaxed, a hand-held instrumented hammer was used to tap the patellar tendon and record the tapping force, while knee extensi on torque and quadriceps EMG were recorded isometrically as measures of the reflex response. Setting: Research laboratory in a rehabilitation hospital. Subjects: Ten spastic MS and 14 healthy subjects. Main Outcome Measures: Tendon tapping force (designated as system input), r eflex torque (as output), their dynamic relationship (characterized as syst em parameters tendon reflex gain, contraction rate, and reflex loop delay), Ashworth scale, and tendon reflex scale. Results: The system parameters provide more repeatable measures than do inp ut or output parameters alone because they quantify the input and output si multaneously and dynamically. Compared with control subjects, MS patients h ad a significantly lower threshold in tapping force (p = .026), yet their e voked reflex torque was significantly higher (p = .033). Despite significan t quadriceps weakness (p < .0001), MS patients had a significantly higher r eflex gain (p = .0002) and contraction rate (p = .0002), and shorter reflex loop delay (p = .0046), indicating hyperexcitability of motoneurons and pe ripheral receptors, and indicating that relatively more of the muscle was a ctivated reflexively, with greater recruitment of larger fast-twitch fibers . Both the reflex gain and rate measures correlated more closely with the A shworth scale and tendon reflex scale than did the output measures, indicat ing their potential clinical value. Conclusions: With appropriate simplification, the method may be used in cli nical practice to quantify more precisely the tendon jerk than is currently feasible with standard clinical tests.