Deep tendon reflexes: a study of quantitative methods

The deep tendon reflex (DTR) is routinely used by clinicians to evaluate the nervous system. Depressed and hyperactive DTRs suggest peripheral and central nervous system compromise, respectively. Limitations of DTRs are: qualitative nature of the assessments based upon subjective grading, and limited inter-rater reliability. This preliminary study was undertaken to quantify the tendon tap used by clinicians to elicit DTRs and the reflex response elicited.

Tendon taps were applied to a force transducer in hypo-, normo-, and hyperreflexic ranges by 2 clinicians, using 3 different tendon hammers (Babinski, Queen Square, and Taylor). Patellar DTRs, measured as joint angle excursion with an electrogoniometer, were compared in hyper- and normoreflexic individuals.

Median peak tap force was 1 2.8, 38.0, and 85.2 Newtons (Nt), respectively, for eliciting hyper-, normo-, and hyporeflexic DTRs. Peak tap force was similar in the hyper- and normoreflexic ranges for all 3 hammers; in the hyporeflexic range, peak tap forces with the Taylor hammer were lower. A good distinguishing feature between hyper- and normoreflexic patellar DTRs was briskness, measured as the quotient of knee excursion divided by peak tendon tap force. Knee excursion is a non-linear patellar DTR response, when measured sitting.

Peak tap forces used by clinicians fall into 3 ranges: 0-20 Nt for hyperreflexia, 21-50 Nt for normoreflexia, and >50 Nt for hyporeflexia. The Taylor hammer, with small mass and short handle, has a ceiling effect in the hyporeflexic range. We propose a systematic method for DTR testing.