Deep Tendon Reflex: The Tools and Techniques. What Surgical Neurology Residents Should Know

Correspondence: Dr Ooi Lin-Wei, MD (Sechenov First Moscow State Medical University), Department of Neurosciences, School of Medical Sciences, Universiti Sains Malaysia, 16150 Kubang Kerian, Kelantan, Malaysia. Tel: +609 7676300, Fax: +609 7673833, E-mail: moc.liamg@assyla.ioo

This work is licensed under the terms of the Creative Commons Attribution (CC BY) (http://creativecommons.org/licenses/by/4.0/).

Abstract

The deep tendon reflex (DTR) is a key component of the neurological examination. However, interpretation of the results is a challenge since there is a lack of knowledge on the important features of reflex responses such as the amount of hammer force, the strength of contraction, duration of the contraction and relaxation. The tools used to elicit the reflexes also play a role in the quality of the reflex contraction. Furthermore, improper execution techniques during the DTR assessment may alter the findings and cloud the true assessment of the nervous system. Therefore, understanding the basic principles and the key features of DTR allows for better interpretation of the reflex responses. This paper discusses the brief history of reflexes, the development of the reflex hammer, and also the key features of a reflex response encompassing the amplitude of force needed to elicit a reflex response, the velocity of contraction, the strength of contraction, and the duration of contraction and relaxation phases. The final section encloses the techniques of eliciting DTR in the upper extremities, trunk, and lower extremities, and the interpretation of these reflexes.

Keywords: deep tendon reflex, myotatic reflex, muscle spindle, afferent, spinal cord, nervous system, reflex hammer, tapping force, reflex torque, contraction time, relaxation time, striking point, peripheral nerve, effector, technique, biceps reflex, brachioradialis reflex, knee jerk, Achilles reflex

Introduction

A reflex is an involuntary, unlearned, repeatable response to a specific stimulus that does not require any input from the brain (1). The deep tendon reflex (DTR), also known as a myotatic reflex, is a sequence of lengthening, contraction, and relaxation of a group of muscles. A DTR comprises of a reflex arc, which is a neural pathway that controls a reflex. The reflex arc is made up of five components:

The muscle spindle is a receptor within the muscle that detects changes in the length of the muscle. The muscle spindle consists of a noncontractile centre portion and intrafusal muscle fibres which make up the contractile portion. Tapping the tendon will cause stretching of the muscle spindle, activating it, leading to the propagation of an action potential to the spinal cord via afferent Ia fibres through the dorsal horn. In the spinal cord, the afferent nerve fibre synapses with a-motoneurons that supply the agonist muscles and also synapses with an inhibitory neuron that inhibits the antagonistic muscle group. This causes a concomitant relaxation of the antagonistic muscle as the agonist muscle groups contract.

The firing of the afferent fibres (reflecting the sensitivity of the central portion) depends on the length of the intrafusal fibres. The intrafusal fibres are controlled by gamma motoneurons, which are influenced by the cortex, cerebellum and various brainstem nuclei. This forms the suprasegmental control that modulates reflex activity. The higher centres receive information from the muscle spindles. For example, the dorsal spinocerebellar tract conveys the information about the proprioceptive organs from the muscle spindle to the cerebellum. In return, the higher centres modulate the segmental activity through gamma motoneurons. This, in turn, regulates the quality and amount of information received as the ‘sensitivity’ of the central portion depends on the length of the intrafusal fibres.

A Brief History

Reflexes have been studied for centuries and it begins with Aristotle and Galen. French physiologist, François Magendie in Paris and surgeon-anatomist, Charles Bell in London were among the first to identify the sensory and motor function of spinal nerve roots (2). In the late 18th century, Robert Whytt, Johann Unzer and Prochaska set up the reflex concept (1, 3) while the concept of the ‘reflex arc’ was formulated in the 1830s by Marshall Hall (1790–1857). However, the concept underwent multiple developments and refinements conducted by various physicians in the late 1800s to reach the neurological examination concept that is applied today. Charles Sherrington played a central role in understanding the reflex activity from his vast physiologic investigations in the late 1800s to early 1900s (1, 2).

Muscle Stretch Reflexes or Deep Tendon Reflexes

In 1859, Silas Weir Mitchell was the first person to appreciate the response elicited by percussed muscles with a hammer. The term ‘reflex’ was not used at that time and the percussing action was performed on muscles instead of the tendons. Jean-Martin Charcot recognised the diagnostic significance of hyperactive and absent muscle stretch reflex; however, this was only studied systematically by Erb and Westphal 16 years later and the findings were published in the German Archives of Psychiatry and Nervous Diseases.

The Tools

Percussion Hammers (3, 4)

In 1761, Viennese physician, Leopold Auenbrugger (1722–1809) described the use of percussion on the chest, back and abdomen as an aid to medical diagnosis. This technique is inspired by the thumping of wine casks to determine the level of the fluid.

In 1826, French Pierre Adolphe Piorry (1794–1879) invented a pleximeter, a resonator in the form of small ivory, metal, cedar or river disk, that was placed on the chest and struck with a finger. Two years later, a percussion hammer was devised by a Scottish physician, Sir David Barry (1781–1836) to strike the pleximeter. However, the percussion hammer was not widely adopted.

In 1841, a German clinician, Max A Wintrich (1812–1882) introduced a percussion hammer, known as the Wintrich hammer ( Figure 1 ), which was then widely used throughout Europe. The hammer underwent many modifications using different materials, weight and shape. In 1912, the hammer was refurbished by Ebstein into a new model called the ‘reflex and sensibility tester’ for striking the tendons, with a pin on one end.