Reflex testing for intermediate-level beginners
You’re probably wondering about the chubby cherub who is sticking his foot in the fire. [Image available only on the blog homepage.] This was one of the original descriptions of the reflex circuit, and it was described by the French philosopher, René Descartes. It was very Cartesian to imagine that the human body was like a machine, with a bunch of pulleys and levers acting under the control of kind of soul. Quite right, actually. Except the soul bit. Descartes thought the soul was in the pineal glad, which is a step up from Aristotle who thought the soul was in the heart where the blood was warmest. But Descartes was the first to use the term, “reflexion,” when describing a person’s uncontrolled reaction to a frightening situation, like a foot placed over a flame followed by the immediate and unconscious withdrawal of it. But it would be another 200 years before the reflex became part of the every day neuro exam.
In this week’s episode, I chat with Texas A&M Health Sciences Center medical student, Eliana Vasquez about the circuitry
and assessment of reflexes. And she opens the show with Newton’s third law of motion: For every action, there is an equal and opposite reaction. For every stimulus, there should be an appropriate response. A reflex has two components: first, a neuronal input triggered by a stimulus–which is mediated by the muscle spindle, and second, a motor output that address the stimulus. The two essential components that create a complete monosynaptic reflex arc are the: 1a sensory neuron and alpha motor-neuron. But there are other components we often ignore, as in how the sensory neuron also stimulates the inhibitory interneuron in the spinal cord to tell the antagonist muscles to relax.
In general, we grade reflexes from 0-4+. Depending on secondary factors such as anxiety and genetic predisposition to excitation, 1-3 on the grading scale can be considered within normal limits. 0 and 4+ are always abnormal in adults.
Reflexes graded as 4+ are pathological in adults. This means there is either sustained flexion/extension oscillatory contractions (clonus), involvement of adjacent motor units (spread), or crossed responses involving similar muscle groups on the contralateral extremity following a sensory stimulation. You can assess crossed responses of the thigh adductors and the pectoralis muscles most easily. Common things being common, here are just a few considerations in a “healthy” appearing patient:
Reflexes should be evaluated in the context of the rest of the exam. Heightened reflexes in the setting of muscle weakness should suggest to you that there is a central process—something in the brain or spine—because the afferent and efferent limbs of the reflex circuit are spared, but not enough signal is getting from the brain to the muscle in order to use that muscle. If a hyporeflexia noted, and it is accompanied by sensory loss in the distribution of the same dermatome—for instance the anterior aspect of the knee in an L4 dermatome with a reduction in the patellar response—then the lesion is in the afferent limb of the monosynaptic circuit. Also, noting any asymmetry is important. Testing the same reflex on either side sequentially will give you something of an internal control during your exam. And when there is asymmetry, it’s up to you as the clinician to determine which side is the normal side and which side is abnormal.
Hyporeflexia is most commonly due to peripheral sensory nerve impairment. ‘Nuff said.
Sometimes reflexes can be difficult to obtain, as in children, or in a patient who is extremely nervous or who has difficulty following commands. The first trick here, that I always learned, is to distract them. You can do this by making patients draw out their names in the air, count backward from 100 by 7’s, or any other task to distract them—because that’s all it counts on. Distraction. And, as it turns out, this technique has a formal name. I didn’t know this before preparing the show today, but historians have called it the Jendrassik maneuver. As it was originally described in the nineteenth century by the Hungarian physician Jendrassik Erno, the patient would be asked to clench his or her fingers together very tightly and pull. Then the physician would attempt to elicit the reflex. And I’m sure many of you out there have tried it, and it actually works. The elevated tone in the patient would drop and the myotatic reflex could be primed to respond.
Another thing I found interesting in preparing for this episode had to do with spaceflight and astronauts. So astronauts, it should come as no surprise, endure a pretty significant battery of medical testing in order to even qualify for space travel. And even after short trips, a number of adverse effects on their health have been reported—things like reduced muscle mass, changes in bone density, and this condition called space adaptation syndrome, where you have vestibular impairment and nausea. But also, and more pertinent to this discussion, is that astronauts tend to lose their reflexes. After only 5 days, you can see depression of muscle spindle reflexes, and we can quantitate this using EMG. And within 1 day of returning to earth, much of that diminished reflex arc is restored. By 10 days, it’s completely normal. What we think is that the microgravity environment actually reduces the intrinsic excitability of the spinal cord. So those same sensory signals are reaching their respective alpha motor neuron, but the alpha motor neurons will require more of a signal in order to depolarize. Heavy stuff…
BrainWaves podcast and online content are intended for medical education only and should not be used in the routine care of patients. I swear, if someone tries to sue me because their doctor said “Well, Jim told me I should hit your knee with a hammer,” then I’m coming for you.
- Walker HK. Deep Tendon Reflexes. In: H. K. Walker, W. D. Hall and J. W. Hurst, eds. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd ed. Boston; 1990.
- Boes CJ. The history of examination of reflexes. Journal of neurology. 2014;261:2264-74.
- Watt D, Lefebvre L. Effects of altered gravity on spinal cord excitability. American Institute of Aeronautics and Astronautics. 2001-4939.