Physiology of Reflexes
Assessment of reflexes is based on a clear understanding of the following principles and relationships:
Tendons connect muscles to bones, usually crossing a joint. When the muscle contracts, the tendon pulls on the bone, causing the attached structure to move.
When the tendon is struck by the reflex hammer, stretch receptors contained within it generate an impulse that is carried via sensory nerves to the spinal cord. At this juncture, the message is transmitted across a synapse to an appropriate lower motor neuron. An upper motor neuron, whose cell body resides in the brain, also provides input to this synapse.
The signal then travels down the lower motor neuron to the target muscle.
The sensory and motor signals that comprise a reflex arc travel over anatomically well characterized pathways. Pathologic processes affecting discrete roots or named peripheral nerves will cause the reflex to be diminished or absent. This can obviously be of great clinical significance. The Achilles Reflex (see below) is dependent on the S1 and S2 nerve roots. Herniated disc material (a relatively common process) can put pressure on the S1 nerve root, causing pain along its entire distribution (i.e. the lateral aspect of the lower leg). If enough pressure if placed on the nerve, it may no longer function, causing a loss of the Achilles reflex. In extreme cases, the patient may develop weakness or even complete loss of function of the muscles innervated by the nerve root, a medical emergency mandating surgical decompression. The specific nerve roots that comprise the arcs are listed for each of the major reflexes described below.
A normal response generates an easily observed shortening of the muscle. This, in turn, causes the attached structure to move.
The vigor of contraction is graded on the following scale:
0 No evidence of contraction
1+ Decreased, but still present (hypo-reflexic)
2+ Normal
3+ Super-normal (hyper-reflexic)
4+ Clonus: Repetitive shortening of the muscle after a single stimulation
Technique
Deep Tendon Reflexes
Using a reflex hammer, deep tendon reflexes are elicited in all 4 extremities. Note the extent or power of the reflex, both visually and by palpation of the tendon or muscle in question.
The Reflex Hammer
You will need to use a reflex hammer when performing this aspect of the exam. A number of the most commonly used models are pictured below. Regardless of the hammer type, proper technique is critical. The larger hammers have weighted heads, such that if you raise them approximately 10 cm from the target and then release, they will swing into the tendon with adequate force. The smaller hammers should be swung loosely between thumb and forefinger.
Technique:
The muscle group to be tested must be in a neutral position (i.e. neither stretched nor contracted).
The tendon attached to the muscle(s) which is/are to be tested must be clearly identified. The extremity should be positioned such that the tendon can be easily struck with the reflex hammer.
If you are having trouble locating the tendon, ask the patient to contract the muscle to which it is attached. When the muscle shortens, you should be able to both see and feel the cord like tendon, confirming its precise location. You may, for example, have some difficulty identifying the Biceps tendon within the Antecubital Fossa. Ask the patient to flex their forearm (i.e. contract their Biceps muscle) while you simultaneously palpate the fossa. The Biceps tendon should become taut and thus readily apparent.
Strike the tendon with a single, brisk, stroke. While this is done firmly, it should not elicit pain. Occasionally, due to other medical problems (e.g. severe arthritis), you will not be able to position the patient's arm in such a way that you are able to strike the tendon. If this occurs, do not cause the patient discomfort. Simply move on to another aspect of the exam.
This grading system is rather subjective. Additional levels of response can be included by omitting the '+' or adding a '-' to any of the numbers. As you gain more experience, you'll have a greater sense of how to arrange your own scale.
Specifics of Reflex Testing � The peripheral nerves and contributing spinal nerve roots that form each reflex arc are listed in parentheses:
Achilles (S1, S2 � Sciatic Nerve):
This is most easily done with the patient seated, feet dangling over the edge of the exam table. If they cannot maintain this position, have them lie supine, crossing one leg over the other in a figure 4. Or, failing that, arrange the legs in a frog-type position.
Identify the Achilles tendon, a taut, discrete, cord-like structure running from the heel to the muscles of the calf. If you are unsure, ask the patient to plantar flex (i.e. "step on the gas"), which will cause the calf to contract and the Achilles to become taut.
Achilles Tendon:Tendon is outlined in pen on left, grasped by forceps (gross dissection) on right.
Position the foot so that it forms a right angle with the rest of the lower leg. You will probably need to support the bottom of the foot with your hand.
Strike the tendon directly with your reflex hammer. Be sure that the calf if exposed so that you can see the muscle contract. A normal reflex will cause the foot to plantar flex (i.e. move into your supporting hand).
Positions for Checking Achilles Reflex
Patellar (L3, L4 � Femoral Nerve):
This is most easily done with the patient seated, feet dangling over the edge the exam table. If they cannot maintain this position, have them lie supine (i.e. on their backs).
Identify the patellar tendon, a thick, broad band of tissue extending down from the lower aspect of the patella (knee cap). If you are not certain where it's located, ask the patient to extend their knee. This causes the quadriceps (thigh muscles) to contract and makes the attached tendon more apparent.
Patellar Tendon: Outlined in pen on left, grasped by forceps (gross dissection)on right.
Strike the tendon directly with your reflex hammer. If you are having trouble identifying the exact location of the tendon (e.g. if there is a lot of subcutaneous fat), place your index finger firmly on top of it. Strike your finger, which should then transmit the impulse.
Patellar Reflex Testing, seated patient
For the supine patient, support the back of their thigh with your hands such that the knee is flexed and the quadriceps muscles relaxed. Then strike the tendon as described above.
Patellar Reflex, supine patient
Make sure that the quadriceps are exposed so that you can see muscle contraction. In the normal reflex, the lower leg will extend at the knee.
Biceps (C5, C6 � Musculocutaneous Nerve):
This is most easily done with the patient seated.
Identify the location of the biceps tendon. To do this, have the patient flex at the elbow while you observe and palpate the antecubital fossa. The tendon will look and feel like a thick cord.
Biceps Tendon:Tendon is outlined in pen on left, grasped by forceps (gross dissection) on right.
The patient's arm can be positioned in one of two ways:
Allow the arm to rest in the patient's lap, forming an angle of slightly more then 90 degrees at the elbow.
Biceps Reflex Testing
Support the arm in yours, such that your thumb is resting directly over the biceps tendon (hold their right arm with your right; and vice versa).
Biceps Reflex Testing,arm supported
Make sure that the biceps muscle is completely relaxed.
It may be difficult to direct your hammer strike such that the force is transmitted directly on to the biceps tendon, and not dissipated amongst the rest of the soft tissue in the area. If you are supporting the patient's arm, place your thumb on the tendon and strike this digit. If the arm is unsupported, place your index or middle fingers firmly against the tendon and strike them with the hammer.
Make sure that the patient's sleeve is rolled up so that you can directly observe the muscle as well as watch the lower arm for movement. A normal response will cause the biceps to contract, drawing the lower arm upwards.
Brachioradialis (C5, C6 � Radial Nerve):
This is most easily done with the patient seated. The lower arm should be resting loosely on the patient's lap.
The tendon of the Brachioradialis muscle cannot be seen or well palpated, which makes this reflex a bit tricky to elicit. The tendon crosses the radius (thumb side of the lower arm) approximately 10 cm proximal to the wrist.
Brachioradialis Tendon: Tendon is outlined in pen on left, grasped by forceps (gross dissection) on right.
Strike this area with your reflex hammer. Usually, hitting anywhere in the right vicinity will generate the reflex.
Brachioradialis Reflex
Observe the lower arm and body of the Brachioradialis for a response. A normal reflex will cause the lower arm to flex at the elbow and the hand to supinate (turn palm upward).
Triceps (C7, C8 � Radial Nerve):
This is most easily done with the patient seated.
Identify the triceps tendon, a discrete, broad structure that can be palpated (and often seen) as it extends across the elbow to the body of the muscle, located on the back of the upper arm. If you are having trouble clearly identifying the tendon, ask the patient to extend their lower arm at the elbow while you observe and palpate in the appropriate region.
Triceps Tendon:Tendon is outlined in pen on left, grasped by forceps (gross dissection) on right.
The arm can be placed in either of 2 positions:
Gently pull the arm out from the patient's body, such that it roughly forms a right angle at the shoulder. The lower arm should dangle directly downward at the elbow.
Triceps Reflex, arm supported
Have the patient place their hands on their hips.
Triceps Reflex, arm unsupported
Either of these techniques will allow the triceps to completely relax.
If you are certain as to the precise location of the tendon, strike this area directly with your hammer. If the target is not clearly apparent or the tendon is surrounded by an excessive amount of subcutaneous fat (which might dissipate the force of your strike), place your index or middle finger firmly against the structure. Then strike your finger.
Make sure that the triceps is uncovered, so that you can observe the response. The normal reflex will cause the lower arm to extend at the elbow and swing away from the body. If the patient's hands are on their hips, the arm will not move but the muscle should shorten vigorously .
Making Clinical Sense of Reflexes:
Normal reflexes require that every aspect of the system function normally. Breakdowns cause specific patterns of dysfunction. These are interpreted as follows:
Disorders in the sensory limb will prevent or delay the transmission of the impulse to the spinal cord. This causes the resulting reflex to be diminished or completely absent. Diabetes induced peripheral neuropathy (the most common sensory neuropathy seen in developed countries), for example, is a relatively common reason for loss of reflexes.
Abnormal lower motor neuron (LMN) function will result in decreased or absent reflexes. If, for example, a peripheral motor neuron is transected as a result of trauma, the reflex dependent on this nerve will be absent.
If the upper motor neuron (UMN)is completely transected, as might occur in traumatic spinal cord injury, the arc receiving input from this nerve becomes disinhibited, resulting in hyperactive reflexes. Of note, immediately following such an injury, the reflexes are actually diminished, with hyper-reflexia developing several weeks later. A similar pattern is seen with the death of the cell body of the UMN (located in the brain), as occurs with a stroke affecting the motor cortex of the brain.
Primary disease of the neuro-muscular junction or the muscle itself will result in a loss of reflexes, as disease at the target organ (i.e. the muscle) precludes movement.
A number of systemic disease states can affect reflexes. Some have their impact through direct toxicity to a specific limb of the system. Poorly controlled diabetes, as described above, can result in a peripheral sensory neuropathy. Extremes of thyroid disorder can also affect reflexes, though the precise mechanisms through which this occurs are not clear. Hyperthyroidisim is associated with hyperreflexia, and hypothyroidism with hyporeflexia.
Detection of abnormal reflexes (either increased or decreased) does not necessarily tell you which limb of the system is broken, nor what might be causing the dysfunction. Decreased reflexes could be due to impaired sensory input or abnormal motor nerve function. Only by considering all of the findings, together with their rate of progression, pattern of distribution (bilateral v unilateral, etc.) and other medical conditions can the clinician make educated diagnostic inferences about the results generated during reflex testing.
Trouble Shooting
If you are unable to elicit a reflex, stop and consider the following:
Are you striking in the correct place? Confirm the location of the tendon by observing and palpating the appropriate region while asking the patient to perform an activity that causes the muscle to shorten, making the attached tendon more apparent.
Make sure that your hammer strike is falling directly on the appropriate tendon. If there is a lot of surrounding soft tissue that could dampen the force of the strike, place a finger firmly on the correct tendon and use that as your target.
Make sure that the muscle is uncovered so that you can see any contraction (occasionally the force of the reflex will not be sufficient to cause the limb to move).
Reinforcement is accomplished by asking the patient to clench their teeth, or if testing lower extremity reflexes, have the patient hook together their flexed fingers and pull apart. This is known as the Jendrassik maneuver.
Sometimes the patient is unable to relax, which can inhibit the reflex even when all is neurologically intact. If this occurs during your assessment of lower extremity reflexes, ask the patient to interlock their hands and direct them to pull, while you simultaneously strike the tendon. This sometimes provides enough distraction so that the reflex arc is no longer inhibited.
Occasionally, it will not be possible to elicit reflexes, even when no neurological disease exists. This is most commonly due to a patient's inability to relax. In these settings, the absence of reflexes are of no clinical consequence. This assumes that you were otherwise thorough in your history taking, used appropriate examination techniques, and otherwise identified no evidence of disease.
Pathological Reflexes :
The Hoffman response is elicited by holding the patient's middle finger between the examiner''s thumb and index finger. Ask the patient to relax their fingers completely. Once the patient is relaxed, using your thumbnail press down on the patient's fingernail and move downward until your nail "clicks" over the end of the patient's nail. Normally, nothing occurs.
A positive Hoffman's response is when the other fingers flex transiently after the "click". Repeat this manuever multiple times on both hands.
Abdominal reflexes = contractions of the abdominal muscles on stimulation of the abdominal skin.
Cremasteric reflex = stimulation of the skin on the front and inner thigh retracts the testis on the same side.
Anal reflex = contraction of the anal sphincter on irritation of the anal skin.
Oppenheim reflex = dorsiflexion of the big toe on stroking downward along the medial side of the tibia, seen in pyramidal tract disease.
Babinski Response
The Babinski response is a test used to assess upper motor neuron dysfunction and is performed as follows:
Use the handle end of your reflex hammer, which is solid and comes to a point.
The patient may either sit or lie supine.
Start at the lateral aspect of the foot, near the heel. Apply steady pressure with the end of the hammer as you move up towards the ball (area of the metatarsal heads) of the foot.
When you reach the ball of the foot, move medially, stroking across this area.
Then test the other foot.
Some patients find this test to be particularly noxious/uncomfortable. Tell them what you are going to do and why. If it's unlikely to contribute important information (e.g. screening exam of the normal patient) and they are quite averse, simply skip it.
Interpretation: In the normal patient, the first movement of the great toe should be downwards (i.e. plantar flexion). If there is an upper motor neuron injury (e.g. spinal cord injury, stroke), then the great toe will dorsiflex and the remainder of the other toes will fan out. A few additional things to remember:
Babinski Response Present
Newborns normally have a positive Babinksi. It usually goes away after about 6 months.
Sometimes you will be unable to generate any response, even in the absence of disease. Responses must therefore be interpreted in the context of the rest of the exam.
If the great toe flexes and the other toes flair, the Babinski Response is said to be present. If not (i.e. normal), it is recorded as absent. For reasons of semantics, the Babinski is not recorded as '+' or '-'.
Withdrawal of the entire foot (due to unpleasant stimulation), is not interpreted as a positive response.
Chaddock's reflex in lesions of the pyramidal tract, stimulation below the external malleolus causes extension of the great toe.
Finally, test clonus if any of the reflexes appeared hyperactive. Hold the relaxed lower leg in your hand, and sharply dorsiflex the foot and hold it dorsiflexed. Feel for oscillations between flexion and extension of the foot indicating clonus. Normally nothing is felt.
http://meded.ucsd.edu/clinicalmed/neuro3.htm
http://edinfo.med.nyu.edu/courseware/neurosurgery/reflexes.html
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