Interpreting a Peripheral Painful Stimulus Response

Emergency medical service personnel often work under conditions that can be best described as “extremely uncontrolled.” Under these conditions, patient assessment is expected to be conducted in a rapid manner, in order to collect as much history and physical exam information as possible.

This information is used during the critical thinking process to develop a differential field diagnosis. Further assessment and emergency care is based on the differential field diagnosis; thus, the information collected must be as accurate as possible. Inaccurate information can lead to improper care. However, the results of the exam may not always provide the most accurate information.

Last month’s column discussed the possible misinterpretation of a sternal rub response in a patient with an altered mental status. In addition to the sternal rub, there are a few other situations where the results from a physical exam conducted on a patient with an altered mental status may be misinterpreted.

One particular situation is when a painful stimulus is applied to an extremity to elicit a response, if the patient does not respond to verbal stimuli. There are two different anatomic locations where a painful stimulus is applied: centrally and peripherally. Central stimuli are applied to the core of the body, such as the sternal rub, trapezius pinch, or supraorbital pressure.Peripheral stimuli are applied to the extremities. This method commonly involvescompressing the fingernail bed or pinching the web between the thumb and index finger.

When a painful stimulus is applied to the periphery, the examiner expects a response from the patient. If no response is elicited, the examiner suspects the brain function is significantly impaired. If the patient withdraws his hand when pain is applied, the examiner interprets this finding as an indication of higher brain function.

In the latter case, the examiner assumes the brain is triggered. the pain is sensed by the receptors, travels to the spinal cord via an afferent (sensory) nerve tract, up the spinal cord and to the brain via a spinothalmic tract, and is interpreted correctly by the brain. Then a response from the brain is sent out through the cerebral cortex with the impulse traveling back down the spinal cord via a corticospinal tract to the muscle byway of an efferent (motor) nerve, causing the patient to withdraw or move his hand or arm. Thus, withdrawal of the hand or arm to pain might cause the examiner to assume the brain was able to receive the impulse, interpret it correctly, and send out a proper response, equating to better brain function.

However, a body response to stimuli does not always mean that the brain is functioning properly. For example, the spinal cord contains reflex arcs that can play a part. If a sensory impulse, such as pain, from an afferent nerve tract enters the spinal cord and triggers a reflex arc, the impulse does not travel up the spinal cord to the brain where it is interpreted. Instead, it is immediately returned to the muscle via an efferent (motor) nerve tract causing the patient to respond with movement such as withdrawal. Thus, the pain impulse never travels to the brain and is never interpreted by the brain; however, the patient still exhibits an appropriate response. Keep in mind, the purpose of applying a painful stimulus is to test the integrity and level of brain function.

Be careful in your interpretation of a painful stimulus applied to the extremities. If a reflex arc is triggered, the patient may respond appropriately without invoking brain involvement. As a result, you would most likely award the patient a four on the Glasgow Coma Score, when the patient should be given a one in actuality. Some protocols may require a change in destination or treatment for trauma patients with a GCS of 8 or less; therefore, this inaccuracy may lead to less aggressive emergency management than needed.

It is important to understand the limitations of this exam and the possibility of misinterpreting the results. By applying a central painful stimulus, you should be able to obtain an accurate response to pain. Just keep in mind that the sternal rub has limitations of its own as well.

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