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Pain Modification Theories

Gate control theory: The gate theory of pain control was postulated by Melzack and Wall in 1965 and is still a strong influence on our understanding of pain today. It states that the dorsal horn synapses can be inhibited by increased activity of the larger A-beta fibers coming into the cord. These larger fibers carry impulses received from mechanoreceptors other than nociceptors. Movement, massage, vibration, adjustments, and exercise can stimulate these mechanoreceptors. As they are stimulated, the A beta fiber input to the cord is increased and the dorsal horn pain synapses are inhibited. According to this theory, by increasing A-beta fiber input to the cord, transmission of pain is inhibited. Electrotherapy is an extremely effective therapy for achieving this result, however correct protocols must be used.

When using an electrical current to take advantage of this inherent pathway, correct frequency and intensity settings must be used. These settings can be used with these electrical currents: high volt, low volt, interferential, pre-mod, sinusoidal, and variable muscle stimulation (VMS).

The settings are designed to stimulate the A beta fiber directly, thereby increasing A-beta fiber input to the cord. To optimize the effect, the pads should be placed around the point of pain, the frequency should be set to sweep between 70 and 150 Hz, and the intensity should be set to patient comfort. The patient’s perception should be that of a comfortable paresthesia. The sweeping of the frequency is used to prevent accommodation to the stimulus. If your machine does not permit you to sweep, then set it to a fixed amount between 70 and 150 Hz and check on the patient every 5–7 minutes to see if the patient is still perceiving the stimulus. If not, then change the frequency to a higher setting until sensed. The optimal treatment time for this protocol is 20 minutes and it can be performed up to three times per day.

Descending inhibitory pathway theory: This is a more advanced theory that describes descending neurons from the brain stem that when activated, can presynaptically inhibit the same dorsal horn pain synapses that have been previously mentioned. These descending neurons are believed to originate in the periaqueductal gray matter (PAG) of the brain stem. Stimulation of the PAG causes descending neurons to release inhibitory neurotransmitters in the dorsal horn, thus inhibiting pain synapses and decreasing the patient’s perception of pain. The question is what stimulates the PAG?

According to this theory, three types of stimuli can stimulate the PAG: large fiber mechanoreceptor input to the cord, nociceptive bombardment, and power of suggestion.

The same electrical currents described for the gate control theory can be used in this protocol as well. The frequency and intensity settings, however, will be different. The electrical current is designed to stimulate the PAG through nociceptive bombardment of the spinal cord, meaning the current must be optimized to stimulate the smaller A-delta and C fibers. These fibers will be stimulated with frequencies sweeping between 1 and 10 Hz with intensities that the patient describes as painful or uncomfortable. Sometimes the intensities will be high enough to cause muscle contractions. Once again, the pads should be around the point of pain and the treatment time should be twenty minutes. Because this therapy is so uncomfortable, it should not be performed more than once per day.

In what situation should the gate theory protocols or descending theory protocols be utilized? There are no rules set in stone. They are both effective protocols, however, in my experience, some patients respond to one but not the other; therefore you need to experiment between the two. Typically, I begin the more acute patients with the gate control theory protocols and chronic patients with the descending theory protocols. However, I will switch protocols when I feel the patient is no longer responding.

The electrotherapies are only as effective as the doctor using them. Electrotherapy devices require the proper settings in order to achieve desired results. There are other protocols that are used for reducing edema and spasm, and still other protocols for strengthening and reeducating muscles. Too many doctors use electrotherapy without following the correct protocols and as a result, it is not producing the expected outcomes and the therapy is blamed for the failure. An unfortunate conclusion considering the results that are achievable when the machines are used correctly.