‘Gate’ Control of Pain and Pulsed Radio Frequency Therapy
This is a simplified and easy to understand explanation of the so called “Gate Theory of Pain” As this name implies there is a so called gate that controls the flow of pain impulses to the brain and, therefore the awareness of pain. As with any gate this gate can either open – in other words pain impulses can flow freely to the brain and the intensity and awareness of the pain is intensified, or, closed, meaning that the pain impulses reaching the brain are restricted and of less intensity lessening the experience of pain.
This gate is located in the spinal cord in the dorsal horn (see figure 1)
The pain pathway from the periphery to the cerebral cortex (awareness or experience of pain) involves three neurons – See Figure 2.
1. Primary: from the ‘nociceptors’ (receptors in the tissue that respond to noxious stimuli) to the dorsal horn of the spinal cord. The posterior horn is made up of these primary neurons synapsing with the secondary neurons and all the interconnections between them (See Figure 3)
2. Secondary: from the dorsal horn to the thalamus NOTE: pain impulses reach the thalamus before the cortex – so an emotional response to pain is evoked before we are even aware of the pain.
3. Tertiary: from thalamus to cortex and awareness.
The important thing to notice in Figure 3 is that between the fibres conducting sensations like touch,temperature and vibration and those that conduct pain, there a tiny ;little interneurons. These tiny neurons will, when activated, inhibit conduction in the pain fibres OR, when switched off will facilitate conduction in the pain fibres. So the interneurons either open or close the gate thus controlling the flow of pain impulses.
To inhibit pain conduction, the interneurons are activated (See Figure 4):
i. Activity in the A fibres that conduct other sensations. This is why rubbing or stroking a painful area really does help to reduce the pain. Rubbing causes increases activity in the a-fibres, which activates the interneurons which then inhibits the pain fibres. This is also the theory behind Spinal Cord Stimulators. They are placed so that they stimulate the fibres that carry touch, temperature etc which lie in the dorsal columns of the spinal cord. This increased activity in these fibres leads to inhibition, very effectively, of the pain fibres
ii. Activity in the descending neuromodulatory fibres from the brain and higher centers. This why the associations with pain, emotions, thoughts about pain, cultural differences, variations in pain thresholds etc can all lead to reduced conduction of pain. This is also where opiates have some of their effect in reducing the conduction of pain.
In order to facilitate the conduction of pain, or open the gate, the interneurons are inhibited (See Figure 4):
i. The same descending neuromodulatory fibres that activate the interneurons can also inhibitthe interneurons, so opening the gate. So, emotions, experience, association, thoughts and expectations can all make pain worse.
ii. In case of acute intense pain that is not treated, the constant afferent barrage on the gate can cause theinterneurons to die and this then, of course, leaves the gate wide open – one of the causes of chronic pain.
So, some chronic pain states are due to dysfunction or death of interneurons. This can also lead to permanent changes in the nerve fibres in the dorsal horn. Some of the pain can be perpetuated or worsened by the emotional state of the patient and its effect on the descending modulatory fibres.
This is why it is vitally important to recognize and treat emotional issues in chronic pain patients. In severe chronic pain states Fig 4. Schematic of interneuron activation in dorsal horn of spinal cord Synapses between Primary and Secondary Neurons Descending Modulatory Fibres INTER-NEURON inhibitory to pain fibres C-fibres carrying pain from periphery A-fibres carrying touch, temperature and vibration from periphery the interneurons that die off can be replaced by excitatory neurons which makes the treatment and resolution of the pain states all but impossible.
In order to address the changes in the nerves, the neuroplasty, one therapy involves applying Pulsed Radio Frequency (P-RF) to the nerves. This is a non-destructive not-ablative procedure, but is treatment as opposed to Thermal-RF which uses heat to be destructive of the nerve. P-RF has the effect of switching on genes and reversing some of the changes in the nerve.
Unfortunately one cannot apply the P-RF directly to the dorsal horn as one cannot stick a needle into the spinal cord. One can, however, apply the therapy to the Dorsal Root Ganglion (DRG) which is close to the dorsal horn and the effect of the therapy applied to the DRG will find its way to the dorsal horn. This is why the clinical effect of a successful DRG treatment is not immediately seen but can take up to four weeks to manifest.
Also, when doing the DRG procedure, to locate the nerve, it is stimulated. The patient just needs to feel a sensation in the distribution of the nerve and not necessarily pain in order to have a successful therapy. Why this is has been explained above.
Pulsed Radio Frequency is a useful treatment modality is a pain practitioner’s repertoire when treating neuropathic pain including phantom pain and CRPS, but modulating the gate control of the pain conduction.