March 26, 2008

Toward An Objective Correlate Of Pain

visual analog scaleWhile taking a hot water bag to find relief for a severe spondylosis pain, I wondered why pain could not be expressed in a way different from the generally used Visual analog scale. The patient is asked to look at a chart (shown in the figure) and told to rate his sensation, that tallies most well with his pain. If that hot bag were to be applied to a person not having any pain, he would jump almost instantly. The fact that I tolerated it so well (and benefited from it), only shows its countering (counter irritant property) property, which should be somewhat proportional to the severity of one's pain. The relationship of a counter-irritant to pain severity, whether linear, logarithmic or exponential, needs to be established and quantified.

A patient's own account of pain may be subjectively modified according to the personality of the patient and many other factors. As such, this type of quantification is liable to be erroneous. Pain should better be measured in an objective manner, free of bias. In this instance cited above, one could use the formula: Q(heat)=m(mass) x s(specific heat of the substance) x t(temperature), to know the amount of heat energy transferred to the patient. I am assuming that the pain relieving techniques will, kind of, obey Newton's Third law. Amount (intensity) of counterirritant that just suffices pain relief will be equal to the degree of pain. But it is not actually so, as we will see later.

By noting the difference in local temperatures before and after the procedure, one could get
Mass or "m" could be measured by estimating the volume of the body tissue that was actually heated by infrared mapping, for example; and the expected density of that area. Specific heat for the tissue in question could be easily known and standardized using some cross-sectional studies. A suitable nomogram may later be drawn by plotting values obtained from such observations, for quick estimates. It seems logical that pain so measured, will have its units in British Thermal Units (btu), calories or their work (mechanical) equivalents like ergs, Joules, foot-pounds etc..

In pain therapies using mechanical energies (Ultrasound), electromagnetic devices (laser, short wave diathermy, high frequency photons such as X rays) , a similar formula may be used to obtain the pain equivalent. For example, in laser or short wave therapy, we may design a device that will measure the amount of energy in Watt.seconds/Joules the given area of tissue is supposed to absorb, over a given period of time. The chemical analgesics (pharmaceuticals e.g. Non Steroidal Anti Inflammatory Drugs or steroids; counter irritants such as capsaicin) may be quantified using Scoville scale or by evaluation on the degree of relief from algesia.

Calculating pain may be quite painful in itself. Pain sensation does not tally linearly with noxious stimulus. Rather, a logarithmic relationship was proposed in the Weber Fechner law, which held that the magnitude of pain (or a sensation) felt, was proportional to the log of the intensity of a stimulus. In other words, to feel twice as much pain, you needed to hurt 10 times! To complicate matters further, our present knowledge suggests that the magnitude of a sensation is related by a power factor to the intensity of that sensation. R=KS^A; where R is the sensation felt, S the intensity of stimulus, K and A are constants for that particular tissue. The brighter side is, we get a preformulated relationship for pain calculation.

Pain (musculo skeletal/ visceral, exogenous/endogenous) is generally of two types: fast pain and slow pain. Fast pains such as sharp pain of pricks, stabs are usually carried by Ad (A delta) nerve fibres, while slow aching pains are carried by type C nerve fibers. Ad fibres can carry impulses rapidly as these fibers are myelinated and are of large caliber. C fibers, on the other hand, are unmyelinated and narrow. A-delta fibres release glutamate and C fibers secrete substance-P. A way to measure these chemicals could be a step closer to quantifying pain.

The signals from these fibers travel to the thalamus, a part of the lower brain, on their way to the cerebrum for the localization of pain. Measuring the metabolic activity in thalamus, arising out of increased neural discharges there, by fMRI or PET scan may also shed some light on the intensity of pain stimuli (the stimulus at this level is unmodified by the higher brain) that reaches thalamus. We can also measure the blood levels of endogenous opioids (enkephalins, endorphins) which are secreted in body's response to the pain and adrenaline, secreted in response to increased sympathetic discharge, which is an usual accompaniment of pain. Their blood/plasma levels may correspond with pain severity. Other pain markers like bradykinin, histamine, potassium ions and proteolytic enzymes could be probed too.

True, that the patient may or may not feel as much pain as has been measured this way, because pain perception may not be proportional to the physical/chemical parameters thus described and it is not uniform in all subjects. The brain sees pain in its own mathematical terms, and everyones' brain is different in this regard. A soldier may overlook his gushing wounds, whereas pampered girls of rich persons may feel a lot of pain from an apparently trivial injury. But, quantification of pain in this way (by measuring the physical/chemical yardsticks) may correctly establish the severity of pain in silent myocardial infarction of neuropathy (pain sensation is dulled here due to neural malfunction), decubitus ulcers, trophic ulcers, malingering and in similar situations. In this way we may be able to find a better and objective correlate of pain in clinical practice and develop more efficient analgesics.

P.S. In a recent development, some objective physiological correlates of pain has been tracked. These include measurements from the nonlinear composite of heart rate, heart rate variability, amplitude of the photoplethysmogram, skin conductance, fluctuations in skin conductance, and their time derivatives.  Algorithms can then convert the data into a real-time, continuous index on a bedside monitor. This has resulted in the fabrication of a wearable sensing device can be mounted on a finger.

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