The Lost Pain System

Ask the average medical student to name all the pain systems of the body and he will likely mention the somatic pain apparatus carried via the spinothalamic tracts and the sensory division of the autonomic nervous system. He may not be sure if there are one or two spinothalamic tracts (anterior and lateral), but even if he mentions the dual tracts, he may have difficulty remembering which pain is carried in which division.

The one pain system which nearly all forget is the sensory arm of the gamma motor system, or pain sensation from the muscle spindle apparatus. Even if a physician encountered a patient with nociceptive muscle spindle pain, he might not recognize it. This goes double for neuropathic pain from the spindle.

To the physician, muscle spindle pain isn't a problem and so he often forgets it exists. It is a big problem for many Central Pain patients. Patients with intact motor units who are functionally paralyzed are occasionally seen, and short of full paralysis there are many who suffer with any movement. Neuropathic gamma pain represents one of the clearest opportunities to really study the muscle spindle and learn how to identify its dysfunction clinically. Unfortunately, because it is so little studied, and so easily forgotten, almost no one with Central Pain is asked about motor pain, although muscle pain is a very definite Goliath which the patient must confront.

The muscle spindle is a marvel of servo control. Perhaps one in every two hundred muscle fibers has embedded a swollen bulb that informs the brain of the state of the muscle. For Nature's purposes, the muscle spindle also has a very effective pain sensing apparatus. The pain characteristics in Central Pain suggests the spindle is not shortchanged in the pain sensors which are supplied. It is fairly complex since it gives off both allodynic and hyperpathic pain, just as the skin does.

Deep pressure which stretches the spindle relieves the pain of "confinement cramps" (pseudocramp), but it does not relieve the dysesthetic allodynia which occurs with muscle loading. A very effective test for gamma pain is to ask the patient to go down into a squat position. This causes a dysesthetic burning "cramp" in the thigh, which accompanies a very definite sensation of "pulling" or "load on the muscle". It becomes unbearable in seconds to minutes. Many patients complain of great soreness when trying to lay in a bed and awaken feeling as if they had been beaten. This pain is not dysesthetic, but probably represents hyperpathic response to pressure, such as one might feel sitting on a chair for a very long time.

Gamma Pain Can Cause Functional Paralysis

Permanent Cramps Are Usually in Localized Muscles,
or Even Divisions of a Single Muscle

A number of patients have a permanent cramp of hyperpathic quality which never leaves. The pain tends to localize, such as behind one thigh. It is the most dramatic of the muscle pains and makes concentration on other matters difficult.

Muscle pain is very disabling, but like many other Central Pain components is invisible. It is one of the hardest Central Pain group of pains for clinicians to accept because patients spend so much time getting over the hurdle of trying to describe the nature and severity of their dysesthetic burning is that they never address the muscle pain. Just when the physician thinks he finally has an idea about what the burning dysesthesia is like, the Central Pain patient complains of this very severe muscle pain of which the physician was unaware.

Its late mention may cause the physician to relegate muscle pain to minor status. This is a mistake and ignores one area where the physician can make a difference. The problem is simply that the patient cares more if he burns than if he has to sit still. At home, however, the patient sits immobile, progressively withdrawing from society and its demands.

This is unfortunate, because hyperpathic muscle pain is the one component of Central Pain which is amenable to therapy. The problem is that the therapy must be truly massive. In those patients with permanent cramp, it must be nearly continuous. The medical system simply cannot envision a sensory disability which consumes such resources. Such intensive treatment is generally reserved for those with motor problems and paralytics. No matter how bad the pain is, it is difficult to persuade third party payers to allocate the appropriate resources to treat severe gamma pain, even if it means the patient will otherwise stop moving to a very great extent.

The Ineffectiveness of Endogenous Opiods

Muscle Stretch, Not Muscle Rub

I Feel Like My Legs Are Going To Collapse, But They Don't

Impaired proprioception, combined with a minor motor weakness from whatever CNS injury precipitated the Central Pain, may cause the feeling that the legs are going to collapse. In addition, the patient with distal incrementation of dysesthesia on the feet has diminished sensation from the area, causing a sensory ataxia. This combination creates a fear of falling which is often realized. A cane can help, but increasing muscle strength in the legs by accommodations in therapy to accomplish exercise should be attempted.

If the above deep muscle massage can be provided and exercise performed, the symptom becomes less frightening as strength is built into the leg muscles. There is a price to be paid; namely greater gamma pain, but it is in the patients best interest to maintain mobility as much as is practicable. Activity which takes the patient to various locations can serve as a distraction to the dysesthetic burning, if she can stand the movement.

Motor Strength Is Not Just About the Corticospinal Tracts

What You Always Wanted To Know About The Muscle Spindle,
But Were Afraid To Ask

It also possesses a system to control tension of the muscle based on Golgi tendon organs at the polar ends of the spindle which return via Ib fibers. In addition, the nuclear chain fibers may have secondary "flower spray" endings associated with the tonic stretch reflex, which travel back to cord via II fibers to interneurons, which are also impacted by messages from above via the reticulospinal or pyramidal tracts (Duus 1989).

Nonconscious deep sensibility refers to coordination between agonists and anatagonists and travels via the spinocerebellar tracts. This is not thought responsible for the perceived dysesthesia in Central Pain. As to conscious sensibility, the body distribution here is roughly analogous to light touch sensation where the greater the loss of touch , the greater will be the dysesthesia. i.e. in motor pain, dysesthetic burning from stretch of muscles is greatest distally in extremities as is impaired proprioception.

In Central Pain, while proprioceptive impairment, as measured by vibratory sense, is greatest where sensory loss is greatest (i.e. distally); the patient may perceive dysesthetic burning  to extend further centrally than is the perception of proprioceptive loss. Isolated areas of nondysesthetic constant cramping sensation are also seen in Central Pain, possibly most commonly in extensors (i.e. behind the right thigh in a patient whose symptoms are otherwise bilateral).

As already emphasized, the muscle pains of Central Pain can be extremely severe but are often overlooked because the examiner is fascinated by, or the patient is fixated on, the dysesthetic burning. From a research standpoint, however, here again, the Central Pain patient shines as a laboratory subject and deserves our careful attention.

While there are exceptions, the most common pain is associated with tonic and not dynamic activity. The patient describes tonic pain as a "burning cramp", which is dysesthetic, or unfamiliar. Patients are unable to localize dysesthetic cramping sufficiently to discriminate between muscle belly and muscle tendon as the place of origin. It manifests most readily when the muscle is under stretch, such as in the quadriceps when descending to a squat and remaining there. Its onset is within a few seconds of assuming the position and becomes unbearable in a minute or more.

Because dysesthetic cramp occurs when the muscle is under tension, it may derive from the Golgi tendon organs. The pain from dynamic activity can range from the discomfort of sore muscles to unbearable torture rendering the patient a functional paralytic. In the most common situation, if a Central Pain patient attempts to exercise, the tonic soreness in the muscles will be unbearable the next day. The onset of dynamic pain is immediate.

Beric reported of a Central Pain patient so agonized by movement pain severe enough to functionally paralyze the patient although they possessed an intact motor unit. This should alert clinicians to the likely presence of similar distress of a lesser degree in many Central Pain patients. (Beric 1993) The physician should look upon dread of movement by the patient as a measure of the almost overwhelming severity of Central Pain, and not as an indicator of patient weakness.

It is not known whether the "diminished proprioception" seen in Central Pain originates exclusively in the posterior columns or whether it is actually a reflection of spindle dysfunction. The physiological connections between the spindle and the posterior columns is poorly understood.

The debate continues about whether the posterior columns are involved in Central Pain. Historically, researchers thought the posterior columns carried epicritic (discriminatory) pain, and protopathic (poorly localized burning) followed the spinothalamic tract. Some researchers opined that the epicritic suppressed (or gated) the protopathic, and that Central Pain reflected a protopathic spinothalamic tract which had become a "loose cannon" after posterior column injury caused failure of epicritic damping of the protopathic. Alternatively, virtually all Central Pain patients retain some degree of function in the posterior columns. Some regard this as evidence the posterior columns are not involved, while others regard it as a manifestation of what happens when a partially injured epicritic posterior column improperly regulates the protopathic spinothalamic tracts.

Patients variously describe gamma pain as "like I have been forced in a torture session to remain in one uncomfortable position until my muscles are burning", "pulling", "stretching", "tearing", "crushing", or "cramping". These descriptors often fail to distinguish causation and quality. It appears patients sometimes confuse questions about gamma pain to be about the position most likely to cause the pain, rather than its quality.(Boivie1989, and in Wall 1989)

Beric has made great contributions in illuminating the muscle aspects of Central Pain. He is a leader in such research and his terminology is gaining acceptance. Beric terms tonic/tension pain "proprioceptive pain" (other researchers have distinguished patients who complain of "pain from sense of position" from those who complain of "confinement cramps"); and the dynamic pain he terms "kinesthetic pain". (Beric 1993) Kinesthetic pain is also called "motion allodynia", inasmuch as ordinary movement is not painful in people without Central Pain. Some patients report that deep muscle pressure which would have been neutral as to sensation feels "pleasurable" in Central Pain.

A "withdrawal reflex" has been described in Central Pain, which consists of slight but sudden, involuntary, adductor movement when unexpected touch of affected extremities is experienced. Just as a person may not tickle himself, the patient cannot make himself do this, nor will it display if the patient knows the examiner is attempting it. According to one theory, this simply represents the unmasking of spinal protective reflexes in medullary (cord) injury (Armen Haerrer, 1993 personal communication); or, it may originate in spindle dysfunction. The "withdrawal reflex" may be related to the nociceptive flexion reflex noted in evoked potential studies. ( Bougosslavsky 1995, i.e. Barth, 505).

A common muscle complain in Central Pain is that of limb heaviness of the feeling of impending collapse. The result is a painful fatigue, which greatly interferes with mobility. "One patient described as feeling that the signal to move wasn't really getting to my legs". Deep pressure is the only known possible beneficial therapy. The patient cannot work through this by exercise, indeed, ill-directed attempts to encourage or require exercise may put the patient into a psychological crisis over the pain induced.

Although loss of motor memory is not a pain, it should be noted that loss of motor memory in limbs which are dysesthetic is common in central pain

• Situation: Patient complains of severe agony with Central Pain dysesthesia. Physician, thinking of endogenous opiate release (endorphins), prescribes exercise.
  
  
• Error: This approach is not entirely wrong, but it requires such time consuming deep massage that it may not be manageable. Central Pain patients generally cannot bring themselves to endure more pain, such as unassisted exercise brings.

  Central Pain patients with severe gamma pain (pain from the muscle spindle) have both pain on movement (Berich's kinesthetic allodynia) and severe "lactic acid buildup pain" (proprioceptive allodynia) following any attempt at exercise. If the patient suffers primarily from the latter, then the patient can endure exercise, provided that a great deal of deep massage to stretch the spindle is provided at intervals during and following the exercise. This is such a massive effort that it generally requires the support of volunteers. Theoretically, physical therapists could perform such assistance but the expense would be overwhelming.

  As to endorphins, Central Pain does not respond to opioids, at least in conventional dosages. We have yet to find the Central Pain patient who could endure enough exercise to generate a superendorphin release sufficient to stop the pain, and we probably never will. Exercise programs which benefit nociceptive pain are not comparable and probably irrelevant.

  As to the theory behind endorphine induction, it is not at all clear that the allodynic state so easily induced in rats is the same as the complex syndrome of Central Pain. Baclophen and other medicines which showed promise in rats have yielded disappointing results in humans.

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• Situation: Physician prescribes hydromassage for gamma pain.
  
  

• Error: Gamma pain requires deep stretching of the muscle spindle, not conventional massage. Hydromassage is inadequate to accomplish spindle stretch. It must generally be done by hand, pushing into the muscle to fatigue the muscle and damp the spindle, followed by elongation parallel to the direction of the muscle fibers.

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• Situation: Physician prescribes TENS unit for Central Pain.
  
  

• Error: This again is not necessarily wrong, but clearly the physician should provide it only after weighing benefit against risks and following informed consent. Central Pain patients do show some response to TENS insofar as it refers to spindle (gamma) pain in superficial muscles. It is unknown whether this is conventional TENS response or represents some sort of blocking which reaches the spindle. The problem is that it risks worsening Central Pain or reactivating Central Pain which has abated. Beric reports of patients where resolved Central Pain recurred after TENS application.

  Another serious consideration is that the pads necessary to deliver current across the skin are certain to cause evoked dysesthesia during the period of their application. The patient must decide whether muscle pain relief outweighs the evoked dysesthesia in a small area. TENS does not benefit dysesthesia. no one knows whether it benefits lancinating pain.

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• Situation: Patient has neck pain and headaches in the area supplied by cranial nerve V (trigeminal nerve). The physician will then prescribe cervical traction.
  
  

• Error: A patient with evoked dysesthesia cannot tolerate the cervical traction device because the sling contacts the face in areas supplied by the trigeminal nerve. Trigeminal nerve sensation covers approximately the front half of the head and the brain dura. The physician should remember the dural supply by the trigeminal nerve when evaluating headache in Central Pain patients. The trigeminal line or boundary approximately follows along the angle of the lower mandible and swings up through the ear auricle toward a point at the vertex of the head.

  Cervical nerves supply the back of the head, and these areas may be missed in injury which affects the trigeminal pain tracts. Theoretically some sort of traction could be devised by clamping onto the hair on the back of the head, but as you may imagine, this would be difficult to effect practically.