From:
Chronic
Pain: Reflex Sympathetic Dystrophy Prevention and Management
CRC Press,
Boca Raton, Florida
H. Hooshmand, M.D.
Referred Pain
Referred pain usually accompanies RSD. The acute
non-RSD (somesthetic) type of pain usually is not accompanied by referred pain.
Referred pain is quite common in visceral pathology.
Whereas burning, crushing, and cutting of skin cause severe pain, the same stimuli do not
cause any sensation in the gut.432 The noxious stimuli that cause pain and
referred pain in viscera distention,431 anoxia,436 and acidity.434
Lewis433 demonstrated that stimulation of
"myofacial" structures, i.e., muscle, periosteum, and ligaments, cause pain
quite similar to visceral pain. Lewis433 stimulated referred pain in 94% of
experimental injection of 6% normal saline into deep skeletal structures of 28 normal
volunteers. The pattern of referred pain was quite consistent, although not confined to
the injected dermatomal segment. Cohen,429 Theobald437 and White and
Sweet438 demonstrated that superficial stimulation of peripheral lesions (e.g.,
a fracture elbow or a stump neuroma) can instigate recurrence of angina pectoris. It is
obvious that cutaneous stimulation causes efferent visceral changes such as
vasoconstriction. This referred pain, typical of the
function of the sympathetic system, is quite common in RSD (e.g., shoulder-hand syndrome).
It may explain the higher incidence of heart attack in RSD patients. Obviously
stress-induced pain (SIP) is another feature of RSD that results in distressful strain on
the cardiovascular system.
Sterling phenomenon435 has been
experienced by the majority of normal population. This referred pain is a sharp pain -
always ipsilateral and in a distant dermatome - after scraping the skull with a fingernail
or pulling of an unwanted hair (e.g., nostril hair). The same phenomenon is noted
pathologically on dilatation of abdominal aneurysm, which can cause pain in the testicle
prior to sudden death. Squeezing the testicle can cause excruciating pain in the nipple.
Most commonly, referred pain occurs in cervical
spondylosis (see Etiology of RSD) and cervical sprain with complex symptoms outline in the
section of cervical spondylosis in Table 13.
The two factors that are important in the
development of referred pain are (1) wide dynamic range(WDR) distribution of c fibers at
the point of input in the spinal cord and (2) Sherrington's phenomenon573
(Figure 16).
As the physiologist Sherrington demonstrated,573
referred pain is principally caused by the input of multiple sensory nerves from different
dermatomes and different parts of the body (skin as well as viscera) into substantia
gelatinosa in the dorsal horn of the spinal cord (Figures 10,11,13,and 16).
The large number of sensory nerve endings entering
the substantia gelatinosa stimulate fewer number of internuncial nerve cells. This results
in an overlap of sensory input - be it proprioceptive or nociceptive - on the same
secondary neurons (Figure 16).
The overlap results in the stimulation of one nerve
cell by multiple nerves. For example, a nociceptive input from an inflamed appendix may
overlap the proprioceptive input of proximal portions of the ileum. As a result, the
appendicitis pain may be felt in the epigastric region.
The dorsal branches of the posterior cervical nerve
roots overlap the distal branches of the same nerve roots at the are of entrance to the
substantia nigra. As a result, a patient who has nerve root irritation may not feel the
pain in the hand or arm, but in the posterior aspect of the shoulder (Figure 16). This
results in mistaken diagnoses such as bursitis of the shoulder or rotator cuff injury.
The same principle of referred pain can result in an
increase or decrease in temperature in the trigger points in the remote areas of sensory
nerve endings. For example, damage over the dorsum of the hand to the radial nerve many
cause increased temperature in the distribution of sensory nerve fibers of the radial
nerve in the dorsum of the hand, and may also cause a trigger point of hot or cold nature
in the posterior aspect of the shoulder.
Usually the trigger point is cold (Figure 2a and 3).
It is commonly seen in the posterior aspect of the shoulder, trapezius muscle, scalene
muscle, sacroiliac joint region, or frontal region of the head in the case of upper
cervical nerve irritation (Figure 5) where they are cold rather than hot spots. The cold
trigger point is formed by longstanding myofascial reaction to the accumulation of
substance P and other irritants in the area of referred pain.
Message or insertion of a needle in the cold spot
results in release of the entrapped irritants. The superimposed skin becomes reddish, and
after several minutes the irritants are absorbed. This may be the reason for therapeutic
effects of massage.
