Neuromediator and hormonal perturbations in fibromyalgia syndrome: results of chronic stress?

Since the first comprehensive description of the symptoms of FMS by Yunus et al (1981), numerous investigations have confirmed that FMS is a clinical entity. However, the aetiology of the syndrome is still not fully elucidated. It seems, however, logical to place the origin of the disorder in the muscle. Muscle pain, especially at the muscle-tendon junctions, fatigue and stiffness are the first symptoms. A malfunction of energy metabolism has been detected in part of the muscle fibres. However, it has to be considered that the muscle is not an isolated entity. Its activity is controlled by segmentally arranged motor units of the ventral horn of the spinal cord in response to proprioceptive afferent signals arising in the muscle spindles or in other sensory elements including nociceptors. Together with supraspinal descending inputs, the spinal motor neurone pool is the common final pathway for segmental and suprasegmental inputs, making the motor system extremely powerful for adaptive adjustments but also vulnerable if deficits occur in either of these input levels. A second, recently discovered abnormality seen in FMS is a lowered serotonin level in peripheral and most likely also central structures. The underlying mechanism seems to be defective absorption of the precursor amino acid tryptophan from the gut. Serotonin is involved centrally in the regulation of the sleep pattern, and at the spinal level it acts as a 'gain setter' of motoneurone excitability and suppresses signal transmission of noxious stimuli in dorsal horn neurones. Either of these two disturbances, muscle energy depletion or serotonin deficiency, could by itself evoke many of the symptoms of FMS, and their combined appearance will perpetuate the disease. Depressed levels of somatomedin C, caused by a deficit of stage 4 sleep-dependent release of GH, might represent an additional factor in preventing proper development or repair of myoskeletal structures. Malabsorption of certain amino acids, possibly due to a genetic disorder of gut transport mechanisms, may constitute an additional deleterious factor. The abnormalities found in the HPA and HPT axis may be seen as an attempt of the organism to restore homeostasis. The stimulus eliciting this counter-regulatory reaction may be pain or other afferent signals which normally do not reach the central nervous system. It is doubtful whether the unspecific activation of the HPA axis in a non-inflammatory disease is beneficial.(ABSTRACT TRUNCATED AT 400 WORDS)