Early and subtle signs in low-back sprain.

The authors have previously reported myalgic hyperalgesia as a useful localizing sign in "low-back sprain" patients with no physical findings. This paper describes some other subtle signs related to the phenomenon of denervation supersensitivity which is well known to physiologists and clinicians involved in peripheral nerve disease, yet its related signs have not been applied to low-back pain. Following denervation of some neurons, muscle and peripheral receptors become supersensitive to transmitter substances and to different forms of stimuli. Since the peripheral nerve is a mixed nerve, findings are multiphasic and include autonomic dysfunction, trophic changes, cutaneous and myalgic hyperalgesia, and increased muscle tone. One or more of these signs occurred in 30 patients with secondary low-back pain but less often in 30 patients with primary or mechanical low-back pain; their presence, though slight, in asymptomatic controls may identify those individuals with a vulnerable back.