In the chronic tension-type headache patient, a needle EMG into an extrafusal muscle fiber shows normal activity of the musculature. When the needle EMG is moved only a few millimeters into a trigger point, EMG activity increases significantly (3) (O). If the needle is left in the trigger point, patients' symptoms continue to worsen as long as the needle remains (4).
In asymptomatic controls, (5) when the needle
EMG is placed into a "latent" trigger point (tender to palpation, but the
patient reports no symptoms) and the subject is placed in a stressful situation
(asked to quickly count backwards by 13s, without mistakes), EMG activity
of the intrafusal fibers increases significantly (P).
Injecting a sympathetic alpha-blocker into the site eliminates the contraction
activity. Additional animal studies has shown that the intrafusal
fibers of the spindles are sympathetically innervated.
Conclusion: The "trigger points" that
are responsible for myofascial pain and the "tension" that a tension-type
headache patient report are due to chronic sympathetically tensed intrafusal
fibers of the spindles (O). The intensity
of nocturnal clenching may be propagated by the increased intrafusal tension.
Upon exposure to stressful events, either physical or emotional, sympathetic
innervation increases, intrafusal fibers tense more, and the patient reports
an increase of symptoms. The most common areas of spindual dysfunction
are temples (temporalis), neck and shoulders (trapezius).