Pain, including muscle pain (myalgia), is
not a compulsory symptom for ME under the International Consensus Criteria (ICC).
http://www.co-cure.org/MEICC.pdf
Pain is one of four symptoms categories under the Neurological Impairment section.
At least one symptom from just
three of the four categories below is required, so pain could be absent.
1. Neurocognitive Impairments
2. Pain
3. Sleep Disturbance
4. Neurosensory, perceptual and motor disturbances
Even then, the pain need not be muscle pain (myalgia):
2. Pain
a. Headaches: e.g. chronic, generalized headaches often involve aching of the eyes, behind the eyes or back of the head that may be associated with cervical muscle tension; migraine; tension headaches
b. Significant pain can be experienced in muscles, muscle-tendon junctions, joints, abdomen or chest. It is non-inflammatory in nature and often migrates. e.g. generalized hyperalgesia, widespread pain (may meet fibromyalgia criteria), myofascial or radiating pain.
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Although I had both flu-like muscle aches and joint pain early on and was actually diagnosed with "post infectious myalgia" at a time when M.E. (not to mention CFS) was unheard of in the U.S., I never considered it a major symptom compared to what else was going on. Much more significant in terms of pain were the "migraine" headaches and a sort of "inflamed" feeling just under the skin in certain areas where there was no apparent inflammation.
Never-the-less, despite the name
Myalgic Encephalomyelitis, "myalgia" is not a required symptoms according to the ICC.