Neck strain, spondylotic changes in cervical vertebrae, bone disease or bad posture should be looked into.
Nerve infections as in Lyme’s disease and
nerve disorder such as Multiple sclerosis too should be investigated. I would suggest you get the blood levels of the following checked if they have not already been checked: potassium, calcium, sodium, magnesium, vitamin B complex, or vitamins B1, B3, or B6 and Vitamin D.