Why is there numbness in face, arm, and hand on one side?

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Is it sudden onset or gradually develops from last many days. If it is gradual and it is there since long time period, then it is most probably spondylosis both cervical spine. It is a spine problem where you get numbness, pain, and other symptoms due to compression of nerves between spine. Do you have any history of trauma to your spine, does this pain increase on neck movement, is it associated with tingling, needles and pins sensation, if yes then it further supports the diagnosis. You will need vitamin D level, X-ray cervical spine, spine AP/lateral view and further may need MRI if X-rays are not informative. The treatment for this is physiotherapy to release the nerve compression, traction, cervical collar, medicine and if none helps, then surgery. Till the time you can have tablet Neurobion forte once a day which is a nerve vitamin. You need to visit a neurologist for all these investigations. If the problem is suddenly and of short duration, then it can be a minor stroke and needs further investigation.