The thinning of the arm is mainly due to the following reasons: First, if the arm is thinning on one side, accompanied by a decrease in hand strength, it is necessary to ask the patient whether he or she has taken drugs or been exposed to chemicals in the past, and it is possible that the nerve damage is caused by drug or chemical poisoning. Second, if the arm thinning is progressive, from one limb to bilateral limbs, accompanied by muscle pain. This time to consider the patient may have muscle disease, need to go to do kinase and electromyography, but also to do muscle MRI, to understand whether there is polymyositis or myotonic dystrophy. Thirdly, if the arm becomes thin, if the disease starts on one side, also accompanied by hand muscle atrophy, this time to do cervical spine magnetic resonance examination, to understand whether there is no cervical spine compression of nerves after the symptoms, but also to do cranial magnetic resonance and electromyography examination, to understand whether there is the possibility of motor neuron disease.