Alzheimer's is a degenerative brain condition which is the leading cause of dementia
in the world. The condition leads to progressive loss of cognitive function coupled with social impairment. Importantly, although Alzheimer's disease does produce many general symptoms, it rarely produces focal symptoms, such as twitching of a single arm. This suggests that there may be something else going on. Numerous things can produce isolated twitched. It could be a benign tremor, Parkinson's disease, a tumor, a partial seizure, or a medication side effect, among other things. All of these possibilities are things that should be evaluated relatively urgently to make certain that what your brother has is not something that is potentially life threatening. As always the diagnosis and the management of your brother's particular concerns will require a physical examination by his personal physician. Setting up an office visit with his primary care doctor
to discuss this concerning symptom as soon as possible is recommended.