Rotator Cuff Injury or Tendinitis: This condition is likely due to the repetitive motion and strain on the shoulder muscles when performing activities like combing hair or brushing teeth, which can exacerbate existing rotator cuff issues.

Muscular Dystrophy: Certain types of muscular dystrophy can lead to muscle weakness, particularly in the proximal muscles, which could manifest as difficulty in maintaining arm position during daily activities.
Peripheral Neuropathy: Damage to the nerves supplying the arm muscles can cause weakness and drooping, especially if the neuropathy affects the nerves controlling shoulder and arm movements.
Cervical Radiculopathy: Nerve root compression in the cervical spine can lead to arm weakness, pain, and numbness, which might be exacerbated by certain positions or movements.

Stroke or Transient Ischemic Attack (TIA): Although less common, a stroke or TIA can cause sudden weakness or paralysis of one side of the body, including the arm. Early recognition is crucial for timely intervention.
Amyotrophic Lateral Sclerosis (ALS): ALS is a progressive neurological disease that affects nerve cells in the brain and spinal cord, leading to loss of muscle control. Early symptoms can include muscle weakness or twitching, which might be noticed during routine activities.

Myasthenia Gravis: An autoimmune disorder that leads to fluctuating muscle weakness, which can be exacerbated by activity and improved by rest. It's rare but should be considered in the differential diagnosis due to its potential for significant morbidity if left untreated.
Guillain-Barré Syndrome: A rare autoimmune disorder where the body's immune system mistakenly attacks part of its peripheral nervous system, leading to muscle weakness and sometimes paralysis. It typically follows a minor infection and can present with ascending weakness, starting in the lower limbs but potentially affecting the arms.