Spasticity due to Upper Motor Neuron Lesion: This is the most common cause of hypertonicity in arms in stroke patients, resulting from damage to the upper motor neurons in the brain or spinal cord, leading to increased muscle tone and stiffness.

Parkinson's Disease: Although less common in the acute stroke setting, Parkinson's disease can cause hypertonicity in the arms due to dopamine deficiency and can be a consideration in patients with a history of the disease.
Dystonia: A movement disorder characterized by involuntary muscle contractions, which can cause hypertonicity in the arms and can be seen in some stroke patients, particularly those with basal ganglia involvement.
Serotonin Syndrome: A condition caused by excessive levels of serotonin, which can occur in patients taking certain medications, including antidepressants, and can lead to hypertonicity, among other symptoms.

Tetanus: A rare but life-threatening condition caused by the bacterium Clostridium tetani, which can cause severe muscle stiffness and hypertonicity, and requires prompt diagnosis and treatment.
Meningitis or Encephalitis: Infections that can cause increased intracranial pressure, leading to hypertonicity, and require urgent medical attention.
Neuroleptic Malignant Syndrome: A rare but potentially fatal condition caused by an adverse reaction to neuroleptic medications, which can cause hypertonicity, fever, and changes in mental status.

Stiff Person Syndrome: A rare autoimmune disorder characterized by progressive stiffness and hypertonicity in the muscles, which can be considered in patients with unexplained hypertonicity.
Ischemic Dystonia: A rare condition caused by ischemia to the basal ganglia, which can lead to dystonic movements and hypertonicity in the affected limbs.
Wilson's Disease: A rare genetic disorder that can cause neurological symptoms, including hypertonicity, due to copper accumulation in the brain.