Acute ischemic stroke: This is the most likely diagnosis due to the sudden onset of neurological deficits, which is a hallmark of ischemic stroke. The symptoms can include sudden weakness, numbness, or paralysis of the face, arm, or leg, which are also common in hemiplegic migraine but the severity and rapid progression often point more towards a stroke.

Hemiplegic migraine: This condition presents with similar symptoms to ischemic stroke, including hemiparesis (weakness on one side of the body), but it is typically accompanied by a headache and sometimes visual or sensory auras. The key to differentiating it from a stroke is the presence of these migraine features and a slower onset of neurological deficits.
Transient ischemic attack (TIA): Often referred to as a "mini-stroke," TIA has symptoms similar to those of a stroke but they last for a shorter period, typically less than 24 hours. It's a warning sign of a future stroke and requires immediate medical attention.

Subarachnoid hemorrhage: Although less common than ischemic stroke, subarachnoid hemorrhage can present with sudden severe headache and neurological deficits. Missing this diagnosis can be fatal due to the risk of rebleeding and vasospasm.
Cerebral venous sinus thrombosis: This condition involves a blood clot in the brain's venous sinuses and can present with headache, seizures, and focal neurological deficits. It's less common but can be deadly if not treated promptly.
Posterior reversible encephalopathy syndrome (PRES): Characterized by headache, seizures, and visual disturbances, PRES can sometimes mimic stroke or migraine but is associated with specific imaging findings and often occurs in the context of hypertension or certain medications.

Mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS): A rare genetic disorder that can cause stroke-like episodes, often accompanied by other systemic symptoms such as seizures, muscle weakness, and developmental delays.
CADASIL (Cerebral Autosomal Dominant Arteriopathy with Subcortical Infarcts and Leukoencephalopathy): An inherited condition that affects the blood vessels in the brain, leading to subcortical strokes and dementia, typically presenting in mid-adult life.
Reversible cerebral vasoconstriction syndrome (RCVS): Characterized by sudden, severe headaches and sometimes neurological deficits due to transient constriction of cerebral arteries, which can mimic stroke or hemiplegic migraine.