Todd’s paralysis is caused by temporary dysfunction in the brain’s motor regions after a seizure. During a seizure, neurons fire intensely and rapidly, consuming enormous amounts of energy and oxygen. Once the seizure stops, those overworked neurons essentially shut down for a recovery period, leaving the body parts they control weak or paralyzed. The weakness typically resolves on its own, with a median recovery time of about 15 hours and most cases clearing completely within 36 hours.
How Seizures Exhaust the Brain
The core trigger is a seizure, most often one that originates in or spreads to the motor cortex, the strip of brain tissue responsible for voluntary movement. During a seizure, neurons in this area fire at an extraordinary rate. That burst of activity demands a sharp spike in oxygen, glucose, and blood flow. When the seizure ends, those neurons are metabolically depleted. They’ve burned through their fuel supply and need time to restore normal function.
Think of it like a muscle that’s been worked to complete exhaustion. Just as your legs might give out after an all-out sprint, the neurons controlling movement become temporarily unable to send signals after the extreme exertion of a seizure. This concept, first proposed in the mid-1800s by the neurologist Robert Bentley Todd, remains one of the leading explanations for the condition that bears his name.
Reduced Blood Flow After Seizures
More recent research points to a second mechanism working alongside that metabolic exhaustion: reduced blood flow to the affected brain region. After a seizure, oxygen levels and blood flow in the involved area can drop and stay low for up to an hour. Brain imaging studies have captured this in real time, showing measurably decreased blood volume and flow in the motor areas on the same side of the brain where the seizure occurred. When those same scans are repeated roughly 24 hours later, blood flow patterns return to normal, matching the patient’s clinical recovery.
The typical pattern appears to be a brief surge of extra blood flow during and immediately after the seizure, followed by a period of reduced perfusion. That secondary drop in blood supply may starve the already-depleted neurons of the oxygen they need to recover quickly, prolonging the paralysis. The combination of cellular exhaustion and temporary blood flow reduction likely explains why the weakness can last anywhere from minutes to, in rare cases, several days.
Increased Inhibitory Brain Activity
A third contributing factor involves the brain’s own braking system. Some researchers have proposed that Todd’s paralysis is not just passive exhaustion but also an active process. After a seizure, inhibitory signaling in the motor areas ramps up dramatically, essentially telling those neurons to stop firing. This may be the brain’s protective response to prevent another seizure from starting. The downside is that this heightened inhibition also suppresses normal movement signals, producing weakness or paralysis on the affected side of the body.
These three mechanisms, metabolic depletion, reduced blood flow, and increased inhibitory signaling, likely work together rather than independently. The relative contribution of each may vary from person to person and seizure to seizure, which could explain why some episodes of Todd’s paralysis last 20 minutes while others persist for a full day.
Which Seizures Cause It
Not every seizure leads to Todd’s paralysis. It occurs most often after focal seizures, meaning seizures that start in one specific area of the brain rather than involving the whole brain at once. When the seizure focus sits in or near the motor cortex, the resulting weakness shows up on the opposite side of the body. A seizure originating in the left hemisphere, for example, produces right-sided weakness. This contralateral pattern is so reliable that in one study of 29 patients with postictal weakness, the paralysis occurred on the side opposite the seizure origin in 93% of cases.
Generalized seizures (those involving both hemispheres simultaneously) can also trigger Todd’s paralysis, though the weakness is more commonly one-sided. The condition can affect an arm, a leg, one entire side of the body, or occasionally speech and vision, depending on which brain region was most involved in the seizure.
Why It Resolves on Its Own
Todd’s paralysis is temporary because the underlying brain tissue is not damaged. Unlike a stroke, where blood flow is blocked long enough to kill neurons, the blood flow reduction after a seizure is brief and self-correcting. Neurons gradually restore their energy reserves, blood flow normalizes, and the excessive inhibitory signaling fades. The vast majority of people experience full, spontaneous recovery. The median resolution time is about 15 hours, and symptoms almost always clear within 36 hours.
There is no specific treatment to speed up recovery. Management focuses on preventing further seizures through existing seizure control plans and, critically, on ruling out stroke. Because one-sided weakness is also the hallmark of a stroke, emergency evaluation is important any time new paralysis develops, even if the person has a known seizure disorder. The distinction matters because stroke treatment is time-sensitive in a way that Todd’s paralysis is not.
Why the Distinction From Stroke Matters
The overlap in symptoms between Todd’s paralysis and stroke creates a genuine diagnostic challenge. Both can produce sudden weakness on one side of the body, difficulty speaking, or facial drooping. If someone is found with these symptoms and no one witnessed a preceding seizure, the initial assumption is often stroke until proven otherwise. Brain imaging, particularly perfusion studies that map blood flow patterns, helps clinicians tell the two apart. In Todd’s paralysis, the imaging either looks normal or shows transient blood flow changes that resolve within hours. In stroke, imaging reveals a blocked vessel or a region of brain tissue that is not receiving adequate blood supply.
For people with epilepsy who experience Todd’s paralysis repeatedly, the pattern itself becomes a useful clinical tool. Because the weakness so reliably appears on the side opposite the seizure focus, it helps neurologists pinpoint where seizures originate. This localization information can be valuable when evaluating whether someone is a candidate for surgical treatment of epilepsy.