What Causes Todd's Paralysis?
Todd's paralysis is caused by transient neuronal dysfunction following a seizure, with the underlying mechanism believed to involve cerebral perfusion abnormalities and neuronal exhaustion in the motor cortex after ictal activity. 1
Pathophysiological Mechanism
The exact pathophysiology remains incompletely understood, but the leading theory centers on post-ictal cerebral perfusion abnormalities that develop after seizure activity. 1 The motor weakness results from temporary dysfunction of neurons in the motor cortex that have been exhausted by the preceding seizure activity, leading to transient paralysis contralateral to the epileptogenic zone. 2
- The condition represents neuronal exhaustion and metabolic depletion in cortical areas involved in the seizure, particularly affecting motor pathways. 1
- Cerebral hyperperfusion syndrome can also trigger seizures followed by Todd's paralysis, as documented in post-surgical cases. 3
Seizure Types That Cause Todd's Paralysis
Todd's paralysis most commonly occurs after partial (focal) seizures or generalized tonic-clonic seizures. 1 The Canadian Stroke Best Practice guidelines specifically identify "seizure with post-ictal Todd's paralysis" as a condition that can mimic acute stroke and should be excluded when evaluating patients for thrombolytic therapy. 4
- The paralysis typically manifests as contralateral weakness to the epileptogenic zone after focal or focal-to-bilateral tonic-clonic seizures. 2
- Bilateral Todd's paralysis is exceedingly rare but can occur when seizure activity spreads to both primary motor cortices. 2
Risk Factors and Predisposing Conditions
Old stroke is an independent risk factor for developing Todd's paralysis, with seizures having remote etiologies (including old stroke) showing a positive likelihood ratio of 11.2 for developing post-ictal paralysis. 5
- Advanced age significantly increases the risk of Todd's paralysis following seizures. 1, 5
- Convulsive status epilepticus and longer duration of convulsion are associated with higher incidence. 5
- Pre-existing structural brain lesions, including old infarcts, significantly predispose patients to developing Todd's paralysis—occurring in 19.7% of seizures with remote etiologies. 5
- Patients with epilepsy associated with stroke history have greater incidence of post-ictal paralysis. 1
Clinical Pitfall: Distinguishing from Acute Stroke
The critical diagnostic challenge is that Todd's paralysis presents identically to acute ischemic stroke with hemiplegia, making it a dangerous stroke mimic. 4, 1 The Canadian Stroke guidelines explicitly list "stroke symptoms due to seizure with post-ictal Todd's paralysis" as an absolute exclusion criterion for thrombolytic therapy. 4
- The diagnosis becomes particularly difficult in patients without a known history of epilepsy who present with new-onset weakness. 1
- The presence of premorbid structural CNS lesions (especially old stroke) should raise suspicion for Todd's paralysis when weakness follows witnessed or suspected seizure activity. 5