Definition of Tonic-Clonic Seizure
A tonic-clonic seizure is a type of epileptic seizure characterized by two distinct sequential phases: an initial tonic phase with sustained forceful muscle contraction causing rigid extension of the extremities, followed by a clonic phase with massive synchronous jerking movements of the arms and/or legs. 1
Clinical Phases and Characteristics
Tonic Phase
- Sustained muscle contraction causing forceful extension of the extremities that lasts from seconds to minutes 1, 2
- Patient may keel over like a falling log due to the rigid posture, rather than collapsing flaccidly 1
- The tonic posture can occur before the fall, distinguishing it from syncope where movements occur after collapse 1
- Characterized by increased amplitude of muscle activation on electromyography 2
Clonic Phase
- Massive synchronous jerks of the arms and/or legs, not the limited asynchronous movements seen in syncope 1
- Clonic movements are symmetrical and synchronous in generalized seizures 1
- These movements begin at the onset of unconsciousness in epilepsy, whereas in syncope they typically start 20 seconds after loss of consciousness 1
Duration and Sequence
- Mean duration of generalized tonic-clonic seizures is approximately 62 seconds, though individual phases show high variability 3
- Only 27% of seizures include all five possible phases (onset of generalization, pretonic clonic, tonic, tremulousness, and clonic) 3
- The complete seizure typically includes many movements (described as "100" or "cannot count"), contrasting with syncope which has approximately 10 movements 1
Associated Features
During the Seizure
- Tongue biting occurs much more frequently than in syncope, typically on the lateral side of the tongue (uni- or bilateral) 1
- Cyanotic face may be observed 1
- Eyes remain open during the seizure 1
- Stertorous (snoring) breathing may occur 1
Post-Ictal Period
- Prolonged confusion lasting more than a few minutes, distinguishing it from syncope where clearheadedness is usually immediate 1
- Muscle pain lasting hours or days 1
- Headache and elevation of creatinine kinase and prolactin levels 1
Key Distinguishing Features from Syncope
Critical pitfall: Movement during loss of consciousness is often misinterpreted by both medical personnel and laymen as automatically indicating epilepsy, but syncope can also have movements ("convulsive syncope"). 1
The distinguishing features are:
- Timing of movements: Epileptic movements begin at onset of unconsciousness; syncopal movements begin after collapse (mean 20 seconds later) 1
- Type of movements: Epileptic movements are synchronous and massive; syncopal movements are asynchronous and limited 1
- Duration of unconsciousness: <30 seconds strongly suggests syncope; >1 minute suggests epilepsy 1
- Post-event state: Immediate clearheadedness suggests syncope; prolonged confusion suggests epilepsy 1
Subtypes
Primary Generalized Tonic-Clonic Seizures
- Occur in idiopathic generalized epilepsy syndromes 4, 5
- Typically show symmetric tonic phase and minimal asymmetry during clonic phase 6
- May have brief focal features at onset (such as adversive head turn) but these are inconsistent in direction 6