What are tonic and clonic phases in the context of seizures?

Definitions of Tonic and Clonic Phases in Seizures

Tonic refers to sustained, forceful muscle rigidity with extension of the extremities, while clonic refers to massive, synchronous jerking movements of the arms and/or legs. 1

Tonic Phase

The tonic phase is characterized by specific motor and physiologic features that distinguish it from other seizure phenomena:

Motor Characteristics

• Sustained muscle contraction with forceful extension of the extremities that causes the patient to become rigid and stiff 1
• The patient may "keel over like a falling log" due to the rigid posture, rather than collapsing in a flaccid manner 1
• Movements are symmetrical and synchronous, beginning at the onset of unconsciousness 1
• The tonic phase typically lasts 10-20 seconds before transitioning to the clonic phase 2

Associated Features

• An "ictal cry" may occur at onset as air is forcefully expelled through partially closed vocal cords 2
• The face may become cyanotic (blue) due to respiratory arrest 2
• Eyes typically remain open during this phase 2
• Complete loss of consciousness coincides with the onset of tonic movements 2

Pathophysiology

• Research demonstrates that the tonic phase involves a significant shift toward higher frequency muscle activation (100-500 Hz), with increased signal amplitude compared to voluntary muscle contraction 3
• The mechanism of muscle activation during epileptic tonic activity differs fundamentally from physiologic voluntary contraction 3

Clonic Phase

The clonic phase follows the tonic phase and has distinct characteristics:

Motor Characteristics

• Massive, synchronous jerking movements of the arms and/or legs (not to be confused with "myoclonus," which describes different phenomena) 1
• Movements consist of EMG discharges of remarkably constant duration (approximately 0.2 seconds) separated by silent periods of exponentially increasing duration 4
• The jerking movements are symmetrical and synchronous in primary generalized seizures, though asymmetry may occur transiently, particularly in focal-onset seizures 5
• Many movements occur (often described as "100" or "cannot count"), unlike the few movements (approximately 10) seen in syncope 1

Duration and Progression

• The clonic phase shows progressive lengthening of silent periods between jerks, eventually leading to seizure termination 4
• Mean duration of the entire tonic-clonic sequence is approximately 62 seconds, though marked variability exists 6

Critical Distinguishing Features from Syncope

Understanding these definitions is clinically essential for differentiating seizures from syncope:

• In epilepsy, clonic movements can occur before the fall, whereas in syncope, movements occur after the patient has collapsed due to brain ischemia 1
• Tonic posture causes the patient to fall stiffly, while syncope typically involves flaccid collapse 1
• Syncopal movements are asynchronous and limited in scope (termed "myoclonic" in neurology), contrasting with the massive synchronous jerks of true clonic activity 1
• The onset of movements relative to loss of consciousness differs: epileptic movements begin at the onset of unconsciousness, while syncopal movements typically begin approximately 20 seconds after loss of consciousness 1

Terminology Clarification

The term "clonic" in everyday medical use is restricted to epilepsy, while "myoclonus" describes movements in syncope as well as certain epilepsy types 1. The word "convulsions" is best reserved specifically for epilepsy, and "myoclonic jerks" has little diagnostic specificity 1. This distinction prevents unwarranted diagnostic conclusions based on imprecise terminology.