Electromyographic (EMG) burst duration is a critical neurophysiological marker used to identify the anatomical source of myoclonus, as the simplicity and brevity of cortical discharges differ significantly from the more complex pathways involved in non-cortical forms (Riva et al. 2024; van der Veen et al. 2024).

Statistical differentiation and diagnostic accuracy

Recent comparative research has established specific quantitative thresholds for distinguishing between cortical and non-cortical origins:

• Mean burst durations: In direct clinical comparisons, the mean burst duration for cortical myoclonus (CM) is 31.1 ms (±6.7 ms), which is significantly shorter than the 56.7 ms (±11.5 ms) found in non-cortical myoclonus (NCM) (van der Veen et al. 2024).
• The 45.0 ms threshold: Using a threshold of 45.0 ms, burst duration distinguishes CM from NCM with 100% sensitivity and 89.5% specificity (van der Veen et al. 2024).
• Area under the curve (AUC): The diagnostic value of this feature is classified as "excellent," with an AUC-ROC of 0.947 (van der Veen et al. 2024).
• Data requirements: A minimum of 10 randomly selected bursts from a single patient is sufficient to achieve reliable diagnostic accuracy (van der Veen et al. 2024).

Burst duration by anatomical origin

The duration of the EMG burst varies according to the location of the myoclonic generator:

• Cortical myoclonus: Classically presents with very brief bursts, typically ranging from 10 to 50 ms (Riva et al. 2024; Latorre et al. 2023). In conditions like primary generalised epileptic myoclonus, durations are uniformly within this 10–50 ms range (Latorre et al. 2023).
• Cortical–subcortical myoclonus: Bursts are slightly longer but still relatively brief, typically 25–100 ms, and are time-locked to EEG spike/poly-spike discharges (Riva et al. 2024).
• Subcortical–non-segmental myoclonus: This heterogeneous group (including reticular reflex myoclonus) typically features long discharges, reaching up to 300 ms (Riva et al. 2024).
• Segmental myoclonus: This includes spinal and brainstem forms, where bursts are widely variable, ranging from 50 ms to 500 ms (Riva et al. 2024).
• Peripheral myoclonus: Bursts are often focal and show marked variability, generally ranging from 50 ms to 200 ms (Riva et al. 2024).

Pathophysiological mechanisms for duration

The difference in duration is attributed to the complexity of the neural pathways involved:

• CM brevity: The brevity of a cortical jerk is thought to resemble the action potentials evoked by a reflex jerk (like a tendon tap), reflecting a relatively direct and rapid discharge from the sensorimotor cortex (van der Veen et al. 2024).
• NCM longevity: The longer duration in NCM (particularly in myoclonus-dystonia) is attributed to the involvement of multiple subcortical structures such as the basal ganglia, thalamus, and brainstem (van der Veen et al. 2024). These abnormal electrical activities travel through more complex circuits, leading to prolonged muscle contractions (van der Veen et al. 2024).

Clinical considerations and limitations

• Condition-specific ranges: In multiple system atrophy, burst durations of 25–50 ms have been recorded (Latorre et al. 2023). In patients with myoclonus-dystonia, while the mean is ~57 ms, some studies have reported higher averages of 95 ms to 102 ms, potentially due to the inclusion of longer dystonic movements (van der Veen et al. 2024).
• Lack of specificity: While short duration (<50 ms) is a supportive criterion for CM, it is not pathognomonic; fasciculation potentials can be shorter than 50 ms, and some forms of spinal myoclonus can also manifest with short-lasting discharges (Latorre et al. 2023).
• Voluntary activity: Discriminating between CM bursts and voluntary ballistic movements can be difficult, as single motor unit activations in healthy subjects can also produce very short bursts (Latorre et al. 2023).