Learning insight: the "big three" characteristics 

To distinguish myoclonus from other movement disorders like tics, tremors, or chorea, we focus on three specific markers:

1. Brief duration: These are exceptionally fast events, with cortical bursts typically lasting less than 50 ms.
2. Non-rhythmic nature: Unlike the regular oscillation of a tremor, myoclonus is typically irregular (arrhythmic).
3. Involuntary origin: These movements are not suppressible and occur without the premonitory urge associated with tics. While these jerks are the visible "output," they are merely the tip of the iceberg, representing a sudden electrical discharge within the brain's complex circuitry.

2. The neocortex: the "hyper-excitable" surface

In the "blueprint" of the brain, the sensorimotor cortex acts as the primary generator of cortical myoclonus. This condition occurs when abnormal neuronal discharges originate on the cortical surface and descend to the muscles.

The spread of excitation 

The electrical "fire" can follow two distinct architectural paths:

• Focal: The firing remains localised, causing a jerk in a single muscle group, such as the face or a distal extremity.
• Bilateral: The excitation spreads rapidly through cortico-cortical and transcallosal pathways, crossing the bridges between hemispheres to trigger synchronous movements on both sides of the body.

Typical triggers for hyperexcitability

• Action-induced: Jerks precipitated by voluntary movement or the intent to move.
• Stimulus-sensitive (reflex): Jerks triggered by external "shocks" to the senses (tactile, acoustic, or visual).
• Spontaneous: Jerks occurring unexpectedly at rest without an external trigger. To diagnose this state of hyper-reactivity, clinicians utilise neurophysiological stress tests to measure the cortex's response to sensory input.

3. Somatosensory evoked potentials (SSEPs) and the "giant" response

Somatosensory evoked potentials (SSEPs) measure the brain's electrical response to a sensory stimulus, typically an electrical pulse delivered to the median nerve. 

Instructor's Note

The N20 distinction: A critical diagnostic "trap" is the N20 wave. In cortical myoclonus, the N20 component is typically normal. This proves that the sensory signal arrives at the primary somatosensory cortex (S1) correctly. The pathology occurs after arrival, where the brain over-processes the signal, resulting in the enlarged P25 and N33 components. 

The concept of the "giant SEP": In a hyperexcitable brain, the standard response is amplified into a "giant SEP.

Clinical Definition: A "giant SEP" is an electrical response 5 to 10 times larger than the norm. This reflects a profound lack of inhibition in the primary somatosensory area, where the cortex "over-processes" sensory input, turning a minor touch into a massive electrical discharge.

Technical refinement: To successfully record these signals, the stimulus frequency must be kept low (approximately 1 Hz). Faster stimulation can cause "extinction," where the enlarged components disappear, leading to a false-negative result. If the brain is over-responding to input in this manner, it suggests a failure in the cortical "braking system" that normally prevents excessive output.

4. Intracortical inhibition: the missing brake

A functional brain maintains a precise balance between excitation and inhibition. In myoclonic conditions, we often see an impairment in short-interval intracortical inhibition (SICI).

The role of GABA: The brain's primary inhibitory neurotransmitter is GABA. Specifically, GABA-ergic interneurons act as the "brakes" of the motor system. 

Concept Box

The GABA-A brake mechanism: GABA-A-mediated inhibition. 

Function: SICI serves as a filter. When this GABA-A system is impaired, the brain loses its ability to suppress accidental motor commands, allowing the cortical "generator" to fire unchecked and produce involuntary jerks. This loss of "braking" power creates a short-circuit where a simple sensory stimulus can immediately trigger a motor response.

5. The C-reflex: the exaggerated loop

The C-reflex (or long-latency reflex) is a pathologically enhanced transcortical reflex. In a healthy subject, this reflex is usually only seen during active muscle contraction. In a hyperexcitable patient, it can appear even at rest.

The path of a C-reflex:

1. Sensory stimulus (input): An electrical pulse to the median nerve travels to the cortex.
2. Rapid cortical processing (the generator): Due to the lack of GABA-A "brakes," the signal is instantly amplified and converted into a motor command.
3. Immediate motor discharge (the jerk): The command travels down the corticospinal tract, causing a jerk in the thenar (thumb) muscles. To prove that a muscle jerk is the direct result of a specific brain spike, we use "detective tools" that sync brain waves with muscle activity.

6. Detective tools: JLBA and polygraphy

Neurophysiologists use two primary methods to link the brain to the movement: EEG-EMG polygraphy and jerk-locked back-averaging (JLBA). 

The "time-travel" of JLBA: JLBA is a computer-assisted "look back." Because a single brain spike is often too small to distinguish from background noise, the computer records multiple jerks and looks backward in time. By averaging these segments, the noise cancels out, revealing the cortical "spike" that preceded the movement. 

Technical precision

The latency gap: For a jerk to be confirmed as "cortical," the EEG spike must precede the muscle activity by a specific window:

• 10–20 ms for jerks in the arm.
• 30 ms for jerks in the leg