Subcortical myoclonus originates from abnormal activity in neural circuits and sites ranging from the thalamus and basal ganglia to the brainstem and spinal cord. It is generally divided into nonsegmental (involving multiple sites or circuits) and segmental (limited to specific spinal or brainstem segments) forms.

The common causes and associated statistics for subcortical myoclonus include:

1. Neurodegenerative and dementia syndromes

Neurodegenerative disorders frequently manifest with subcortical or cortical–subcortical myoclonus as the disease progresses.

• Dementia with Lewy Bodies (DLB): Statistics show that patients with DLB have a significantly higher incidence of myoclonus than other dementias, with a cumulative probability of 58.1%.
• Corticobasal Syndrome (CBS): Myoclonus in CBS is often asymmetric and movement-induced, with a prevalence of approximately 41.0% in this patient group. While it can have cortical features, advanced connectivity studies suggest prominent cerebellar and subcortical involvement.
• Creutzfeldt–Jakob Disease (CJD): Jerks in CJD are often generated by a cortical–subcortical loop. The disease presents with a rapid cognitive decline and generalised myoclonus, showing a 69% to 92% sensitivity for MRI signal abnormalities in the basal ganglia and cortex.
• Huntington’s Disease: While it can be cortical, it often involves subcortical structures like the basal ganglia.

2. Metabolic and toxic disorders

Systemic metabolic derangements often exert toxic effects on subcortical structures.

• Uraemia (kidney disease): Severe kidney disease often produces reticular, stimulus-sensitive myoclonus due to the toxic effect of uraemia on the medulla oblongata in the brainstem.
• Hepatic encephalopathy: This condition typically presents with a "negative myoclonus" (asterixis) and is associated with diffuse triphasic waves on EEG, often involving cortical–subcortical generators.
• Drug-induced: Many medications, particularly antiseizure medications at high doses, can induce subcortical jerks.

3. Post-hypoxic cerebral damage

• Reticular reflex myoclonus: This is a classic subcortical–nonsegmental form often following hypoxic encephalopathy. It involves a localised source in the brainstem (reticular formation) spreading rostrocaudally to involve the cranial nerves and then the trunk and limbs.

4. Infectious diseases

• Subacute sclerosing panencephalitis (SSPE): A rare complication of measles that causes rapidly progressive dementia and subcortical myoclonus. EEG typically shows Radermecker complexes—high-voltage periodic polyphasic sharp and slow wave complexes lasting 0.5 to 2 sec and recurring every 4 to 15 sec.
• Viral encephalitis: Herpes simplex virus (HSV) encephalitis can cause focal or generalised jerks of subcortical or cortical origin.

5. Genetic and isolated syndromes

• Myoclonus–dystonia syndrome: Formerly known as "essential myoclonus," this is a subcortical–nonsegmental disorder often caused by a mutation in the epsilon-sarcoglycan (SGCE) gene.
• Opsoclonus–myoclonus syndrome: This involves rapid, multidirectional eye movements and multifocal subcortical jerks, often of an autoimmune or paraneoplastic nature.
• Propriospinal myoclonus: The primary source is located in the cervical or thoracic spinal cord, causing axial flexion of the trunk and hip muscles.

6. Structural and segmental lesions

Segmental myoclonus is caused by localised pathological lesions in the brainstem or spinal cord.

• Palatal myoclonus: Caused by lesions in the Guillain–Mollaret triangle of the brainstem, resulting in rhythmic jerks of the palatal muscles at a frequency typically between 1–3 Hz.
• Spinal segmental myoclonus: Resulting from local spinal cord damage, these jerks are rhythmic and unaffected by sensory stimuli or sleep.

Statistics on burst duration

A key neurophysiological differentiator for subcortical (non-cortical) myoclonus is the duration of the muscle burst:

• Non-cortical myoclonus (NCM): The mean burst duration is 56.7 ms (±11.5 ms), which is significantly longer than cortical forms (31.1 ms).
• Threshold accuracy: Using a threshold of 45.0 ms, burst duration distinguishes subcortical/non-cortical from cortical myoclonus with 100% sensitivity and 89.5% specificity.

Incidence: Pathological myoclonus (all types) has an estimated annual incidence of approximately 1.3 cases per 100,000 persons.