see also:

• neurology
• upper motor neuron weakness
• weakness of the lower limb(s) / weak in the knees

• clonus is a series of involuntary, rhythmic muscle contractions and relaxations triggered by a sudden stretch of a muscle, most commonly elicited by the clinician rapidly dorsiflexing the ankle joint and holding it dorsiflexed, but can also be elicited at the knee, wrist, fingers or jaw
• normal physiologic clonus fades away after 1-4 beats
• persistent clonus more than 4 beats is pathologic and usually implies an upper motor neuron (UMN) pathology
• in contrast, myoclonus is a brief, involuntary, irregular twitching of a muscle or a group of muscles

• usually occurs along with hyperreflexia, spasticity and muscle weakness as part of upper motor neuron weakness

• neonates (due to immature nervous system) - disappears soon as the brain matures
• cerebral palsy
• traumatic brain injury
• motor neurone disease (MND)
• serotonin syndrome
• spinal cord injury
• spinal cord ischaemia
• transverse myelitis
• encephalitis
• leukoencephalopathy (ADEM) - acute, rapidly progressive, mainly in children
• pseudobulbar palsy - involves muscles innervated by the cranial nerves IX, X and XII, may have labile emotions and bilateral upper motor limb signs

• stroke (CVA)
• other focal intracranial pathology causing unilateral weakness eg. tumour, abscess, multiple sclerosis (MS)
• spinal cord injury
• spinal cord ischaemia