clonus

Introduction

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

decorticate posturing
- occurs due to severe cerebral cortical dysfunction
- results in arms are bent inward at the elbows and held tightly against the chest with wrists and fists clenched, while the legs are stiffly extended and rotated inward
decerebrate posturing
- occurs due to severe brainstem dysfunction below the red nucleus
- results in all four limbs are rigidly straightened and extended out, with the arms pronated (palms facing away), toes pointed downward, and the head and neck arched backward

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