A site for medical students - Practical,Theory,Osce Notes

Clonus and its clinical signifiance

Rhythmic reflex monophasic contractions and relaxations of hypertonic muscles in response to an abruptly applied and sustained strecth stimulus
Clonus is a rhythmical series of contractions in response to the maintenance of tension in a muscle, associated with increased gamma efferent discharge. It is elicitable when tendon reflexes are brisk after a corticospinal lesion
The word Clonus  comes from the Greek word  meaning "violent, confused motion. Significance  of clonus is that it is a sign of neurological conditions, epecially associated with upper motor neuron lesions involving descending motor pathways
Clonus is a condition that produce involuntary muscle spasms and most commonly it affects the muscles at the end of a limb. Clonus mainly affects the knees, ankles, upper and lower arm muscles, and the jaw.
The basic abnormality in clonus is
1. alternate involuntary muscular contraction and relaxation in rapid succession.
2. a continuous rhythmic reflex tremor that is initiated by the spinal cord below an area of spinal cord injury, when set in motion by reflex testing.
Mechanism of clonus
The exact mechanism of clonus remains unclear. Two different hypotheses have been asserted regarding the development of clonus.
  • The most widely accepted explanation is that hyperactive stretch reflexes in clonus are caused by self-excitation. 
  • Another alternative explanation for clonus is central generator activity that arises as a consequence of appropriate peripheral events and produces rhythmic stimulation of the lower motor neurons.

What are the association of clonus?
Clonus is always associated with
  • Brisk tendon reflex. 
  • Spasticity and
  • Bablnski's sign-It is a very reliable sign of pyramidal tract lesion. 
There arc two types of clonus :
  • Sustained clonus (true clonus).
  • Unsustained clonus (pseudoclonus).
Unsustained clonus (≤5 beats): may be physiological
Sustained clonus (>5 beats): regarded as abnormal

Sites for clonus examination  in clinical neurology :
  • Ankle clonus.
  • Patellar clonus.
  • Jaw clonus (elicit the Jaw jerk to see a scries of closure and opening of the mouth).
  • Wrist clonus (elicited by sudden passive extension of the fingers).
3 and 4 are not routinely practised in clinical neurology.

Medical conditions that can cause clonus include:
  • Multiple sclerosis
  • Huntington disease
  • Spinal cord or brain injuries
  • Stroke
  • Meningitis
  • Cerebral palsy
Difference between a organic and non organic clonus

Organic clonus          Functional clonus
Sustained                         Non sustained
Bilateral equal                 Unequal
Exaggerated reflexes       Absent
Plantar extensor          Flexor