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Jaw jerk - A complete guide

Jaw jerk is a deep tendon reflex that help to localise the lesion to above the level  of pons. This is a stretch reflex used to test the status of a patient's trigeminal nerve (CN V).Jaw jerk  is a classic monosynaptic stretch (myotactic) reflex, common to many somatic joints. The jaw jerk reflex can be classified as a dynamic stretch reflex
The jaw jerk reflex was originally described by Lewis in 1882
Other names of jaw jerk
Chin reflex, Chin-jerk reflex, Mandibular reflex. masseter reflex
How to perform jaw jerk?
  • Ask the patient to open the Mouth partially and the jaw relaxed
  • Tongue remains inside the mouth.
  • Left  index finger of the examiner is placed over the middle of the patient's chin.
  • The index finger is then tapped with a reflex hammer, delivering a downward stroke.
  • The normal response is slight and constists of sudden closure of the mouth.(the masseter muscles will jerk the mandible upwards).
This reflex is sometimes absent in health.
Normal Response:
The jaw should not deviate to either side. The jaw-jerk is usually absent or weakly present.
Abnormal Response:
The jaw deviates towards the side of weakness.
the jaw-jerk is exaggerated and pathologically brisk with lesions affecting the pyramidal pathways above the 5th nerve motor nucleus, especially if the lesions are bilateral.

Clinical significance of jaw jerk
The Jaw jerk is increased In tJMN lesions above the Vth cranial nerve nucleus occur in the following conditions
  • Pseudobulbar palsy
  • Multiple sclerosis. 
In patients with spastic quadriparesis , an exaggerated jaw jerk excludes cervical cord disease and it points to pyramidal tract disease above the pons .This reflex will help to distinguish an upper cervical cord compression from lesions that are above the foramen magnum.
The mandibular reflex, or jaw jerk, is the only monosynaptic reflex available for electrophysiologic testing in the cranial muscles
How does the jaw jerk reflex help in localizing lesions in patients with hyperreflexia?
The jaw jerk is a reflex that involves the contraction of the masseter and temporalis muscles when the patient's lower jaw is tapped. The jaw jerk is exaggerated with bilateral lesions above the level of trigeminal nerve but will not be affected by lesions below it in the spinal cord. This is helpful in patients who have hyperreflexia in all four limbs  because an exaggerated jaw jerk reflex suggests that the lesion is above the level of the spinal cord (i.e., high brain stem or brain).
Hyper reflexia may be  seen in cervical spondylotic myelopathy can be similar to multiple sclerosis (MS) or amyotrophic lateral sclerosis (ALS), however, a hyperactive jaw reflex suggests the pathology is above the foramen magnum. In other words, a normal jaw jerk reflex points the diagnosis toward cervical spondylotic myelopathy and away from MS or ALS.
Path way for jaw jerk
Both the afferent and efferent fibres are present in the trtgeminal  nerve.The afferent impulse for this reflex is the sensory portion of the trigeminal nerve ie the afferent limb travels via the mandibular branch of the trigeminal nerve
Then reach the mesencephalic nucleus of the trigeminal nerve situated in pons.
The efferent limb arises from the motor nucleus of the trigeminal nerve and also travels via the mandibular branch. The efferent limb is through the motor (V3) branch of the trigeminal nerve.

Factors affecting the reflex
Gender, age