Objective: To highlight the importance of distinguishing jaw clonus from jaw tremor which can be a useful sign in clinical diagnosis by reporting a case of ALS.

Background: Jaw clonus is an involuntary rhythmic contraction induced by the jaw jerk stretch reflex and is suggestive of upper motor neuron dysfunction. When it occurs spontaneously, it may be mistaken for the jaw tremor that is associated with movement disorders such as Parkinson’s disease (PD), essential tremor (ET), SCA12 or tardive tremor.

Method: We present a case report (with video) of a 65 year old woman who was initially referred for evaluation of Parkinson’s disease who had experienced a jaw tremor at rest for one year.

Results: Physical exam was pertinent for spastic dysarthria, left hemiparesis associated with hyper-reflexia, slowness (but not bradykinesia) and gait circumduction with the left hip. The patient was able to suppress her ‘jaw tremor’ by opening the mouth. The tremulous jaw movement could be elicited by performing the jaw jerk reflex. MRI Brain revealed mild volume loss pronounced in the right anterior temporal region. Further investigation with an EMG/NCV revealed diffuse active and chronic denervation suggestive of motor neuron disease and the study met criteria for the diagnosis of ALS.

Conclusion: Not every rhythmic oscillatory movement is representative of tremor. Jaw clonus may occur spontaneously and may be confused for tremor. It can be elicited and must steer the examiner towards consideration for motor neuron disease rather than a classic movement disorder.

References: 1. Saifee TA, Macerollo A. Jaw clonus in motor neuron disease: an interesting case and review of literature. Neurological Sciences. 2017 Dec 13; 39(5): 949–50. DOI: https://doi.
org/10.1007/s10072-017-3216-y
2. Williams L, Ryan M, Kilbride R, Llamas-Osorio Y. Jaw Clonus: A Rhythmic Oscillatory Movement, but Not Tremor. Tremor Other Hyperkinet Mov (N Y). 2020 Sep 29; 10: 34. DOI:
https://doi.org/10.5334/tohm.538
3. Kumar R, Blackband J, Shukla AW. Rhythmic JAw Moveents in Amyotrophic Lateral Sclerosis: Is it Clonus or Tremor? Tremor and Other Hyperkinetic Movements. 2024; 14(1): 8, pp. 1–3. DOI: https://doi.org/10.5334/tohm.845

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MDS Abstracts - https://www.mdsabstracts.org/abstract/jaw-clonus-masquerading-as-jaw-tremor-in-a-case-of-amyotrophic-lateral-sclerosis-als/