Objective: Evaluate the presence of Hung-up Knee jerk reflex (HUKJR) in premanifest and symptomatic Huntington’s Disease (HD) patients and its neurophysiological correlate. 

Background: The HUKJR is a classical clinical sign specific of HD that can be observed at physical examination along the disease course. HUKJR happens when testing the knee jerk reflex if the extended leg does not relax straightaway and remains elevated. This singularity of HD, known as “Gordon’s reflex phenomenon”, is due to a sustained contraction of the quadriceps femoris muscle and its origin is still unknown. Electromyography register shows a high sensibility detecting the abnormal continuous quadriceps contraction that follows the knee-jerk examination in HD, even if the HUKJR is not seeing at physical examination. As far as we know, the presence of HUKJR has never being evaluated systematically in premanifest individuals. 

Methods: Prospective study of the knee-jerk reflex in genetically confirmed HD patients compared to controls. Patients were classified as premanifest (preHD) if their total motor score, measured through the Unified Huntington’s Disease Rating Scale (UHDRS), was <5 and symptomatic if their UHDRS was ≥5. Controls were enrolled from partners of HD patients. The presence of HUKJR was evaluated by 2 neurologists at physical examination during a needle electromyography register, performed at the quadriceps muscle. We achieved 10 trials in each leg, including Jendrassik maneuver to facilitate the Knee-jerk reflex. We classified the individuals according to the different clinical and electromyography patterns observed. 

Results: We included 35 individuals, 24 genetically confirmed patients -14 preHD, 10 HD- (median age 43±6 years, 6 males, CAG 42±3) and 11 controls (41±4 years, 4 males). HUKJR was present at physical examination in 14% of preHD, 80% of HD and none of the controls; 2 HD also presented a contralateral HUKJR while exploring the presence of ipsilateral HUKJR. We could identified 3 different electromyography patterns: monophasic (controls n=11, preHD n=5); dichrotic (preHD n=7, HD n=1) and sustained (preHD n=2, HD n=9). Patients with the sustained pattern also presented the HUKJR at physical examination.  

Conclusions: HUKJR is frequent in HD and it can be observed in premanifested individuals. Electromyography can detect changes many years before motor phenoconversion and could be used as a marker of disease progression. 

References: Obeso JA, Rothwell JC, Marsden CD. Hung-up tendon jerks in Huntington’s chorea—study of a little sign. Arch Neurobiol 1984; 47:151–6.

Claus D, Lang C, Neundorfer B. Gordon’s reflex phenomenon in Huntington’s Disease. Eur Arch Psychiatr Neurol Sci 1987; 236:303–8.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/hung-up-knee-jerk-reflex-in-huntingtons-disease-a-clinical-and-neurophysiological-study/