Knee jerk responses in infants at high risk for cerebral palsy: an observational EMG study

Following our clinical observation of tonic responses in response to the knee jerk in infants at very high risk for cerebral palsy (VHR infants), we systematically studied tonic responses, clonus, and reflex irradiation. We questioned (i) whether these responses occurred more often in VHR infants than in typically developing (TD) infants, and (ii) whether they were associated with abnormal general movement quality.

Twenty-four VHR and 26 TD infants were assessed around 3 mo corrected age. Surface electromyograms of leg, trunk, neck, and arm muscles were recorded while eliciting the knee jerk. All assessments were video-recorded.

VHR infants more often than TD infants showed tonic responses in the ipsilateral quadriceps and hamstring (Mann-Whitney U; P = 0.0005 and P = 0.0009), clonus (Chi-square; P = 0.0005) and phasic responses in the contralateral quadriceps and hamstring (Mann-Whitney U; P = 0.002 and P = 0.0003, respectively). Widespread reflex irradiation occurred in VHR and TD infants. Definitely abnormal general movements and stiff movements were associated with tonic responses (Mann-Whitney U; P = 0.0005, P = 0.007, respectively) and clonus (Mann-Whitney U; P = 0.003 and P = 0.0005) in the ipsilateral quadriceps.

Similar to clonus, tonic responses may be regarded as a marker of a loss of supraspinal control. Reflex irradiation primarily is a neurodevelopmental phenomenon of early ontogeny.