The tonic response to the infant knee jerk as an early sign of cerebral palsy

Background: Early identification of infants at risk of cerebral palsy (CP) is desirable in order to provide early intervention. We previously demonstrated differences in knee jerk responses between 3-month-old high risk and typically developing infants. Aims: To improve early identification by investigating whether the presence of tonic responses (continuous muscle activity occurring after the typical phasic response), clonus or contralateral responses to the knee jerk during infancy is associated with CP. Study design: Longitudinal EMG-study. Subjects: We included 34 high-risk infants (median gestational age 31.9 weeks) who participated in the LEARN2MOVE 0-2 years trial. Outcome measures: Video-recorded knee jerk EMG-assessments were performed during infancy (1-4 times). Developmental outcome was assessed at 21 months corrected age (CA). Binomial generalized estimating equations models with repeated measurements were fitted using predictor variables. Results: Infants who later were diagnosed with CP (n = 18) showed more often than infants who were not diagnosed with CP i) tonic responses - from 4 months CA onwards, ii) dorms - from 13 months CA onwards, and iii) contralateral responses - from 15 months CA onwards. Limitations: The main limitation is the relatively small sample size. Conclusions: The assessment of tonic responses to the knee jerk using EMG may be a valuable add-on tool to appraise a high risk of CP.