Contribution of reflex hyper-excitability to muscle stiffness in children with cerebral palsy

We aimed to investigate the contribution of reflex hyper-excitability to muscle stiffness in children suffering from cerebral palsy. To address this major controversial clinical issue, we studied the relation between the sonoelastography image and the Hoffman reflex (H-reflex) of lower limb muscles in fifteen children with spastic hemiplegic cerebral palsy. Muscle stiffness was quantified using the sonoelastography images of the affected ankle plantar-flexor muscles. The major parameters were autocorrelation, variance, difference entropy, energy, and histogram ratio. Hyper-excitability of stretch reflexes was evaluated using H-reflex responses of the ankle plantar-flexors. The important features were peak-peak amplitude of maximum H response, maximal M-wave, H/M ratio, latency of H, M, the time interval between H and M, and the stimulus intensity elicited max-H and max-M. The correlations between the major parameters quantifying sonoelastography images and H-reflex responses were calculated using Pearson-correlation analysis. The correlation analysis showed a relation between two features of sonoelastography images (i.e. autocorrelation and variance) and the maximal H/M ratio of the H-reflex response. These findings indicate that an abnormal increase in muscle stiffness, which typically occurs in CP patients, is at least partially due to hyper-excitability of reflexes. The clinical implication is that abnormal reflex responses may play a significant role in neuromuscular abnormalities, which are caused by spasticity in children with cerebral palsy.