medwireNews: Randomised trial findings show a positive effect of growth hormone (GH) therapy on muscle thickness in infants with Prader-Willi syndrome (PWS).

The trial involved 22 infants with PWS, aged an average of 12.9 months, of whom 10 received daily GH (1 mg/m2) from baseline and 12 did not receive it or did so after a 6-month delay. All the infants received physical training.

At baseline, muscle thickness of the biceps brachii, forearm flexors, quadriceps and tibialis anterior was below normal, with standard deviation (SD) scores of around –1.5, and the infants’ muscle strength was 58% of that expected for their age, weight and height.

After 6 months, both groups had improved muscle thickness in the tibialis anterior, so that SD scores had improved to –0.8 and –0.6 in the control and GH groups, respectively. But only the GH group had improved thickness SD scores for biceps brachii and forearm flexors, at –0.5 for both compared with –1.4 and –1.3, respectively, in the control group. Quadriceps thickness improved in the GH group, albeit nonsignificantly to a SD score of –0.9, but not in the control group.

The effect of GH on muscle thickness was independent of age and baseline muscle thickness.

The variable catch-up growth observed between the different muscle groups is probably related to the frequency with which they are used, suggest  Linda Reus (Radboud University Medical Center, Nijmegen, the Netherlands) and study co-authors.

“In muscles that are used a lot in the acquisition of fundamental skills in early motor development, there was a naturally occurring catch-up in growth independent of GH treatment, suggesting a training effect”, they write in Pediatrics.

After 20 of the children had received 12 months of GH therapy, their SD scores ranged from –0.8, for tibialis anterior thickness in both groups, to –0.1 for forearm flexor thickness in the GH group.

Muscle thickness significantly correlated with the infants’ muscle strength, assessed every 3 months using the Infant Muscle Strength metre, and also with motor function on the Gross Motor Function Measurement. Muscle strength and motor performance also correlated.

“We showed for the first time that in infants with PWS, decreased muscle thickness in specific muscle groups is strongly associated with decreased muscle strength and motor performance”, say the researchers.

However, they note that the infants’ motor development remained “seriously delayed”, despite muscle thickness improving to near normal, “which means that although decreased muscle thickness contributes to the motor performance problems, it can also be hypothesized that innate brain pathology affects motor and cognitive development.”

By Eleanor McDermid, Senior medwireNews Reporter

Pediatrics 2014; 134: e1619–1627

Free abstract