In spina bifida aperta, muscle ultrasound can quantify the "second hit of damage"

Purpose: In spina bifida aperta (SBA), the "second-hit hypothesis" addresses consequences by delayed neurological damage superimposed upon the congenital myelomeningocele (MMC). This secondary damage is postulated to underlie the disappearance of leg movements shortly after birth. Innovative fetal surgery might prevent this, but results are methodologically hard to prove in small and heterogeneous treatment groups. We reasoned that delayed postnatal alterations in muscle ultrasound density (MUD = muscle echogenicity) could quantitatively reflect consequences by "the second hit" of damage. In the present study, we investigated whether delayed postnatal leg-MUD alterations are associated with postnatal muscle function loss. Methods: We cross-sectionally assessed leg-MUD in 16 postnatally operated SBA children (MMC-L5; at 0, 6, and 12 months; in n = 11/16; 11/16, and 15/16 children, respectively) and compared outcomes with 13 healthy control children. Additionally, we assessed SBA MUD caudal and cranial to the MMC and calculated MMC-L5 impact by: dMUD_(MMC-L5) = [MUD _{calf muscle/S1-2}] - [MUD_{quadriceps muscle/L2-4}] and associated outcomes with leg muscle function caudal to the MMC. Results: At 0 month, clinically discernible dMUD was more often increased in SBA than in control newborns (p <.05), but a relationship between absolute quantitative differences and leg muscle dysfunction was still lacking. At 6-12 months, additionally increased dMUD outcomes coincided with SBA leg muscle dysfunction (p <.05). Conclusions: In post-neonatal SBA, secondarily increased dMUD (i.e., MMC impact) coincides with leg muscle dysfunction. This may implicate that muscle ultrasound could provide a quantitative tool to assess the neuromuscular impact by the second hit of damage.