De novo twig-like middle cerebral artery after suspected M1 dissection in early childhood with coexisting KRAS-mutated brain arteriovenous malformation: illustrative case.

backgroundTwig-like middle cerebral artery (T-MCA) is an uncommon configuration in which the M1 segment is replaced by a plexiform network reconstituting the distal middle cerebral artery (MCA). It is often labeled congenital, yet adult de novo cases suggest acquired formation after focal M1 disease. OBSERVATIONS: A male neonate underwent angiography at 6 days of life for a suspected right brain arteriovenous malformation (AVM); the contralateral left M1 segment was normal. After staged AVM treatment, follow-up angiography at approximately 1 year showed a new left M1 irregularity and narrowing consistent with dissection, without ischemic lesions on serial MRI. Over the ensuing 2-3 years, serial angiography documented progression to left M1 occlusion with replacement of the M1 trunk by a plexiform collateral network that reconstituted the distal MCA, forming a classic T-MCA configuration by 4.5 years. The child remained neurologically intact. Molecular analysis of AVM tissue identified a likely pathogenic somatic KRAS in-frame duplication. LESSONS: This pediatric case shows that T-MCA can form postnatally after focal M1 injury and may be misclassified as congenital on a single late angiogram. The coexisting KRAS-mutated AVM raises a testable role for somatic vascular genetics in remodeling responses beyond RNF213-focused testing. https://thejns.org/doi/10.3171/CASE2611.