Recessive congenital methemoglobinemia type II: Hypoplastic basal ganglia in two siblings with a novel mutation of the cytochrome b5 reductase gene.

Recessive congenital methemoglobinemia type II is a very rare autosomal recessive hematologic disorder due to NADH-cytochrome b5 reductase deficiency, usually caused by full-stop mutations or deletions. This disease classically presents with mild neonatal cyanosis, early onset severe progressive developmental delay, movement disorders, and progressive microcephaly. We report two siblings with recessive congenital methemoglobinemia type II whose evaluation revealed a novel p.Arg92Trp missense mutation of the CYB5R3 gene and a peculiar imaging finding of basal ganglia hypoplasia. Brain magnetic resonance imaging was performed at age 10 months in the older sibling and at age three months in the younger sibling. It revealed similar findings of bilateral small size of the lentiform and caudate nuclei and reduced frontotemporal brain volume. Our patient cases highlight that basal ganglia hypoplasia is an interesting clue to the very rare and frequently unsuspected diagnosis of recessive congenital methemoglobinemia type II, that may explain the associated movement disorders. The novel missense mutation is one of very few identified missense mutations known to cause severe type II recessive congenital methemoglobinemia.