casal collar casal collar the casal collar or casal necklace is a clinical sign in which there is an erythematous pigmented skin rash in the distribution of a broad collar (dermatomes c3 and c4). it is seen in patients with pellagra, as a result of niacin (vitamin b3) deficiency. a boy with casal collar caused by pellagra. the sign is named after gaspar casal, who first described pellagra.