Definition of Simian crease

Simian crease: A single transverse palmar crease, a minor variation associated with Down syndrome (trisomy 21). Also called a four-finger crease, single palmar flexion crease, single upper palm crease, or line.

There are most often two transverse palmar creases, a proximal one and a more distal one, neither of which extends all the way across the palm. Fusion of these two creases, an event that occurs prior to the 12th week of fetal life, creates a single crease extending horizontally across the entire palm.

A single transverse palmar crease is present in normal people with a frequency of 0.8% on the right hand and 1.3% on the left hand whereas it is present with a frequency of 71% on the right hand and 66%% on the left hand in Down syndrome. It is therefore useful in the clinical diagnosis of Down syndrome.

Like many minor anatomic variations, a single transverse palmar crease is associated with a number of different dysmorphic disorders, including Aarskog syndrome; de Lange syndrome; the deletion syndromes involving chromosomes 4p, 4q, 5p (cat cry syndrome), and 18q; the fetal alcohol syndrome; Seckel syndrome; Smith-Lemli-Opitz syndrome; the syndromes due to trisomy 13 (Patau syndrome) and trisomy 8; and Zellweger syndrome.