In humans, a single transverse palmar crease is a single crease that extends across the palm of thehand, formed by the fusion of the two palmar creases that people typically have. Because it resembles the usual condition of non-human simians, it is also known as a simian crease or simian line, although these terms have fallen out of favor due to its pejorative connotation.
A simian crease is a single line that runs across the palm of the hand. People normally have three creases in their palms.
The term “simian crease” is not used much anymore since it tends to have a negative meaning (it refers to monkey or ape). The crease is usually just refered to as a single palmar crease.
The presence of a single transverse palmar crease can be, but is not always, a symptom associated with abnormal medical conditions, such as Fetal alcohol syndrome, or with genetic chromosomal abnormalities, including Down syndrome(chromosome 21), cri du chat syndrome (chromosome 5), Noonan syndrome (chromosome 12), Patau syndrome (chromosome 13), Edward’s syndrome (chromosome 18), and Aarskog-Scott syndrome (X-linked recessive). Bilateral or unilateral single palmar creases are also associated with aberrations on chromosome 9. Also sometimes found on hand of affected side of patients with Poland Syndrome.
Males are twice as likely as females to have this condition, and it tends to run in families. In its non-symptomatic form, it is more common among Asians and native Americans than among other populations, and in some families there is a tendency to inherit the condition unilaterally, that is, on one hand only
Strong lines (called palmar flexion creases) appear on the palms of the hands and soles of the feet. The palm normally has three of these creases. But sometimes, the horizontal creases join together to form a single one.
Palmar creases develop while the baby is growing in the womb, usually by the 12th week ofgestation.
A single palmar crease appears in approximately 1 out of 30 people. Males are twice as likely as females to have this condition. Some palmar creases indicate problems with development and are associated with disorders like Down syndrome.