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 referred to as a single palmar crease.

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 of gestation.

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.

A single palmar crease is often a normal finding. However, it may also be associated with:

• Down syndrome
• Aarskog syndrome
• Cohen syndrome
• Fetal alcohol syndrome
• Trisomy 13
• Rubella syndrome
• Turner syndrome
• Klinefelter syndrome
• Pseudohypoparathyroidism
• Gonadal dysgenesis
• Cri du chat syndrome

An infant with a single palmar crease may have other symptoms and signs that, when taken together, define a specific syndrome or condition. Diagnosis of that condition is based on a family history, medical history, and complete physical exam.

Your doctor may ask questions such as:

• Is there a family history of Down syndrome or other disorder associated with a single palmar crease?
• Does anyone else in the family have a single palmar crease without other symptoms?
• Did the mother use alcohol while pregnant?
• What other symptoms do you have?

Based on the answers to these questions, the medical history, and the results of the physical exam, further testing may be necessary.