The single transverse palmar crease is a variation in the lines of the hand often referred to by the outdated term, “simian crease.” Although frequently linked to medical conditions, it is a common physical trait in the general population. The original name, which references non-human primates, has largely fallen out of favor in the medical community due to its negative connotation. Understanding this variation requires acknowledging its correct medical nomenclature and biological origin.
Defining the Palmar Crease Variation
The typical human palm contains three distinct major creases: the distal transverse palmar crease, the proximal transverse palmar crease, and the thenar crease. The distal and proximal creases normally run somewhat parallel across the upper and middle sections of the palm. When this variation occurs, the two upper creases merge into a single, continuous line that spans the width of the palm.
The preferred medical term for this anatomical feature is the Single Transverse Palmar Crease, or STPC. This description accurately reflects the physical reality of the single line, distinguishing it from the typical two-crease pattern. The STPC can appear on one hand, which is known as a unilateral presentation, or it can be present on both hands, referred to as a bilateral presentation.
How Palm Creases Develop
Palmar creases form early in fetal development and are not created by the movement or folding of the hand after birth. These skin folds begin to appear in the palm between the eighth and thirteenth weeks of gestation and are fully established by about the twelfth week.
The development of the STPC is considered an alteration in the normal pattern of folding that occurs in the developing hand. It results from a variation in the timing or degree of the initial skin fold formation during this specific embryonic period. Any disruption to the developmental process during the first trimester can influence the final crease pattern.
Prevalence and Genetic Factors
The Single Transverse Palmar Crease occurs in a small percentage of the overall population, with reported prevalence rates generally falling between 1.5% and 5.0%. This variation is more frequently observed in males, who are approximately twice as likely as females to exhibit the trait. Population studies also indicate differences in frequency based on ethnicity, with the STPC being more common in populations of East Asian and Native American descent.
For many individuals, the presence of an STPC has no medical significance and is considered a normal familial trait. Research suggests that genetic factors play a role in the embryogenesis of palmar creases. The STPC can be passed down through families, demonstrating that it is often a benign anatomical variation.
Association with Health Conditions
The vast majority of people with an STPC are healthy and have no underlying medical conditions. However, the Single Transverse Palmar Crease is noted in clinical settings because its incidence is significantly higher in individuals with certain chromosomal disorders and genetic syndromes. The STPC can serve as a soft clinical marker, prompting a deeper medical evaluation when other physical or developmental markers are also present.
The STPC is strongly associated with Down Syndrome (Trisomy 21), occurring in an estimated 45% to 60% of affected individuals. This rate is substantially higher than in the general population, making it a well-known feature of the condition. Other chromosomal and genetic disorders are also linked to an increased frequency of the crease.
These conditions include Fetal Alcohol Syndrome, which results from prenatal alcohol exposure and can cause developmental delays. The crease is also seen more often in individuals with Turner Syndrome, a genetic condition affecting females, and Aarskog Syndrome, which affects development in multiple parts of the body. Medical evaluation is typically warranted only when the crease is accompanied by other physical anomalies, developmental concerns, or a known family history of a related condition.