Hair Growth Patterns — Significance & embryology
Scalp hair distribution provides important clues to early development. The hair bulb forms at 14 weeks and the growth of the scalp, which is strongly influenced by brain growth, stretches the hair shaft from its original perpendicular orientation to more vertical, so the hair “lies down” and doesn’t “stand on end”. Brain growth doesn’t exert a uniform pull on the skin and the most rapidly growing area, between 16 to 19 weeks, is capped by the posterior hair whorl. Generally it is off center and back at around the position of the posterior fontanel. The third influence is the suppression of hair growth in a circle around the face, around the ears and less distinctly along the back of the neck. Finally, the posterior hair line is influenced by growth of the neck or neck edema. This is commonly observed in Turner syndrome where prenatal swelling of the neck occurs due to dysplasia of the lymphatics which usually recovers by the end of gestation. The result is a wide neck, a low posterior hair line and upsweep of the hair line. Examination technique:
Look at the child from the front, the back and from above, noting the hair lines, hair whorls and cowlicks.
Abnormalities
a. The position of the posterior hair whorl is not exact, but multiple hair whorls, widely spaced (>3cm) double hair whorls, markedly displaced hair whorls or no posterior hair whorl are abnormal.
b. A frontal cowlick is an accessory hair whorl and though relatively common it indicates a subtle alteration in fetal brain growth. A marked upsweep, especially in conjunction with other hair growth pattern abnormalities is abnormal.
c. A low anterior hairline especially approaching the lateral eyebrows is abnormal.
d. A widow’s peak is seen in patients with hypertelorism and reflects a lack of hair suppression around the eyes as they are laterally displaced. A marked widow’s peak is abnormal.
e. Upsweeps of the posterior hair line or slightly low posterior hair lines are common and if not pronounced are considered normal variants.
Normal variants
a. A central hair whorl which occurs in 5% is a normal variant.
b. A mild frontal upsweep which occurs in 15% of people is a normal variant.