Random Dermatology MCQ - Uncombable hair syndrome

A 3-year-old child is brought to the clinic because the parents cannot manage his "wild" hair. They report the hair is extremely dry, frizzy, and stands away from the scalp, making it impossible to comb flat. It has been this way since the hair grew in after birth. The hair is silvery-blond in color. On examination, the hair is spun glass-like in appearance, growing in multiple directions from the scalp. The child is otherwise healthy with normal development. Microscopic examination of a hair shaft under light microscopy would most likely reveal which of the following?

A. Pili torti, with the hair shaft flattened and twisted on its axis.
B. A longitudinal groove, characteristic of pili canaliculi.
C. A trichorrhexis nodosa, with nodes causing the hair to break easily.
D. A triangular or reniform (kidney-shaped) cross-section with a longitudinal groove, best seen on scanning electron microscopy.
E. Monilethrix, with a beaded appearance due to nodes of normal thickness separated by constricted, weak internodes.

Correct Answer: D. A triangular or reniform (kidney-shaped) cross-section with a longitudinal groove, best seen on scanning electron microscopy.

Answer and Explanation

The correct answer is D. This question describes the classic presentation of uncombable hair syndrome (UHS), also known as pili trianguli et canaliculi or "spun glass hair syndrome". The key clinical clues are the onset in early childhood, the characteristic hair that is dry, frizzy, and stands away from the scalp in multiple directions, and the silvery-blond color. The diagnosis is confirmed by microscopic examination, where the pathognomonic finding is a hair shaft with a triangular, reniform (kidney-shaped), or even heart-shaped cross-section, which often has a longitudinal groove. This irregular shape causes light to scatter, giving the silvery sheen, and prevents the hairs from lying flat. Scanning electron microscopy (SEM) is the best method to visualize this cross-sectional shape, though light microscopy can show the longitudinal groove.

Why the Other Options are Incorrect:

• A. Pili torti: This causes the hair to be flattened and twisted 180 degrees along its axis, leading to spangled, brittle hair. It is associated with Menkes disease and other syndromes, not the described "spun glass" appearance.

• B. A longitudinal groove, characteristic of pili canaliculi: While "pili canaliculi" is part of the older name for UHS, the longitudinal groove alone is not pathognomonic and may be seen with light microscopy. The key diagnostic feature is the abnormal cross-sectional shape, which requires SEM for definitive confirmation.

• C. Trichorrhexis nodosa: This is a common acquired hair shaft defect where nodes form, causing the hair to break easily. It does not cause the hair to stand away from the scalp or have a spun-glass appearance.

• E. Monilethrix: This presents as brittle, beaded hair due to elliptical nodes of normal thickness separated by constricted, weak internodes. The hair is typically short and breaks easily, but does not have the uncombable, multi-directional growth.

Additional High-Yield Information for Exams:

• Genetics: Often autosomal dominant, but recessive forms exist. Mutations have been identified in genes involved in hair shaft formation (PADI3, TGM3, TCHH).

• Clinical Features:

  • Typically manifests between 3 months and 3 years of age.

  • Hair is dry, shiny, and grows slowly.

  • The characteristic finding is that the hair cannot be combed flat against the scalp; it resists alignment.

  • Most commonly affects the scalp hair, but eyebrows and eyelashes are normal.

• Differential Diagnosis: The main differential includes other hair shaft disorders that cause unruly hair, such as woolly hair syndrome (tightly coiled hair) and pili torti. The specific clinical and microscopic findings distinguish UHS.

• Associated Conditions & Prognosis:

  • Benign Isolated Finding: In the majority of cases, UHS is an isolated, benign condition with no associated abnormalities.

  • Syndromic Associations: Rarely, it can be associated with ectodermal dysplasias, retinal dysplasia, or other syndromes (e.g., Angel-shaped phalango-epiphyseal dysplasia).

  • Prognosis: Excellent. The condition often improves spontaneously around puberty, with hair becoming more manageable, though the structural abnormality may persist.

• Management & Rationale:

  • Rationale: The primary goals are to confirm the diagnosis (to provide reassurance) and to offer supportive hair care advice.

  • First-line: Clinical diagnosis confirmed by hair microscopy (light microscopy and ideally SEM). This rules out other, potentially syndromic causes.

  • Treatment: Reassurance is the mainstay. Gentle hair care is advised: use soft brushes, avoid harsh chemicals and heat styling, and consider conditioning treatments. There is no definitive medical treatment. Biotin supplementation is often tried, though evidence is anecdotal.