see also:

• dermatology

• hyperkeratosis is excessive keratin buildup specifically in the outermost skin layer (stratum corneum), often from friction, pressure, or inflammation, forming rough, scaly patches like calluses
  • thickened skin may be due to repeated local traumas or an underlying (usually rare) dermatosis
  • it often causes deep fissures which severely impair the natural barrier function of the skin and then allow infections to develop
• lichenification is broader epidermal thickening (acanthosis) with hyperkeratosis, creating leathery skin with exaggerated lines, often due to chronic scratching
• ichthyosis is a group of genetic skin disorders causing dry, thickened, scaly skin resembling fish scales due to defective skin cell shedding

• hyperkeratosis only:
  • calluses / corns
    • due to repeated rubbing, usually occurring on feet
  • cracked heels
  • actinic keratosis from many years of UV exposure
• lichenification:
  • atopic eczema
  • psoriasis
  • lichen simplex - chronic scratching
  • lichen sclerosis
  • lichen planus
  • pityriasis rubra pilaris (PRP) (rare)

• ichthyosis vulgaris
  • most common (up to 1 in 250 people)
  • mild, with fine white/brown scales on legs, arms, and trunk; worsens in dry weather
  • usually spares flexures like elbows
• X-linked recessive ichthyosis:
  • affects males, large dark scales, often on neck and trunk
• congenital forms:
  • lamellar ichthyosis
  • harlequin ichthyosis
    • very rare congenital disease results from a mutation in the ABCA12 gene responsible for lipid transport
    • babies born with harlequin ichthyosis have thick armor-like skin plates separated by deep red fissures; they easily become dehydrated, infected, and may suffer from respiratory distress due to the rigidity imparted by the skin surrounding the chest
    • the disease was almost always fatal in the neonatal period, but outcomes have greatly improved due to advances in neonatal care