Author: Dr Anthony Honigman, Monash University, Victoria, Australia. DermNet NZ Editor-in-Chief: A/Prof Amanda Oakley, Dermatologist, Hamilton, New Zealand. February 2018.
Harlequin ichthyosis is a rare and severe form of ichythyosis that results in hard, thickened armour-like plates of skin covering the entire body from birth.
Harlequin ichthyosis is also called harlequin-type ichythosis, and harlequin fetus. Note that harlequin syndrome refers to a different condition characterised by asymmetrical, progressive and segmental sweat loss (also known as progressive isolated segmental anhidrosis), and to unilateral sweating and flushing on the chest, neck and face. Harlequin change describes rapidly fluctuating unilateral erythema in neonates.
Harlequin ichthyosis is rare and males and females are affected in equal numbers. It affects approximately one in 300,000 births 
No racial predilection is known for harlequin ichthyosis. A higher incidence may be encountered in cultures where parental consanguinity is common .
The inheritance of harlequin ichthyosis is autosomal recessive. It is due to mutations in the ABCA 12 (ATP-binding cassette sub-family A member 12) gene. The ABCA 12 gene is believed to encode a transporter protein involved in the transport of epidermal lipids across cell membranes and is essential for normal skin development.
Some mutations in the ABCA12 gene prevent the cell from making any ABCA12 protein. Other mutations lead to the production of an abnormally small version of the protein that cannot transport lipids properly. A loss of functional ABCA12 protein disrupts the normal development of the epidermis, resulting in the hard, thick scales characteristic of harlequin ichthyosis .
The diagnosis of harlequin ichthyosis relies on physical examination and genetic laboratory investigations.
Genetic testing for a loss of function mutation on the ABCA12 gene is the most specific diagnostic test for harlequin ichthyosis.
Biopsy of skin may reveal hyperkeratotic skin cells on histology.
The differential diagnosis includes the other forms of inherited ichthyosis:
X-linked chondrodysplasia punctata (Conradi-Hünermann-Happle syndrome)
There is no cure for harlequin ichthyosis and treatment is centred around protecting the skin and preventing infection.
After birth, the thick plate-like outer layer of skin eventually splits and peels, leaving vulnerable inner layers of the dermis exposed. Most harlequin infants will need one-on-one nursing care for the first several weeks of life. Antibiotic treatment may also be necessary to prevent or treat infection during this time.
Softening emollients, especially those containing urea, salicylic acid or alpha hydroxyl acids, are particularly effective after bathing while the skin is still moist. These products work to keep the skin moisturised and pliable, while preventing cracking and fissuring that can lead to secondary bacterial infection.
In the past, harlequin ichthyosis infants rarely survived the first few days of life. With advancements in neonatal care, the survival rate has improved, with worldwide figures approaching 50% .
Children who survive the neonatal period usually evolve to a less severe phenotype but continue to develop fish-like scales, retention of waxy, yellowing material in seborrhoeic areas, generalised poor hair growth and scarring alopecia. Harlequin ichthyosis survivors may continue to suffer from temperature dysregulation, have contractures of digits, arthralgias, failure to thrive, hypothyroidism, and short stature.
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