8.9 Infantile skin

• Newborn refers to a baby from birth to ca. 2 months of age.
• Infant denotes children anywhere from birth to 1 year old.
• Children from 1 year to age 4 years old are called toddlers.

• Embryogenesis: epidermis begins to stratify, complete at 34 weeks of gestational age.
• Pre-term neonates have decreased epidermal and stratum corneum thickness.
• After birth/Newborns: maturation process with adaptation of the skin to a relatively dry environment. Risk of dehydration, hypothermia, and percutaneous absorption of toxic agents.
• Transepidermal water loss (TEWL) decreases along with gestational and postnatal age and TEWL is a good marker of gestational age at birth.

Neonates and infants have a characteristic spectrum of skin problems and associated skin diseases.

• In pre-term neonates the epidermis is thin, the dermo-epidermal junction is relatively loose and the anchoring structures susceptible to traumatic or infectious blister formation.
• The skin of newborns is much more permeable than the skin of older children.
• Newborn skin lacks functionally mature metabolic systems and the superficial microvasculature capillary network is immature and will be accomplished by the age of 3-4 months. 
• Relatively high sebum production in the first weeks of life due to  mostly maternal androgens. 
• Lower amounts of melanin in the epidermis in infants, warranting effective UV-protection for prevention of melanoma. 
• The ratio of body surface to body mass in newborns is more than twice that in adults.

The acid mantle plays an important role in skin defense and the composition of the microbiome.

In term babies, surface skin acidity (pH) is neutral at birth but drops during the first months to 5 and 5.5. The vernix caseosa is a protective covering, which is lacking in preterm neonates which thus renders them more susceptible for skin infections and hypothermia.  Due to the important function of the vernix, bathing of the newborn during the first six hours of life is no longer recommended.

Immediately postpartum, skin microbiota is homogeneously distributed across the human body regardless of delivery method or gestational age. The skin of vaginally delivered newborns is colonized by vaginal Prevotella and Lactobacillus species, while the skin of neonates born by Caesarean section show a diverse community of cutibacteria, corynebacterial and micrococcae. Anatomically site-specific composition of the microbiome starts to develop between 1-3 months, but still differs significantly from the adult skin microbiome.

The abundant skin-associated immune tissue orchestrates the defense against pathogens, responds to environmental changes, and has also functions in homeostasis. In newborn skin the amount of antimicrobial peptides, lysozyme and lactoferrin are higher than in adults. Newborn skin contains fewer immune cells than adult skin but typically anti-inflammatory regulatory T cells (Tregs) occur at higher density than in adult skin.

The neural network develops early in the fetus, originating from the neural crest and organizing with increasing postnatal age.  Neuro-cutaneous responses and sensitivity to touch are important for overall development and skin function but also skin-skin contact and bonding with caregivers.

Due to the immaturity of the innate and adaptive cellular immunity, neonates and infants have a greater risk of skin infections.

The thin skin of a pre-term infant is most vulnerable. During admission to NICU, skin assessment is an essential part of care to reduce the risk of acquired skin injuries, the main types of which are bruises, blistering, excoriation, erythema and pressure injuries.

Full-term newborns may receive tub baths while daily cleansing of pre-term infant skin is not recommended and swaddle baths are recommended. Particular attention should be devoted to topical agents with excipients due to the the increased permeability of neonatal and infant skin and the risk of accumulation of potentially harmful excipients.

Most skin problems in the newborn period and infancy are benign and transient: eczematous eruptions (e.g. atopic eczema (AE) and seborrheic dermatitis (SD), reactive pustular rashes (e.g. erythema toxicum neonatorum) and acne (acne infantum) are discussed more in detail in the respective chapters.

Skin infections (e.g. bullous impetigo, staphylococcal scaled skin syndrome) and rare systemic diseases (e.g. Langerhans cell histiocytosis, cutaneous mastocytosis, Hyper IgE-syndrome) can also cause skin rashes. Neonatal lupus erythematosus caused by trans-placental maternal autoantibodies should be recognized to rule out congenital heart block.

Milia and miliaria in newborns and small infants are based on the immaturity of adnexal structures in newborn skin. Treatment is not needed.

• Milia are characterized by 1-2-mm sized white to yellow papules commonly on the face
• Miliaria are usually red 1-2 mm sized papules concentrated on the trunk and are aggravated by high ambient temperatures.   
• Sebaceous hyperplasia on the nose and upper lip 
• Neonatal cephalic pustulosis (neonatal acne) is a facial papulopustular eruption during the first weeks or up to 6-8 weeks are common and transient.
• Reactive pustular rashes are relatively frequent in the neonatal period. 
• The most common transient pustular rash is Erythema toxicum neonatorum, which disappears within 1-2 weeks after birth. 
• Cutis marmorata is the most common transient skin change in newborns. It consists of a symmetric reticular erythema resulting from a physiological response to cold exposure (vasoconstriction of superficial dermal vessels) in infancy.  
• Neonatal jaundice (icterus) is common and physiological and typically peaks at 3-4 days of age or in pre-term infants. It is caused by tissue deposition of bilirubin. 

Most genodermatoses manifest neonatally or during infancy.

• Incontinentia pigmenti manifests neonatally with linear blistering following the lines of Blaschko, turning into linear plaques on the trunk and extremities.
• Café-au-lait macules indicative of Neurofibromatosis and hypopigmented macules associated with Tuberous sclerosis.
• Genetic mutations in proteins associated with the dermo-epidermal junction and the dermal anchoring cause different genetic blistering diseases (Epidermolysis bullosa).
• Collodion baby is a common presentation of several congenital ichthyoses (e.g. lamellar ichthyosis, self-healing collodion baby and Harlequin ichthyosis) and the most severe forms associate with significant morbidity and mortality. Epidermolytic ichthyosis presents with blistering erythroderma.
• Acrodermatitis enteropathica is a very rare disorder of infancy associated with zinc deficiency in zinc absorbance . It presents at the time of weaning with perioral and /or perianal pustular dermatitis.
• Some palmoplantar keratodermas (PPK), especially diffuse PPKs manifest during infancy.

Laboratory investigations are rarely needed in skin diseases of infancy.

• A skin biopsy is warranted if systemic diseases (e.g. Langerhans histiocytosis, mastocytosis) are contemplated. 
• In erythrodermic and cases of severe ichtyoses and epidermolysis bullosa intensive care blood tests are mendatory.
• Genetic profiling should be carried out in genodermatosis.

The diagnosis of skin problems in newborns and infants is usually based on the clinical findings and a thorough history and evaluations of symptoms.