Punctate Keratoderma (Punctate Palmoplantar Keratoderma, PPKP)

Punctate palmoplantar keratoderma (punctate PPK) is a group of rare hereditary (and occasionally acquired) disorders characterized by discrete, small keratotic papules or spines confined to the palms and soles, in contrast to diffuse or striate forms. Three main types are recognized.

Punctate palmoplantar keratoderma type I - multiple hyperkeratotic, centrally indented, yellow-brown papules on palmar skin

Punctate PPK type I (AAGAB mutation): multiple hyperkeratotic, centrally indented, yellow-to-brown papules irregularly distributed over the palmar skin. (Fitzpatrick's Dermatology, Vol. 1 & 2)

Classification

Type I - Buschke-Fischer-Brauer (BFB) Type

(OMIM #148600 [PPKP1A] / #614936 [PPKP1B])

Synonyms: Keratosis punctata palmoplantaris, Keratoma dissipatum, Disseminated clavus
Inheritance: Autosomal dominant
Incidence: ~1-3 per 100,000

Genetics and pathogenesis:

AAGAB mutations (PPKP1A) - encodes the alpha- and gamma-adaptin-binding protein p34, involved in intracellular trafficking of clathrin-coated vesicles. The defect impairs endocytic recycling of receptor tyrosine kinases (EGFR, Axl), leading to increased signaling and cellular proliferation. On electron microscopy: large number of small vesicles + dilated Golgi apparatus.
COL14A1 mutations (PPKP1B) - a missense mutation in collagen type XIV alpha-1 chain gene was identified in a large Chinese family (incomplete penetrance).

Clinical features:

Onset: first or second decade (but can range from childhood to sixth decade)
Multiple hyperkeratotic, centrally indented, yellow-to-brown papules distributed irregularly over palmoplantar skin
The central keratotic core is initially translucent, may become opaque or verrucous over time; a pit/depression remains after removal
Lesions increase in size and number with age; can coalesce into plaques over pressure areas
May be painful or tender
Worsened by mechanical stress
Wood lamp: white fluorescence
Dermoscopy: well-demarcated, structureless yellow-orange areas surrounded by whitish halo; no dotted vessels (distinguishes from verruca)
Genetic anticipation (earlier onset in successive generations) has been reported

Histopathology:

Slight epidermal depression with overlying column of compact orthohyperkeratosis
Focal hypogranulosis with parakeratosis; elongated, curved rete ridges
No cytologic features of HPV infection

Special mention - Punctate keratoses of palmar creases: A variant seen frequently in adults of African ancestry - keratotic papules along palmar creases (digits and soles also affected), worsened by mechanical stress. The relationship to classic punctate PPK is not fully established.

Type II - Spiny Keratoderma (Porokeratotic Type)

(OMIM #175860)

Synonyms: Palmoplantar keratoderma punctata type 2, Music box spine keratoderma, Filiform hyperkeratosis, Digitate keratosis
Inheritance: Autosomal dominant (molecular etiology unknown)
Onset: puberty or early 20s

Clinical features:

Multiple, tiny, firmly attached, skin-colored to yellow, asymptomatic keratotic spines projecting from palms and soles
Discomfort caused by tendency to catch on clothing
May extend onto dorsal and lateral surfaces of fingers
Facial sebaceous hypoplasia reported in males
Wood lamp: white fluorescence resembling "stars under moonlight"

Acquired forms of spiny PPK (usually onset >50 years) are associated with:

Internal malignancies (visceral carcinoma, melanoma, leukemia, multiple myeloma)
Hyperlipidemia type IV
Chronic kidney disease, polycystic kidney disease with liver cysts

Histopathology:

Epidermal depression with overlying column of parakeratosis (in contrast to Type I's orthohyperkeratosis)
Stratum granulosum is preserved
No vacuolization, pleomorphism, or dyskeratosis of keratinocytes (distinguishes from porokeratosis)

Type III - Acrokeratoelastoidosis (AKE) of Costa

(OMIM #101850)

Inheritance: Autosomal dominant; rare, no racial predilection
Onset: childhood or adolescence (infancy or adulthood also reported)

Clinical features:

Asymptomatic, round-to-oval, whitish-to-yellow, translucent papules; rarely nodules and plaques with hyperkeratotic surface or umbilication
Predilection for thenar, hypothenar areas, pressure sites on palms and soles
Linear pattern or "paving stones" arrangement along radial/ulnar margins of hands
Severe cases: dorsal hands and feet, wrists, ankles (including knuckle pads)
Aquagenic PPK, nail dystrophy, and hyperhidrosis may be associated

Differential Diagnosis

Condition	Key distinguishing feature
Verruca vulgaris	Multiple bleeding points on paring; dotted vessels on dermoscopy; HPV features on histology
Punctate porokeratosis	Cornoid lamella on histology (instead of orthohyperkeratosis)
Arsenic/dioxin keratoses	Environmental exposure history
Darier disease	Histology: corps ronds and grains
Acrokeratosis verruciformis of Hopf	Dorsal hands, more verrucous
Cowden syndrome	Trichilemmomas, mucosal lesions, systemic findings
Pitted keratolysis	Bacterial, malodour, pits not true keratoses

Malignancy Association

A 2023 systematic review (PMID 37705065) specifically examined whether PPKP type I is associated with malignancy. Older reports suggested possible links with breast and colon carcinomas, but it remains unclear whether there is truly an excess of malignancies in affected individuals, as noted in [Dermatology 2-Volume Set 5e, p. 1128].

Treatment

Comfortable footwear and mechanical paring/debridement for symptomatic relief
Topical retinoids and keratolytic ointments have limited efficacy
Oral retinoids have been tried
No curative treatment; management is largely symptomatic

Sources: Dermatology 2-Volume Set 5e, pp. 1127-1128 | Fitzpatrick's Dermatology, Vol. 1 & 2, pp. 873-874 | Andrews' Diseases of the Skin Clinical Dermatology, p. 250

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