Table 3.

Knuckle pads in inherited disorders

Inherited disorders	Inheritance/gene involved	Salient features
		Cutaneous features	Extracutaneous features and/or associations
Palmoplantar keratodermas
Epidermolytic PPK	AD/KRT9, KRT1	Diffuse yellowish PPK with sharp erythematous margins; transgradiens and involvement of knees and eblows, fissures, knuckle pads	Very rarely associated with scleroderma, leg ulcers, familial cancers and Ehlers-Danlos syndrome
Loricrin keratoderma	AD/LOR	Diffuse symmetrical honeycomb PPK, starfish keratosis on dorsal aspects of extremities, pseudoaihnum and knuckle pads; generalized ichthyosis	May be associated with microcephaly and neurodevelopmental delay (as well as with vitiligo and atopic dermatitis)
Mal de Meleda	AR/SLURP1	Diffuse thick yellowish waxy transgressive PPK with erythematous margin; keratotic lesions over joints; knuckle pads, hyperhidrosis, nail dystrophy, digital constrictions; lesions are prone to develop malignancy
Gamborg-Nielsen PPK	AR/SLURP1	Similar to Mal de Meleda but with less severe PPK, no keratotic lesions and presence knuckle pads and tapered fingers
Vohwinkel syndrome	Diffuse symmetrical honeycomb PPK, starfish keratosis on dorsal aspects of extremities, pseudoaihnum and knuckle pads; lesions are prone to develop basal cell and squamous cell carcinoma	High frequency sensorineural deafness. The disorder may also be association with craniofacial anomalies, deaf-mutism, mental retardation, spastic paraplegia with myopathy, developmental delay and seizure disorders
Bart-Pumphrey syndrome	AD/GJB2	Honeycomb PPK, knuckle pads and leuconychia	Sever sensorineural deafness
PPK and deafness syndrome	AD/GJB2	PPK ranges from diffuse transgradient with fissuring to mild skin fold accentuation over joints	High frequency, bilateral, prelingual and slowly progressive sensorineural deafness
Striate PPK	AD/DSG1, KRT1	Linear hyperkeratotic plaques on the palmar aspect of the hands and fingers; localized hyperkeratotic plaques over the soles; hyperkeratotic plaques on knees and ankles; knuckle and toe pads [Figure 1a]

PPK: Palmoplantar keratoderma, AD: Autosomal dominant, AR: Autosomal recessive