Acral surfaces, such as the feet, have unique characteristics that make them prone to certain dermatologic conditions. The plantar surface of the foot has the thickest keratin layer, a high concentration of eccrine sweat glands as well as sensory nerves, Pacinian corpuscles and other mechanoreceptors. The combination of abundant keratin and sweat creates an ideal environment for fungal infections. Friction and contact with footwear also make the feet susceptible to contact dermatitis. In addition, the feet are disproportionately affected by vascular disorders, due to their gravity-dependent anatomical location, and by peripheral small fiber sensory neuropathies. Since the feet are a site of frequent injury, vascular disorders and sensory neuropathies predispose this area to recurrent and difficult to manage wounds.
Skin diseases involving the feet can be broadly placed into the following categories starting with the most common diseases (Table 40-1).
Table 40-1.Differential diagnosis for skin diseases of the feet ||Download (.pdf) Table 40-1. Differential diagnosis for skin diseases of the feet
|Disease ||Epidemiology, History, and Physical Examination |
|Infectious Diseases |
M > F.
Age: Occurs after puberty. Increasing prevalence with age.
Asymptomatic or pruritic. Lasts months to years. Can occur from contact with infected person or animal or autoinoculation (e.g., from groin). Associated with onychomycosis. Can be associated with tinea manuum, with either two hands and one foot involved or one hand and two feet involved.
Interdigital type: Dry scaling and/ or maceration, peeling, fissuring in toe webs.
Moccasin type: Well-demarcated erythematous patch with fine, white uniform scale on soles and sides of feet. Inflammatory/bullous type: Vesicles or bullae containing clear fluid, erosions on the soles.
Distinguished from plantar psoriasis by poor demarcation and fine scale. Distinguishing from dermatitis may be more difficult and may require KOH exam. Presence of onychomycosis or hand involvement of one hand can be a helpful clue.
(see Chapter 12)
M = F
Age: Any age. When occurring on feet, more common in adults.
Acute onset of pain and swelling, fever, malaise, and chills.
More common in lymphedema and areas of skin breakdown.
Risk increased with obesity, prior surgical site intervention, hepatic or renal disease, connective tissue disease, and malignancy.
Localized warm, red, tender plaque with ill-defined borders.
Concurrent tinea pedis/onychomycosis can be portal of entry.
Distinguish from stasis dermatitis by unilateral nature and systemic symptoms.
(see Chapter 11)
M > F.
More common in children and young adults.
Asymptomatic or painful. May persist for years.
Discrete or confluent hyperkeratotic papules or plaques on soles. May have black or brown dots within the lesions created by thrombosed capillaries.
Distinguish from callus by interruption of dermatoglyphics.
(see Chapter 13)