Dermatitis (inflammation of the skin) presenting as disc- or coin-shaped lesions is called nummular or discoid dermatitis. This type of dermatitis is commonly seen among middle-aged or elderly patients and is associated with the presence of dry skin and cold weather. The cause of this condition is unknown, though id reactions (autoeczematization) may present as nummular dermatitis. Diagnosis is made clinically, and it can be treated with antibiotics, corticosteroids, or ultraviolet radiation therapy.
Nummular dermatitis is a form of skin inflammation characterized by multiple, round or oval erythematous plaques on the upper and lower extremities. It is considered as a form of eczema. Initial lesions start out as papules and coalesce to form oval or discoid lesions, which may or may not contain vesicles with serous exudate. These highly pruritic lesions eventually form plaques with scales, and yellowish crusting may be present in some lesions infected with bacteria. It normally erupts in the extremities and affects both females and males albeit in different age distributions.
Although the etiology of nummular dermatitis is unknown, triggers such as local trauma (abrasions or insect bites), contact dermatitis (irritant or allergic), and exposure to chemicals may cause its development. The etiology is also likely multifactorial, and there have been some reports of sensitivity to metals and drugs such as cobalt, nickel sulfate, nitrofurazone, and neomycin causing nummular dermatitis. Other substances that may be associated with its development include potassium thioglycolate (found in depilating creams) , glue containing ethylcyanoacrylate, tincture of thimerosal , or mercury-containing dental amalgams . Autoeczematization, defined as the spread of lesions from an initial focus, is often associated with the presence of multiple plaques . Interferon and ribavirin therapy for hepatitis C  and tumor necrosis factor inhibitors  may cause severe, generalized nummular dermatitis. Patients with nummular dermatitis often have very dry or xerotic skin.
One of the most common dermatological conditions is dermatitis in all its forms. Nummular dermatitis is a type of dermatitis with a prevalence of 2 cases for every 1000 individuals, and this pattern is observed both globally and in the United States. This condition has been found to affect more males than females but has no racial predilection. It is usually more common during 60-70 years of age in males, with a smaller peak occurring during the second and third decades of life in women, especially those with atopic dermatitis. This condition is rare in children.
The exact pathophysiology of nummular dermatitis is unknown. However, the presence of xerosis (dry skin) may promote its development by causing dysfunction of the epidermal lipid barrier, which facilitates the entry of allergens into the skin to induce an allergic or irritant response  .
Hypersensitivity to metals may trigger nummular dermatitis. It is believed that the mercury present in dental amalgams induces an immune response that causes the development of the lesions.
Histamine and mast cells play a key role in the intense pruritus seen in nummular dermatitis. There are a greater number of mast cells present in nummular dermatitis lesions compared to normal skin, and the mast cells of patients with nummular dermatitis have a lower chymase activity. This leads to the impaired ability of the enzyme to suppress inflammation due to inadequate degradation of neuropeptides and protein, substances that potentiate the inflammatory response.
Nummular dermatitis is a recurrent disease. Lesions may take a year or several years before they resolve, and they may reappear in the same sites as the initial outbreak. Proper treatment may cause complete resolution, though darker or lighter spots may replace the lesions in the affected areas, especially in the thighs, legs, and feet. Control of recurrences may be partially achieved by avoidance of exacerbating factors and hydration of the skin.
Nummular dermatitis may present with a history of pruritic eruptions in the legs lasting for days to months, which may wax and wane with changes in temperature or climate. The lesions and pruritus may improve with the use of moisturizers or exposure to the sun or higher humidity. Patients with nummular dermatitis often have a prior history of atopic conditions such as atopic dermatitis or eczema and may exhibit dry and sensitive skin. The lesions are described as very itchy and may also produce a burning sensation.
Nummular dermatitis is diagnosed through the presence of characteristic round-to-oval erythematous plaques symmetrically distributed in the extensor aspects of the lower extremities or other areas of the body including the trunk, hand, or feet . The initial lesions are papulovesicular in nature, and they may develop yellowish crusts when superinfected with staphylococci. They eventually coalesce to form 2 to 10 cm diameter scaly plaques that exhibit central clearing, a feature also seen in tinea corporis. An important exclusion criteria is the presence of lesions on the face and scalp, which is not typically seen in nummular dermatitis.
Nummular dermatitis is commonly seen in elderly males and younger females. This condition is uncommon in children.
A biopsy of the lesion may be performed after excluding tinea corporis through scraping and microscopic examination of the specimen using potassium hydroxide. Biopsy findings of nummular dermatitis lesions are characterized by nonspecific lymphocytic infiltrates, spongiosis, and eosinophils in the papillary dermis. Hyperkeratosis, epidermal hyperplasia, and a thickened granular cell layer may be seen in chronic lesions.
Swab cultures of lesions with yellowish crusting may be performed to detect the presence of methicillin-resistant Staphylococcus aureus (MRSA) in order to choose the proper antibiotic therapy. Testing for other infections such as H. pylori and Giardia may also be required .
The primary treatment for nummular dermatitis involves skin rehydration in order to restore the epidermal lipid barrier. This is achieved through lukewarm or cool baths or soaking the affected area for 20 minutes in order to rehydrate the skin, followed by the application of a steroid ointment or petrolatum to the wet skin. This method has been reported to produce a greater response in >90% of patients with chronic refractory pruritic eruptions . However, the patient should be instructed not to use soaps when performing this technique. Wet wrap treatments are also helpful, and this involves the procedures mentioned above, followed by occlusion with a damp cloth or a non-breathable material for 1 hour.
Another treatment goal in the management of nummular dermatitis is the control of inflammation using several medications. Steroids are the most commonly used drugs. Low-potency steroids (Class III-VI) are indicated for less pruritic or less erythematous lesions, whereas lesions with severe inflammation or intense pruritus may be treated with high-potency steroids (Class I-II). Severe, generalized nummular dermatitis may require oral, intramuscular, or intravenous steroids. Long-term use of steroids should be avoided. Other medications to decrease inflammation include immune modulators such as tacrolimus and coal tar.
Infected lesions require treatment with antibiotics such as dicloxacillin, erythromycin, tetracycline, or cephalexin. Ideally, swab cultures should be the basis in the selection of an antibiotic.
Patients with severe, generalized, refractory nummular dermatitis may benefit from phototherapy using psoralen + UVA (PUVA) or UVB radiation.
Since the condition is associated with dry skin, prevention requires aggressive skin hydration and avoidance of cleansers that damage the epidermal lipid barrier. Mild cleansers should be used and application of emollients after bathing may decrease the chances of eruptions. Soaps should be avoided since they damage the lipid layer of the skin, the structure responsible for retaining moisture inside the skin.
Patients should wear loose-fitting clothes to prevent overheating, and fibers that have the potential to irritate the skin such as wool or polyester should be avoided. Extremes in temperature should also be evaded and the use of humidifiers may help control the condition.
Nummular dermatitis is a type of dermatitis characterized by red, itchy, coin-shaped rashes that may or may not have blisters or scales. The skin lesions may start out as tiny blisters that eventually come together to form a bigger rash. They usually form at the backs of the arms, legs, or buttocks. However, they can also appear on the torso.
Although the cause of this disease is unknown, it has been associated with cold weather during winter, and is more common in older males (60-70 years old) and younger women (20-30 years old). The rash may spontaneously disappear and reappear without any apparent reason.
This condition may be treated with moisturizers since it is associated with dry, sensitive skin. Antibiotics may be prescribed if the rashes are infected, and corticosteroid creams, injections, or pills may be administered if there is inflammation. Phototherapy using ultraviolet light may help in severe cases. However, the results of the treatment vary and may not be effective for all cases.