Atopic dermatitis is a common skin disease characterized by a chronic, relapsing and inflammatory condition. The disease is also known as eczema and has substantial social and psychological implications. It often develops early on, even during infancy. Hence, it is marked by three age-related stages of infantile, childhood and adulthood. It is also linked with other atopic ailments such as asthma and allergic rhinitis.

This age-wise categorization of atopic dermatitis combined with thorough clinical examination of the patient, facilitates a variable differential diagnosis. It is important to exclude a number of other skin conditions before any prognosis of atopic dermatitis can be made by a doctor.

It is difficult to diagnose atopic dermatitis due to the absence of an exclusive distinguishing feature and any particular laboratory test or biopsy outcome. Hence, the diagnosis is dependent on various clinical features of the disease coupled with analysis of a personal or family medical history.

The differential diagnosis of atopic dermatitis begins with two very important distinguishing features:

1. Pruritus or itchiness: It is a typical trait of atopic dermatitis and is the key factor in a correct prognosis.
2. Lesions: The biology and spread of the atopic dermatitis lesions is also a major distinguishing factor.

This apart the differential prognosis must take into account a comprehensive set of other skin conditions (against the backdrop of the affected individual’s age and degree of occurrence) that must be excluded before confirming atopic dermatitis. They are:

• Chronic dermatoses
• Infections and infestations
• Malignancies
• Metabolic, genetic and autoimmune malfunction

Chronic dermatoses

The various common chronic dermatoses have to be detected on the basis of their age-wise occurrence. For instance, seborrheic dermatitis is common in children and infants. In adults, nummular dermatitis is uncommon and asteatotic eczema is common. Then again, in case of nummular dermatitis though lacerations are frequent in atopic dermatitis, true nummular eczema is rarely seen.

There are certain conditions that occur in infants, children and adults alike and must also be distinguished from atopic dermatitis.

They are:

• Contact dermatitis of the allergic kind, which is a common development. adults who do not show a personal or family history of atopy (asthma and allergic rhinitis) yet develop eczematous eruption must go through a complete screening process to trace any incidence of allergic contact dermatitis.
• Psoriasis, particularly the palmoplantar type is a common condition that a patient must be checked for.
• Lichen simplex chronicus is also a common factor and is second in importance to pruritus associated with various causes.

Infections and infestations

In children, one must differentiate atopic dermatitis with such variable conditions like neonatal mucocutaneous candidiasis, congenital syphilis and HTLV-1 associated dermatoses. HTLV-1 associated dermatoses, is more commonly seen in kids of Jamaican descent. Then there is the rarer chronic mucocutaneous candidiasis in children.

In adults, one must rule out the common conditions of dermatophytosis. Scabies is a common occurrence and HIV-associated dermatoses can be confused with atopic dermatitis. Impetigo is an uncommon development in all ages though. Though the atopic dermatitis lesions can be distinguished as impetiginized, the reverse is rare.

Primary immunodeficiencies

These are mainly seen in children and are linked with eczematous eruptions. In infantile cases, one must essentially check for two rare but potentially fatal primary immunodeficiencies linked with pruritic dermatitis. The first is the Wiskott-Aldrich syndrome, which is genetically linked to a recessive gene. Its key features include thrombocytopenia, irregularities in humoral and cell mediated immunity, repeated bacterial infections and skin conditions very similar to atopic dermatitis. The presence of a greater degree of exfoliative rashes with serosanguineous layers in Wiskott-Aldrich syndrome could help in distinguishing the condition from atopic dermatitis.

The second important disease to consider is the Hyperimmunoglobulin E immunodeficiency syndrome. It is an autosomal (a chromosome other than one that determines sex) gene based malfunction characterized by very high serum IgE count, inadequate immune response, persistent sinopulmonary and cutaneous (relating to the skin) infections and eczematous lesions.

Apart from these two conditions, children show a rare occurrence of the acute combination of immunodeficiencies and another rare ailment called the DiGeorge syndrome or congenital thymic aplasia.

Malignancies

In children, one can start with a differential check-up of the possibility of malignant Langerhans cell histiocytosis. This is a rare disease in infants who do not respond to atopic dermatitis therapy. This is more applicable in patients who develop dermatitis in the diaper area or erosions in the folds of the groin region.

Another vital but uncommon cancer that needs to be excluded is B Cutaneous T cell lymphoma (CTCL) also known as mycosis fungoides or Sezary syndrome. CTCL occurs in children and adults alike. It is marked by chronic dermatitis that fails to be managed by topical glucocorticoid therapy.

Since it may be difficult to differentiate from atopic dermatitis at the basic structural and anatomical level, biopsies of various affected spots can be a vital diagnostic tool.

To ensure optimum results, the biopsy samples should be obtained from skin that has not been subjected to topical corticosteroids therapy for at least a fortnight. This reduces the possibility of an error, since corticosteroids can remove atypical epidermotropic T cells from the epidermis of the skin, which is a primary indicator of CTCL. On the other hand, it does not clear spongiosis.

Additional implements for a differential diagnosis of CTCL are immunoperoxidase and gene rearrangement research. This apart, the hypopigmented variation of CTCL that occurs more in darker skinned people must also be taken into account in the differential diagnosis of truncal pityriasis alba.

Metabolic and hereditary disorders

Among children, one must exclude the presence of certain rare conditions such as Netherton’s syndrome, Ectodermal dysplasias, Ataxia telangiectasia, Hartnup disease and Phenylketonuria.

There are certain ailments that may be present in both kids and adults alike. They are the more common familial keratosis pilaris, the rarer B zinc deficiency (acrodermatitis enteropathica), glucagonoma syndrome, etc. A patient should also be examined for rare incidence of other dietary deficiencies of biotin, essential fatty acids and organic acidurias etc.

Immunologic disorders

In this group, adults must be diagnosed to exclude dermatitis herpetiformis, pemphigus foliaceus and lupus erythematosus though all three are uncommon ailments. In both children and adults, one must check for other uncommon ailments such as the graft-versus-host disease and dermatomyositis. It is common to confuse atopic dermatitis with the latter in children where dermatomyositis occurs.

Miscellaneous

There are some other conditions that also have to be ruled out before determining atopic dermatitis. Though, drug intolerance-related rashes are common there are very few that are identical to atopic dermatitis. Photoallergic contact dermatitis and chronic actinic dermatitis though uncommon must also be differentiated from atopic dermatitis.