Contents
Introduction
The skin can be affected by a myriad of inflammatory disorders, many of which have complicated names derived from Latin or Greek. A postgraduate textbook of dermatopathology will typically require several hundreds of pages to cover these numerous conditions. However, from the perspective of cut up, this diversity has little impact on how the specimens are handled because the histopathological investigation of inflammatory diseases of the skin usually only involves a punch biopsy or incisional biopsy.
The use of small biopsies to investigate skin rashes means that the pathologist is dependent upon the dermatologist to supply a description of the rash. This clinical assessment can be considered to equate to the macroscopic description that would be produced at cut up for larger specimens. The clinical diagnosis or differential diagnosis is often essential to permit a diagnosis to be offered from the biopsy; many inflammatory diseases of the skin have similar microscopic appearances but may have very different clinical features. The crucial contribution of the clinical properties of the rash mean that many diagnoses in non-neoplastic dermatopathology require clinicopathological correlation. In effect, the pathologist's report will employ statements such as 'the appearances are consistent with the clinical diagnosis of' or 'given the clinical context, the appearances are those of', rather than an independent statement of 'the appearances are those of' which implies that the histopathology in isolation is diagnostic.
Although the number of inflammatory disorders of the skin is considerable in absolute terms, the diseases can be grouped into only a few basic patterns. Inevitably some diseases will show an overlap, but the system remains a useful approach for the interpretation of a skin biopsy and can narrow down the list of differential diagnoses.
Lichenoid
The lichenoid pattern of inflammation features chronic inflammation in the superficial dermis. The lymphocytes of the inflammatory infiltrate destroy the basal cells of the epidermis, resulting in an irregular appearance to this layer. Eosinophilic globules that represent degenerate keratinocytes are sometimes seen and are known as Civatte bodies. The inflammatory infiltrate is often described as resembling a band.
Examples of lichenoid conditions include lichen planus, lupus erythematosus and erythema multiforme.
Spongiotic
Spongiosis is the presence of oedema within the epidermis. This causes the cells to be further apart from each other and often accentuates the appearance of the intercellular bridges between the keratinocytes. There is inflammation in the underlying dermis and some of the lymphocytes migrate into the epidermis. This migration of inflammatory cells into the epidermis is referred to as exocytosis.
Eczema is the principle spongiotic disorder. Many subtypes of eczema exist. Another spongiotic disorder is pityriasis rosea.
Psoraisiform
Psoriasiform disorders are characterised by epidermal hyperplasia that manifests as elongated rete ridges. The elongation is regular. The hyperplasia is due to increased proliferative activity of the keratinocytes in the epidermis and is therefore often accompanied by hyperkeratosis and/or parakeratosis.
The main psoriasiform disorder is psoriasis. Other examples include lichen simplex chronicus and pityriasis rubra pilaris.
Vesicobullous
Vesicobullous disorders are blistering diseases. They may be known as just bullous diseases. The blisters (bullae) are the result of loss of cohesion between cells within the epidermis, or the cells at the base of the epidermis with the more superficial layers of the epidermis, or the basal layer of the epidermis with the underlying dermis. Failure of the attachment of the cells causes them to separate and the space which results can become a blister.
Bullous disorders are usually due to an autoimmune process in which the immune system aberrantly targets part of the complex network of molecules which permit cell-cell and cell-basement membrane adhesion in the epidermis. There are multiple targets that may be affected, but any given disease tends to have only one or a few specific targets.
Failure of the different components of the structural integrity of the epidermis characteristically produce blisters at different levels of the epidermis. This allows bullous disorders to be classified pathologically on the basis of where the blister is located.
- Intracorneal or Subcorneal (within or just beneath the keratin layer)
- Intraepidermal
- Suprabasilar (just above the basal layer of the epidermis
- Subepidermal (below the basal layer)
Further information can be added by evaluating the composition of the accompanying inflammatory cell infiltrate. This yields classification schemes which group blistering disorders based on the location of the blister and the cellular contents of the blister.
Blisters that are due to a split/loss of cohesion deeper in the epidermis tend to be more robust than the superficial bullae.
Immunofluorescence can be helpful in bullous disorders because the inflammatory mediators highlighted by the immunofluorescence map to the distribution of the target molecule. In some instances (for example pemphigoid and linear IgA bullous dermatosis) the immunofluorescence may be crucial in making a definite distinction between the possible diagnoses.
Examples of vesicobullous disorders include pemphigus vulgaris (suprabasilar blister) and pemphigoid (subepidermal blister).
Vasculitic Disorders
Vasculitis is inflammation of the blood vessels. In most organs the diagnosis of vasculitis requires more than just the presence of perivascular inflammation. instead, the inflammation should permeate the wall of the blood vessel and/or be associated with damage to the wall of the blood vessel, often in the form of fibrinoid necrosis and/or swelling of the endothelial cells. However, some dermatopathologists will accept a perivascular distribution of the inflammatory infiltrate alone as sufficent to designate a process as vasculitic or vasculopathic in the skin.
Leucocytoclasis is a particular pattern of cutaneous vasculitis in which there are multiple small dark blue dots of nuclear debris, which resemble dust, that have been released by dead white blood cells.
The urticarial conditions are considered within the category of vasculitis. Vasculitis of the skin can also be part of a systemic vasculitic disease.
Granulomatous Disorders
Granulomatous disorders of the skin feature granulomas. They may be a component of a systemic granulomatous disease like tuberculosis or sarcoidosis, or be a specific dermatological disorder, such as granuloma annulare.
Other Patterns
The above classification does not encompass all non-neoplastic diseases of the skin. Infections may not conform to any of the above patterns, while disorders of keratinisation or hair may also have features which do not fall into any of the preceding categories. The group of diseases that are grouped under the heading of panniculitis all display inflammation of the subcutaneous mature adipose tissue and as such also do not fit into the preceding system.