A pustule is a localized collection of yellow–green fluid containing polymor-phonuclear leucocytes and their breakdown products.
Pustules may appear de novo following neutrophil-mediated inflammation, for example in a bacterial folliculitis.
They may also be caused by gradual polymorphaccumulation in a pre-existing clear vesicle, as occurs in herpesvirus infections.
In this case, an initially clear vesicle becomes cloudy (a transitional stage sometimes called a vesicopustule), and subsequently, the more typical yellow–green colour develops.
The distinction between these two methods of formation is useful in differential diagnosis, but is not always easy to determine unless lesions at different stages are present concurrently or unless individual lesions are observed at intervals.
It can also be difficult to distinguishbetween an early vesicopustule with slightly turbid fluid content due to inflammatory cells and a vesicle with clear fluid content but a damaged over-lying epidermis, as both can be grey in colour.
It is therefore simplest to reserve the term pustule for established lesions with the typical yellow or green colour.
Healing pustules may form a scab or crust in some disorders, notably palmoplantar pustulosis where a brown scale develops as the pustule resolves.
Pustules of the same duration tend to be relatively monomorphic, so the presence of resolving lesions at the same time as fresh pustules implies an ongoing process.
Where pustules appear as crops, they are typical of varicella, in which new vesicles coexist with pustules and with necrotic oldlesions; pustules at several stages of evolution are also typical of chronic dis-orders, such as palmoplantar pustulosis.
The converse is not necessarily true, for example, the lesions of pityrosporum folliculitis tend to be fairly monomorphic irrespective of duration.
