Describing a Skin Lesion

Introduction

Dermatology is a visual specialty, but the diagnosis is made through language as much as through the eye - a precise, systematic description is what turns 'a rash' into a differential diagnosis, and it is what allows a colleague, a photograph, or a future note in the same patient's record to convey exactly what was seen. Learning the vocabulary is not a formality; it is the skill itself.

This article sets out the systematic approach used throughout the other articles in this section, and is worth returning to whenever a specific term (macule, plaque, Nikolsky sign) is unfamiliar in context.¹

A systematic approach

Work through the same structure every time, in the same order, so that nothing is missed and every description is comparable to the last:

1. Distribution - where on the body, and the overall pattern
2. Morphology - the primary lesion type
3. Colour
4. Size and shape
5. Border/margin
6. Configuration - how multiple lesions relate to each other
7. Secondary change - scale, crust, ulceration and so on
8. Associated findings - nail, hair, mucosal or systemic involvement, and relevant special tests

Distribution

Where a rash occurs is often more diagnostically useful than what it looks like up close, since many conditions have a strongly characteristic distribution.

• Flexor versus extensor - atopic eczema classically favours flexures (antecubital and popliteal fossae) in older children and adults; psoriasis classically favours extensor surfaces (elbows, knees)
• Sun-exposed versus covered sites - photosensitive conditions (some drug eruptions, lupus, polymorphic light eruption) favour the face, neck, dorsal hands and forearms while sparing covered skin
• Symmetrical versus unilateral - most inflammatory dermatoses are bilateral and roughly symmetrical; a unilateral eruption raises suspicion of a localised cause (contact exposure, infection, or a dermatomal process)
• Dermatomal - a distribution following a single sensory nerve root is characteristic of herpes zoster
• Along Blaschko's lines - a pattern reflecting embryonic cell migration, seen in some naevi and mosaic genetic skin conditions, distinct from a dermatomal pattern
• Acral versus truncal - hands and feet versus the trunk, relevant to conditions such as hand-foot-and-mouth disease or secondary syphilis

Morphology: primary lesions

The primary lesion is the fundamental, earliest change, before scratching, infection or healing alter its appearance - identifying it correctly is the single most useful step in building a differential.²

Secondary changes

Secondary changes develop on top of a primary lesion, through scratching, healing, infection or the passage of time, and are just as important to document since they often reflect chronicity or complication.

Colour, size, shape and border

• Colour - erythematous (red, from vasodilatation - blanches), purpuric (red/purple, from bleeding into the skin - does not blanch), hyperpigmented, hypopigmented or depigmented, violaceous, yellow (xanthomatous or sebaceous), skin-coloured
• Size - measured in millimetres or centimetres, ideally with a ruler; document serial measurements when monitoring a lesion over time
• Shape - round/oval, annular (ring-shaped with central clearing), discoid/nummular (coin-shaped, uniformly involved), target/iris (concentric rings, as in erythema multiforme), linear, serpiginous (snake-like, wavy)
• Border/margin - well-defined (a sharp edge, as in psoriasis) versus ill-defined (a blurred edge, as in eczema); regular versus irregular (an irregular border in a pigmented lesion is a melanoma warning sign)

Configuration

Configuration describes how multiple lesions are arranged relative to one another, which is a separate axis of description from the shape of any single lesion.

• Grouped/clustered - lesions close together, as in herpes simplex
• Linear - lesions in a line, suggesting external trauma, Koebnerisation, or a Blaschko-line pattern
• Annular - lesions forming a ring
• Reticulate - a net-like, lace pattern
• Discrete versus confluent - separate individual lesions versus lesions that have merged into a single area

Special bedside tests and signs

A handful of simple bedside manoeuvres add diagnostic information beyond visual inspection alone, and are worth performing (and mentioning) explicitly.

Associated findings

A skin examination is rarely complete without checking beyond the lesion itself - several conditions are defined as much by their associated findings as by the skin change that prompted the consultation.

• Nails - pitting, onycholysis and oil-drop discolouration support psoriasis; nail fold telangiectasia and ragged cuticles suggest connective tissue disease
• Hair - scarring versus non-scarring alopecia changes the differential and the urgency of treatment; scalp scale or broken hairs point towards a specific cause
• Mucosae - oral, genital or ocular involvement narrows the differential considerably (for example lichen planus, Stevens-Johnson syndrome, pemphigus vulgaris) and should always be checked when the skin findings are severe, blistering, or otherwise unexplained
• Systemic features - fever, joint pain, lymphadenopathy or weight loss reframe a skin finding as a possible sign of systemic disease rather than a primary dermatological problem

Pigmented lesions: a special case

Pigmented lesions warrant a specific structured description because the goal is explicitly to identify features suggesting malignant melanoma rather than a benign naevus. The ABCDE criteria (Asymmetry, Border irregularity, Colour variation, Diameter over 6 mm, Evolution) and the 7-point checklist are covered in detail in the article on malignant melanoma, and should be applied systematically to any pigmented lesion causing concern, alongside dermoscopy where available.

Documentation

• Clinical photography, with consent and a scale marker, is invaluable for monitoring change over time and for remote or asynchronous specialist review
• Serial measurement of a lesion's dimensions supports objective assessment of change, rather than relying on subjective impression alone
• Structured written description, following the systematic order above, ensures the next clinician (or the same one, months later) can compare like with like

Putting it together

A complete description reads as a single, structured sentence rather than a list of isolated observations - for example: 'well-defined, erythematous plaques with silvery scale, distributed symmetrically over the extensor surfaces of both elbows and knees, with a positive Auspitz sign.' Every element of that sentence maps directly onto the framework above, and together they point strongly towards a single diagnosis (psoriasis) before any further test is done - which is exactly the point of learning the vocabulary systematically rather than describing each lesion from scratch.