Skin Examination for Systemic Disease — Written Clinical Reasoning

SAQ 1 — The Systematic Skin Examination (20 marks, 30 minutes)

Prompt: Outline your systematic approach to the skin short case, addressing: (a) the six-step routine; (b) the morphology vocabulary and distribution clues; (c) the hand signs of systemic disease; (d) the purpuric eruption; (e) the causes of erythema nodosum; and (f) the three most common errors. [1]

Model Answer

(a) The six-step examination routine (4 marks): [1]

The routine is a fixed sequence, performed in the same order every time: general inspection, the hands, the face, the trunk, the legs, and the specific patterns with closure. [1]

At the general inspection, I take ten seconds to note the distribution pattern (photosensitive, acral, dependent, dermatomal, flexural, extensor), the morphology class (macular, papular, plaque-like, nodular, vesicular, bullous, pustular, purpuric, erythematous), and the arrangement (annular, linear, grouped or herpetiform, reticular, target). The distribution is the first clue to the mechanism. [1]

In the hands, I examine the nails (clubbing, koilonychia, Beau lines, splinter haemorrhages, nail-fold capillary changes), the palms (palmar erythema, Gottron papules, thenar atrophy, mechanic hands), and the fingers (sclerodactyly, calcinosis, telangiectasia). [1]

On the face, I look for the malar rash (SLE), the heliotrope rash (dermatomyositis), oral ulcers (SLE, Behcet), periorbital xanthelasma (dyslipidaemia), spider naevi (cirrhosis), and the endocrine facies (moon face of Cushing, exophthalmos of Graves). [1]

On the trunk, I look for acanthosis nigricans (insulin resistance or malignancy), striae (Cushing), gynaecomastia (cirrhosis), spider naevi above the umbilicus (portal hypertension), and the phakomatoses (shagreen patches and ash-leaf macules of tuberous sclerosis, cafe-au-lait macules of NF1). [1]

On the legs, I examine the anterior shins (erythema nodosum, pyoderma gangrenosum, necrobiosis lipoidica, pretibial myxoedema), the lower legs (purpura, livedo), and the feet and toes (digital infarcts). [1]

I close with the specific patterns (dermatomal zoster, dermatitis herpetiformis, erythema multiforme, Stevens-Johnson, morbilliform drug eruption), the mucosae (oral, conjunctival), the scalp, and the genitalia if indicated, plus the investigations. [1]

(b) The morphology vocabulary and the distribution clues (3 marks): [1]

The morphology is the exam currency. I describe each lesion before I diagnose it. A macule is a flat colour change you cannot feel. A papule is a raised solid lesion less than one centimetre. A plaque is a raised flat-topped lesion more than one centimetre. A nodule is a solid lesion extending into the dermis or deeper. A vesicle is a fluid-filled blister less than one centimetre; a bulla is more than one centimetre. A pustule is a pus-filled blister. Purpura is bleeding into the skin that does not blanch. Erythema is redness that blanches. Scale is compacted stratum corneum. Crust is dried serum, blood or pus. Atrophy is thinning of the skin. An ulcer is a full-thickness loss of the epidermis and the papillary dermis. [1]

The distribution is the first clue to the mechanism. A photosensitive distribution (face, the V of the neck, the dorsa of the forearms) points to lupus or dermatomyositis. An acral distribution (fingers, toes, nose, ears) points to vasculitis, antiphospholipid syndrome or cryoglobulinaemia. A dependent distribution (lower legs, sacrum) points to stasis or thrombocytopenic purpura. A dermatomal distribution points to herpes zoster. A flexural distribution (antecubital and popliteal fossae, axillae, neck) points to atopic eczema or acanthosis nigricans. An extensor distribution (elbows, knees, shins) points to psoriasis, dermatitis herpetiformis, erythema nodosum or necrobiosis lipoidica. [1]

(c) The hand signs of systemic disease (4 marks): [1]

The hands are the highest-yield region. The nails: clubbing (pulmonary, cardiac, gastrointestinal causes — assess with the Schamroth window sign), koilonychia (iron-deficiency anaemia), Beau lines (a severe systemic illness, dated by the nail growth rate), splinter haemorrhages (infective endocarditis in context, but also trauma), nail-fold capillary changes (dilated, tortuous loops with dropout in systemic sclerosis and dermatomyositis — examined with a dermatoscope or an ophthalmoscope at 10 to 20 dioptres). The palms: palmar erythema (cirrhosis, pregnancy, thyrotoxicosis — it blanches), Gottron papules (pathognomonic for dermatomyositis — violaceous papules over the MCP and IP joints), mechanic hands (dermatomyositis, especially the antisynthetase syndrome). The fingers: sclerodactyly with loss of skin creases (systemic sclerosis), digital pitting scars (systemic sclerosis), calcinosis (the CREST syndrome), telangiectasia (systemic sclerosis) [2][3].

(d) The purpuric eruption on the legs (3 marks): [1]

I classify the purpura by whether it is palpable or non-palpable, because this is the most important discriminator. Non-palpable (macular) purpura indicates a platelet or coagulation disorder — thrombocytopenia (immune thrombocytopenic purpura, leukaemia, aplastic anaemia, drug-induced), platelet dysfunction, or a coagulopathy (liver disease, disseminated intravascular coagulation, warfarin). Palpable purpura indicates small-vessel (leucocytoclastic) vasculitis or an infectious embolus. The immune complex deposition in the postcapillary venules triggers inflammation and red cell extravasation, making the lesion raised and non-blanching. The investigation of palpable purpura includes the ANCA, the cryoglobulins, the complement, the hepatitis B and C serology, the antinuclear antibodies, the urinalysis (for renal vasculitis), and a skin biopsy with direct immunofluorescence. The infectious causes to consider are meningococcaemia, gonococcaemia, Rocky Mountain spotted fever, and infective endocarditis [1].

(e) The causes of erythema nodosum (3 marks): [1]

Erythema nodosum is the commonest form of panniculitis — bilateral, symmetric, tender, subcutaneous nodules on the anterior shins. The causes to screen for are: streptococcal infection (the commonest infectious cause in children, assess with a throat swab and an antistreptolysin O titre), sarcoidosis (the commonest cause in young adults in many series — Lofgren syndrome is erythema nodosum with bilateral hilar lymphadenopathy and ankle arthritis, a self-limiting form of sarcoidosis), inflammatory bowel disease (Crohn disease, ulcerative colitis), drugs (the oral contraceptive pill, sulfonamides, penicillins, bromides), infections (tuberculosis, yersinia, chlamydia, histoplasmosis, coccidioidomycosis), and other (Behcet disease, Hodgkin lymphoma, pregnancy). In approximately 30 to 55 per cent of cases, no cause is found. The workup is a throat swab, an ASO titre, a chest X-ray, and inflammatory markers [5].

(f) The three most common errors (3 marks): [1]

1. The jump-to-diagnosis error. The candidate who looks at the shins and says "diabetes" before describing the lesion has failed the morphology test. I always describe the distribution, the morphology and the arrangement before naming the entity. [1]

2. The nasolabial fold error. The malar rash of SLE spares the nasolabial folds, while rosacea involves them. The candidate who does not check the nasolabial folds cannot distinguish the two most common causes of a red face in a young woman. [1]

3. The palpable-purpura error. Non-palpable purpura is a platelet or coagulation problem, while palpable purpura is vasculitis or an infectious embolus. The candidate who does not palpate a purpuric lesion has not classified it, and has missed the most important discriminator in purpura. [1]