Differential Diagnosis in Dermatology

Introduction to Differential Diagnosis in Dermatology

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There are over 2000 named diseases in dermatology but common things occur commonly. They just sometimes look a bit different! In practice these common conditions can be diagnosed using 4 mnemonics. This approach will not appeal to the purists amongst you but it works. In a busy general practice it gives you a basis on which to approach most of the common rashes you will see.

You just ask yourself the following 3 questions.

1. Is the rash skin coloured or red AND scaly or non scaly?

2. Are there pustules or blisters?

3. Is it a funny shape, colour or distribution?

The mnemonics are as follows. The red rashes are the commonest in white people.

1 PMs PET for red scaly diseases

2. CU DVA EVEI for red non scaly diseases

3. II for pustular diseases

4. ICI for Blistering diseases.

1. Diagnosing skin diseases is not difficult. You look at a rash and decide if it is red and scaly or red and non scaly. If it is red and scaly you use the mnemonic PMs PET (PET is Psoriasis, Eczema and Tinea. This is the Prime Minister's Pet ( I used to always think of Kevin Rudd with a siamese cat called Petal sitting on his lap!) The first P of PM is for Pityriasis rosea or Pityriasis versicolor and the M is for Mycosis fungoides, a T cell lymphoma of the skin.) View red scaly rashes
Now we know that his pet cat is called PETAL. This helps us to remember Psoriasis, Eczema and Tinea but also the less common red scaly diseases of A for Annular erythemas and L for Lupus erythematosus and Lichen Planus.

2. If it is red but not scaly consider Cellulitis, Urticaria, Drug reaction, Viral exanthem or Annular erythema .The mnemonic is C U at the Department of Veterans Affairs Evei (your girlfriend Evei) (CUDVA EVEI) where EVEI stands for Erythema multiforme, Vasculitis and Erythema nodosum and Infiltrates View the red non scaly rashes
3. If there are Pustules then the mnemonic is II
(aye aye) Infective( viral, bacterial, fungal) or Inflammatory eg psoriasis or a pustular drug reaction. Common causes include Staph folliculitis , modified fungal infection or if the vesicles are grouped herpes simplex. Pustules on the face are Acne, Rosacea, Staph folliculitis or H Simplex if grouped. View the pustular rashes
4. If there are Blisters The mnemonic is ICI(Imperial Chemical Industries) Inflammatory including Immunological, Contact dermatitis and Infective.
Inflammatory causes can include drugs but remember Immunological causes in the elderly particularly bullous pemphigoid. Contact dermatitis usually gives smaller vesicles rather than blisters but individual vesicles can join up into blisters. If blisters are linear and itchy it is probably a Plant contact dermatitis. Infective blisters are usually bullous impetigo due to a staph infection. If in a dermatomal distribution blisters are likely to be Herpes Zoster. View the blistering rashes

For other morphologies such as funny shape, colour or distribution, click on the link in Labels opposite to go to the appropriate section for sample images of relevant conditions and a discussion on how to diagnose them. If you need help with skin disease terminology try this tutorial from Logical Images.

The following links are for General Practitioners who have access to the Skin Consult website. If you are a GP and wish access to this site, please register at www.learndermatology.com and provide proof of status as requested ie email a copy of your letterhead.

You need to log into www.skinconsult.com.au before clicking the links below.

Rash on Face- Under Breasts and Flexures - Hands and Feet - Genitalia and Anus - Lower Legs and Arms - Hair and Scalp Problems - Mouth and Lips- Nails- Light and Dark Patches on Skin- Itch Localised and Generalised- Ringworm Like Rashes - Red All Over Patient- Unusual Rashes in Children- Unusual Rashes in Adults

For expert diagnosticians have a look at the Diagnosis Page of Global Skin Atlas and search using the morphology function alone or combine morphology and site.

Hyperpigmentation generalised

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Generalised hyperpigmentation is always worrying. It can be a feature of underlying malignancy particularly ACTH producing carcinoma of the lung or from an underlying melanoma. Also consider Addison's disease with pigmentation of the skin creases and inside the mouth.Drug induced hyperpigmentation is another thought particularly from some chemotherapy drugs. Metabolic disorders such as hemochromatosis and porphyria cutanea tarda can also give marked generalised hyperpigmentation but in PCT it is accentuated in sun exposed areas.

Generalised hyperpigmetation is summarised in the overview below.

SIGN DIP MEN Overview of Hyperpigmentation

S-Squamous Resolved lichen planus

I-Infective

G-Granulomatous

N-Neoplastic Melanoma metastases,Lung carcinoma,Lymphoma

D-Drugs Melasma, Fixed drug eruption,bleomycin,arsenic,gold and cyclophosphamide,Puva

I-Immunological Scleroderma,lupus erythematosus and dermatomyositis

P-Physical Post sunburn,Post taumatic,Racial pigmentary demarcation lines,Phototoxic hyperpigmentation (Plants),Vagabond’s disease

M-Metabolic Addison’s disease,Porphyria cutanea tarda,Hemochromatosis,Renal and hepatic failure,Amyloidosis

E-Endocrine Hyperthyroidism,Pregnancy,Cushings,Acromegally,Thyrotoxicosis,Pheochromocytoma

N-Nutritional Pellagra,Malabsorption

Others

Eyelids Familial,nevoid,metabolic diseases such as ochronosis,chemical such as mercury ointments and psoralens and argyria,lichen planus,lichen aureus,melanoacanthoma and some endocrine diseases.

Reticulate Pigmentation Dowling Degos(flexures),Zosteriform,Dyskeratosis congenita,Naegeli Franceschetti syndrome(neck axillae,keratoderma),Acropigmentations of Kitamura and Dohi

Localised hyperpigmentation DAMN PIG PAPA

Drugs Fixed drug plus antimalarials,minocycline,cytotoxics,bleomycin(reticulate pigmentation),mercury and bismuth.

Autoimmune Scleroderma,lupus erythematosus,dermatomyositis,atrophoderma,

Metabolic Ochronosis,chloasma,gaucher's disease,

Neoplastic Lymphoma,melanoma,mastocytosis,

P erythema dyschromicum perstans,postinflammatory hyperpigmentation lichen planus,

Infective erythrasma,tinea nigra

Granulomatous leprosy,granuloma annulare,syphilis

P Parapsoriasis,A Amyloid,P Pigmented purpuric dermatosis,A acanthosis nigricans

Hyperpigmentation Localised

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Facial hyperpigmentation if localised would be a lentigene or Hutchison's melanotic freckle,Melasma in females or part of an epidermal or congenital nevus.Ochronosis can arise from treatment with hydroquinone.

Localised hyperpigmentation DAMN PIG PAPA

Drugs Fixed drug plus antimalarials, minocycline, cytotoxics, bleomycin(reticulate pigmentation), mercury and bismuth, psoralens, dithranol in psoriasis,

Autoimmune Scleroderma,lupus erythematosus, dermatomyositis, atrophoderma,

Metabolic Ochronosis, chloasma, gaucher's disease, kwashiorkor, pellagra, linea nigra, acanthosis nigricans,

Neoplastic Lymphoma, melanoma, mastocytosis,

P erythema dyschromicum perstans, post inflammatory hyperpigmentation, erythema ab igne, lichen planus,

Infective erythrasma, tinea nigra

Granulomatous leprosy, granuloma annulare, syphilis

P Parapsoriasis,A Amyloid,P Pigmented purpuric dermatosis,A acanthosis nigricans

Hyperpigmentation in the neonate

Blue grey Mongolian spot, Nevus of Ota (Face), Nevus of Ito (Shoulder,neck), Phakomatosis pigmentovascularis(Trunk plus port wine stain)

Brown Cafe au lait spots, Congenital nevus

Small brown macules Peutz Jeghers syndrome, LEOPARD syndrome, Generalised lentiginosis, Inherited patterned lentiginosis, Carney syndrome, Neurofibromatosis(axillae), Centrofacial lentiginosis(central face)Segmental lentiginosis, Mosaicism, Speckled lentiginous nevus, Nevus spilus, Transient neonatal pustular melanosis

Labial brown macules Peutz Jeghers syndrome, Carney syndrome

Swirled or Blaschko pattern Linear and whorled nevoid hyperpigmentation, Incontinentia pigmenti, Epidermal nevus, Goltz syndrome, Conradi Hunermann syndrome, Mosaicism

White skin generalised

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Generalised white patches on the skin usually mean vitiligo if there is no surface scale. In tropical countries also consider tuberculoid leprosy and pinta.

SIGN DIP MEN Overview of Hypopigmentation

S-Squamous Healing areas of eczema and psoriasis, Pityriasis alba, Pityriasis versicolor, Post discoid lupus erythematosus scarring, Tinea corporis,

I-Infective Pityriasis versicolor, Tuberculoid Leprosy, Pinta, Syphilis, Post herpes zoster

G-Granulomatous Sarcoidosis,

N-Neoplastic Morpheic Basal cell skin cancer, Epidermodysplasia verruciformis,

D-Drugs Phenols,

I-Immunological Vitiligo, Halo nevi, Localised Morphea, Lichen sclerosus, Scleroderma,

P-Physical Following liquid nitrogen, Post traumatic in dark skin, Idiopathic guttate hypomelanosis, Chronic radiodermatitis,

M-Metabolic

E-Endocrine

N-Nutritional

V Vascular Malignant atrophic papulosis

Other

Acquired White areas T cell lymphoma, Halo nevus, Lichen sclerosus, Malignant atrophic papulosis, Pinta, Post inflammatory hypopigmentation, Segmental vitiligo, Tinea versicolor, Vitiligo

Localised congenital depigmentation Nemonic is (WANT A DIP) of colour.

Wardenberg's, Alezandreni, nevus anaemicus, nevus depigmentosus, Tuberous sclerosus, Dyskeratosis congenita, Incontinentia pigmenti achromicans, Piebaldism.

White skin localised

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Localised white skin lesions also signify a limited number of diseases. Working down, on the face consider pityriasis alba, a low grade form of eczema, poliosis, a white area on the forehead with a tuft of white hais from birth. If poliosis appears as a new lesion consider localised vitiligo. White spots on the chest or arms with fine surface scale is pityriasis versicolor. White spots on the forearms or lower legs are idiopathic guttate melanosis, a type of inverse freckle from sun damage. Porcelain white spots on the trunk with surface skin wrinkling is due to lichen sclerosus/ morphoea.

White patches present at birth should raise the possibility of tuberous sclerosus.

Hypopigmentation on the trunk includes that on the chest, back and abdomen.

The commonest conditions causing hypopigmentation at these sites include vitiligo, Halo nevi, Pityriasis versicolor and post inflammatory either after liquid nitrogen or surgery to skin cancers. Post inflammatory hypopigmentation is also seen with discoid lupus erythematosus. Localised morphea or lichen sclerosus can also present as hypopigmented patches but the underlying skin will be firm in morphea. Undertreated psoriasis or eczema of the trunk may also present as hypopigmented patches but some evidence of these conditions elsewhere should allow you to make the diagnosis.

In the newborn look for the ashleaf macules of Tuberous sclerosus or pale connective tissue nevi. Segmental vitiligo may also be seen congenitally. Nevus depigmentosus and nevus anaemicus may also present as hypopigmented patches in this age group.

Annular lesions

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Annular lesions are always fun to diagnose. The public invariably diagnose them as tinea or ringworm but that diagnosis is only a possibility if there is scale. If there is no scale then the process is dermal and you should consider granuloma annulare, sarcoidosis, annular erythema and even leprosy!

Annular lesions on the face

Tinea faciei due to a dermatophyte infection would be the commonest, but granulomatous disorders such as sarcoidosis and granuloma annulare and infective conditions such as leprosy should also be considered.

Management - skin scrapings if scaly, check to see if there is a loss of sensation which would be seen in leprosy in the centre of the lesion and biopsy if you consider one of the granulomatous diseases.

SIGN DIP MEN Overview of Annular lesions

S-Squamous Resolving psoriasis,Discoid eczema,Genital lichen planus,Herald patch of Pityriasis rosea,

I-Infective Tinea (Ringworm),Erythema chronicum migrans,Leprosy,Syphilis,Erythema marginatum,

G-Granulomatous Granuloma annulare,Sarcoidosis,Elastolytic granuloma

N-Neoplastic Basal cell skin cancer,Mycosis fungoides,follicular mucinosis,Erythema gyratum repens,Necrolytic migratory erythema,Porokeratosis of Mibelli

D-Drugs Reactive annular erythema after Vit K injections,can also occur after heparin,collagen steroid and anticancer agent injections,

I-Immunological Urticaria,Angioedema,Subacute lupus erythematosus,Neonatal lupus erythematosus,

P-Physical Cupping,ECG suction caps,

M-Metabolic Reticular erythematous mucinosis

E-Endocrine

N-Nutritional

Others - Annular erythema of infancy,

Annular red brown lesions with scale and central scar Tertiary syphilis,lupus vulgaris,lupus erythematosus,sarcoidosis,cicatricial pemphigoid,leishmaniasis(purplish scar in recidivans)

Linear lesions

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Linear lesions are also quite strikingly obvious. Consider plant contact dermatitis, herpes zoster, lichen striatus and ILVEN or inflammatory linear verrucous epidermal nevus! Some conditions form linear configurations after an injury or scratch. This is known as the Koebner reaction or phenomenon and is seen most often in psoriasis. Mosaicism of the skin can also give linear forms of innumerable skin disorders eg linear porokeratoses.

Linear lesions on the trunk include those on the chest,back and abdomen.

The commonest linear lesions on these areas would be epidermal nevi particularly ILVEN and also lichen striatus.Inflammatory linear with blisters would suggest herpes zoster, followed by a plant contact dermatitis or a phytophotodermatitis from a photosensitising sap or juice eg limes.A tender thrombosed vein on the chest wall is known as Mondor's disease.

SIGN DIP MEN Overview of Linear lesions

S-Squamous Lichen striatus,ILVEN(inflammatory linear verrucous epidermal nevus),Psoriasis,Lichen simplex,,Lichen planus,Darier’s disease,Lichen nitidus,

I-Infective Herpes zoster,Cutaneous larva migrans,Warts including molluscum

G-Granulomatous

N-Neoplastic Porokeratoses,Sebaceous nevus,Epidermal nevi,Linear benign tumours Syringomas,Trichoepitheliomas,and Eccrine spiradenomas,Linear porokeratoses,,Leiomyomas,Segmental neurofibromatosis,Linear Basal cell nevus syndrome

D-Drugs

I-Immunological Vitiligo,Linear Morphea,Graft versus host disease,

P-Physical Scratching,Dermatographism,Plant contact dermatitis,Pigmented purpuric dermatosis,Mondor’s disease,Linear fibromatosis,Linear common diseases because of the Koebner effect

M-Metabolic Papular mucinosis

E-Endocrine Pregnancy pigmentary lines

N-Nutritional

Others- Linear lesions following Blaschko’s lines, Linear disease variants due to Mosaicism, Incontinentia pigmenti, Hypomelanosis of Ito, Goltz syndrome, Conradi syndrome,

Skin Coloured non scaly

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Skin coloured non scaly lesions on the skin are usually infiltrates, ie the pathology is in the dermis rather than the epidermis. ( Involvement of the epidermis usually gives scale or crust of some sort).
Hence the lesions are skin coloured papules, possibly red if inflammed, and are not itchy. Consider sarcoidosis and granuloma annulare if the rash is extensive and tumours such as neurofibromatosis or leiomyomas if the lesions are multiple and grouped. However one of the commonest causes of smoothe dome shaped skin coloured papules is molluscum contagiosum, a pox virus usually seen in young children.

Dermnet on sarcoidosis
Dermnet on granuloma annulare

Dermnet on molluscum contagiosum

Skin coloured scaly diseases

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The Skin coloured but scaly conditions are just the various forms of Ichthyosis, a group of genetic diseases of the skin. See DermNet for further information. There is a rare type of ichthyosis that is red and scaly called CIE or congenital ichthyosiform erythroderma.

Blistering diseases

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If there are Blisters The mnemonic is ICI(Imperial Chemical Industries) Inflammatory including Immunological, Contact dermatitis and Infective.
Inflammatory causes can include drugs but remember Immunological causes in the elderly particularly bullous pemphigoid. Contact dermatitis usually gives smaller vesicles rather than blisters but individual vesicles can join up into blisters. Watch for hair dye allergies around the posterior neck and scalp or consider a plant contact dermatitis if the blisters or vesicles are in a linear streaky distribution on exposed surfaces where the patient has brushed up against an offending bush or tree.
Infective causes of blisters are usually staph toxin in origin and go under the name of bullous impetigo. However if the lesions are in a linear distribution but painful and limited to skin dermatomes then consider Herpes zoster or shingles. The Bullous insect bite reaction occurs on the lower legs usually from sand fly bites and the blister is tense and intact.
Blistering drug eruptions are rare but can be seen with the antiepileptic drugs. They are usually explosive in onset and there may be mucosal involvement. Metabolic disorders rarely give rise to skin blisters but a notable exception is seen in porphyria cutanea tarda PCT. These blisters are usually seen on the backs of the hands or feet in sun exposed areas. The blisters are firm and take time to burst. They surrounding skin is not inflammed. An immunological bullous disease that can look very similar is epidermolysis bullosa acquisita, usually seen at sites of trauma.

A more advanced overview of nearly all possible causes of blisters is shown below. A GP would rarely need to refer to it!

Blisters on the trunk include those on the chest,back and abdomen. On these areas blisters are usually due to Herpes Zoster(unilateral and painful) bullous staph infection,bullous pemphigoid,plant contact dermatitis and drugs causing toxic epidermal necrolysis. Pemphigus blisters are fragile and soon form crusts and erosions.Rarer immunobullous diseases such as dermatitis herpetiformis and linear IgA disease will cause blisters in these areas. Fixed drug eruption may cause localised blistering.

Management Culture for bacteria and viruses,do a Tzanck smear,do a gram stain and biopsy and immunofluorescence if you consider an immunobullous disease is likely.

Herpes Zoster

Bullous pemphigoid

Plant contact

SIGN DIP MEN Overview of Blisters

S-Squamous Bullous Dariers disease, Bullous Lupus erythematosus

I-Infective Impetigo, Staph scalded skin syndrome, Herpes zoster, Bullous tinea, Parvovirus B19, Staph infected varicella, Bullous orf, Pseudomonas septicemia, Hemorrhagic bullae with Vibrio vulnificus, Mucor infection in immunosupressed, Blistering dactylitis

G-Granulomatous

N-Neoplastic Paraneoplastic pemphigus, Bullous mastocytosis, Paraneoplastic pemphigus

D-Drugs Toxic epidermal necrolysis, Fixed drug eruption, Numerous drugs causing the immunobullous disaeases, Pseudoporphyria, Barbiturate coma,

I-Immunological Plant contact dermatitis, Phytophoto dermatitis, Erythema multiforme, Bullous pemphigoid, Pemphigus, Dermatitis herpetiformis, Linear igA disease, Chronic bullous dermatosis of childhood, Bullous lupus, Mucosal Pemphigoid, Epidermolysis bullosa aquisita, Herpes Gestationis, Lichen sclerosus , Bullous morphea, Bullous necrotising vasculitis,

P-Physical Friction blister, Burns, Insect bite reaction, Bullous scabies, Lymphedema, Puva blisters, Tanning bed Pseudoporphyria, Polymorphous light eruption, Edema blisters of the leg, Sucking blisters in neonates, Epidermolysis bullosa,

M-Metabolic Porphyria cutanea tarda, Diabetes mellitus, Amyloidosis, Blisters of hemodialysis, Mastocytosis,

E-Endocrine Hypothyroidism,

N-Nutritional Pellagra, Acquired zinc deficiency, Acrodermatitis enteropathica,

Others Pompholyx eczema palms and soles, Congenital syphilis, Kindler syndrome, Neonatal purpura fulminans, Incontinentia pigmenti, Bullous ichthyosiform erythroderma

Hemorrhagic Blisters pemphigus, herpes zoster, leukemia, lichen sclerosus

Erosions Pemphigus, Hailey Hailey, Eczema herpeticum

Sheets of skin Staph scalded skin syndrome,Toxic epidermal necrolysis

Pustular diseases

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If there are Pustules then the mnemonic is II
(aye aye) Infective( viral, bacterial, fungal) or Inflammatory eg psoriasis or a pustular drug reaction. Common causes include Staph folliculitis , modified fungal infection or if the vesicles are grouped herpes simplex. Pustules on the face are Acne, Rosacea, Staph folliculitis or H Simplex if grouped.

When we see pustules we have a tendency to think infection and often limit ourselves to only bacterial infections at that. Remember pustules can occur with fungal and viral infections as well and never forget that some pustules are not due to infection but to infiltration of the skin by neutrophils in Inflammatory skin diseases such as psoriasis and drug eruptions. There are also a few other very rare inflammatory disorders such as acrodermatitis enteropathica (zinc deficiency) and the glucagonoma syndrome from underlying pancreatic malignancy that can have pustules at the advancing edge of the lesion and thereby simulate a fungal infection.

The images below include variants of pustular psoriasis, both localised and generalised and infected pompholyx eczema.

Vesicular Diseases

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Vesicles are small clear fluid filled deep seated lesions in the epidermis. They are a feature of eczema/ dermatitis when acute in onset but can also be seen in delayed type hypersensitivity reactions such as a reaction to tinea on the soles of the feet. They are usually itchy and can subsequently become pustular from secondary bacterial infection. If you have pustules enquire if they were clear fluid filled lesions initially. The developement of herpes simplex superimposed on a background atopic eczema is always a diagnostic problem but look for a sudden painful deteriration in the condition particularly with painful lesions appearing around the eyes.

View this discussion of dermatitis in DermNet NZ

Red non scaly diseases

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If it is red but not scaly consider Cellulitis, Urticaria, Drug reaction, Viral exanthem or Annular erythema. The mnemonic is C U at the Department of Veterans Affairs Evei (your girlfriend) (CUDVA EVEI) where EVEI stands for Erythema multiforme, Vasculitis and Erythema nodosum and Infiltrates

The important thing is to establish there is no scale before using this algorithm. Remember that some scaly disorders lose their scale if treated with topical steroids and scaly diseases such as psoriasis lose their scale in moist flexures such as under the breasts and in the groin and axillae. In Australia it is also important to consider solar damage as a cause of a red non scaly rash especially on exposed areas such as face, forearms and lower legs, but most areas of significant sun damage are red and scaly
The more common disorders will be outlined first and representative photographs of them will be shown.

C Cellulitis Characterised by a hot, red nonscaly fixed tender area of skin which has arisen suddenly, typically on the lower legs over the shins or on the cheeks or around a recent injury to the skin. Lymphangitis ie a red line travelling towards the local lymph glands is a worrying sign!

For more information See DermNet

U Urticaria Raised itchy plaques in the skin that last a few hours and disappear only to reappear elsewhere. Individual lesions never last longer than 24 hours ecept for urticarial vasculitis where the lesions mat resolve but leave residual bruising at the site and papular urticaria secondary to an insect bite reaction where the mechanism causing the lesion is a delayed type cell mediated hypersensitivity reaction rather than the immediate IgE mediated histamine release from mast cells.

For more information See White Diseases of the Skin

D Drugs You have to consider a drug reaction for virtually every red non scaly rash. Typically sudden onset, no fever, itch prominent and the rash is florid! Compare it with a virus infection where the onset can be just as fast and the rash just as florid but there is fever, little itch and the patient feels unwell. A drug reaction to an antibiotic given to a patient with a viral infection is always difficult to diagnose! Look for a viral enanthem.

For more information See White Diseases of the Skin

V Virus Viral infections can have a variety of morphologies but macular or maculopapular are the commonest and they are red and non scaly. The patient usually has a fever, some lymphadenopathy and feels ill. An enanthem is an associated finding often in the mouth. eg Koplick spots on the buccal mucosae in measles.
You can learn to diagnose viral exanthems by viewing this tutorial.

A Annular erythemas The annular erythemas look like urticaria but individual lesions last longer than 24 hours and often slowly join up with each other to form polycyclic rings. They can be easily misdiagnosed as a tinea fungal infection but they usually do not have a scale except sometimes the EAC or erythema annulare centrifugum variant.

For more information See DermNet NZ

E Erythema multiforme This condition looks like a drug eruption which it sometimes is! Again no fever or itch and few systemic features in the minor variant. The morphological feature you look for is the iris, bullseye or target lesion seen on the lower legs or palms of the hands and soles of the feet. Again the rash is red and non scaly, sudden onset lasting days, occassionally blistered in severe cases due to a drug and in the very severe cases will have involvement of the lips and conjunctival surfaces. In these circumstances it goes under the name of the Stevens-Johnson syndrome.

See White Diseases of the Skin or Dermnet NZ for further information.

V Vasculitis The early stages of true vasculitis give a red non scaly rash particularly on the lower legs or buttocks. It does not blanch with pressure and may be purpuric or small bruise like. Again drugs are the commonest cause but the potential range of causes is very great. The first thing to do is check the urine and see if there is any blood in it. If there is then you have a systemic vasculitis that can hit other organ systems including the joints, the gut , the lungs, heart and the brain. Wide ranging investigations are necessary to diagnose the cause.

See White's diseases of the Skin for an exhaustive discussion or DermNet for a more leisurely one.

E Erythema nodosum This red non scaly rash is also quite distinctive presenting as tender deeper nodules on the anterior shins or sometimes on the calves. The lesions may resolve with bruising before disappearing over a 2-3 week period. You have to consider a condition called erythema induratum when the lesions are mainly on the calves. Erythema nodosum is a form of panniculitis or inflammation of the deeper fat tissue. Again it can be a drug reaction but most cases are post streptococcal throat infection.

See Derm Net for more information.

Infiltrates can present as red non scaly rashes eg Generalised granuloma annulare, sarcoidosis, leukemic infiltrates, leprosy, leishmoniasis, mucinoses ie infiltrates of cells, substances or infectious agents (viral, bacterial, fungal and protozoal).

When you are considering the causes of the red non scaly rashes the underlying causes of cellulitis, drugs and viral infections are self explanatory but all the others have multiple potential causes because they are all reaction patterns in the skin, probably due to immune complex formation. As potential causes for each you should consider drugs, infections,collagen diseases eg lupus erythematosus, food colourants and additives and very rarely underlying malignancy!
So if the rash is red and non scaly then C U at the Department of Veterans Affairs Evei (your girlfriend) (CUDVA EVEI) where EVEI stands for Erythema multiforme, Vasculitis and Erythema nodosum and Infiltrates of cells or substances or infectious agents.

Red scaly diseases

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Diagnosing skin diseases is not difficult. You look at a rash and decide if it is red and scaly or red and non scaly. If it is red and scaly you use the mnemonic PMs PET (PET is Psoriasis, Eczema and Tinea. The first P of PM is for Pityriasis rosea or Pityriasis versicolor and the M is for Mycosis fungoides, a T cell lymphoma of the skin.) The only additional problem we have in Australia is the extent of our solar damage which can masquerade as a red scaly rash and even be non scaly. So add solar damage after the nemonics for both the red scaly and non scaly rashes.

Now we know that his pet cat is called PETAL. This helps us to remember Psoriasis, Eczema and Tinea but also the less common red scaly diseases of A for Annular erythemas and L for Lupus erythematosus and Lichen Planus.

In this group there are five conditions that are common. These are psoriasis, eczema, tinea , pityriasis rosea and pityriasis versicolor. Other less common conditions include mycosis fungoides, lupus erythematosus, parapsoriasis, lichen planus and several other pityriasis conditions such as pityriasis rubra pilaris and pityriasis lichenoides
The more common disorders will be outlined first and representative photographs of them will be shown.

Step 1 Once you have decided that a rash is red and scaly look to see if the scale is broken and if there are cracks in the surface with any oozing or weeping. If there are, then it is a form of eczema, probably discoid eczema, rather than psoriasis or one of the other red scaly disorders. If it is not, then Step 2 scratch it first of all and see the nature of the scale. If you get the silvery wax like scale of psoriasis you are home and hosed. If you do not, then Step 3 the first thing you should do before you even look elsewhere is to take some scrapings for fungal culture. You should do this before you treat any red scaly rash rather than afterwards. Pityriasis rosea you will diagnose because of the herald patch, oval shaped lesions and the distribution along the rib lines on the trunk and Pityriasis versicolor because the lesions will either be whiter than the surrounding tanned skin or pinker than the surrounding white skin, depending on whether it is summer or winter!

Psoriasis
This is a composite image of psoriasis.

For more information on psoriasis try this reference.
Additional Diagnostic Features In psoriasis he skin grows 8 to 10 times faster than normal so you get this thick build up of scale that is easily scraped off. The lesions are usually quite sharply defined, the colour is often a salmon pink colour, except in the more inflammatory irritated types of psoriasis where it can be much redder. You should look at the more typical areas where you would expect to see psoriasis such as the elbows, the knees, the scalp. Look at the nails and see if there is any evidence of pitting. Look under the flexures and the elbows or breasts or in the groin area or especially around the anus to see if there is the relatively smooth non scaly lesions that occur in these areas.
Psoriasis on the lower legs can have an eczematous look to it as well because of stasis features and underlying varicose veins because the patient may have rubbed and scratched the psoriasis. This should not put you off making the diagnosis.

Eczema Red and scaly but with small erosions or breaks in the skin surface, sometimes oozing and often secodarily infected with Staph aureus.

For more information on Eczema View this reference
Additional Diagnostic features Eczema is usually itchy. The small erosions or breaks in the skin surface are the crucial diagnostic feature as is the distribution on the front of the elbow and behind the knee. Contact eczema remains localised at unusual areas depending on what and where you have been applying something you are allergic to.

Tinea Red and scaly but with a spreading edge. Scrape this for microscopy and culture. There may be small pustules in the edge in rare cases. Treatment with topical steroids will reduce the redness and scale making diagnosis more difficult.

For more information on Tinea View this reference

Additional Diagnostic Features The slowly spreading scaly edge is a hallmark of the condition. Seldom itchy. Usually not symmetrical like psoriasis or eczema. Fungal infections from animals are usually more inflammed.

Pityriasis rosea Red and scaly but has the herald patch appearing first with it's peripheral scale , followed within the week by the other oval shaped, scaly lesions running along the rib lines. May involve axillae and groin but seldom further down the limbs. Trunkal rash predominantly. Guttate psoriasis is your main differential diagnosis.

For more information on Pityriasis rosea View this reference

Additional Diagnostic Features The peripheral scale is not found on every lesion. It is seldom seen on the face. The sun helps it to go more quickly. No other systemic symptoms despite it probably being a viral infection. Usually not itchy.

Pityriasis versicolor This red scaly disease has fine bran like scale elicited by scratching the surface. It is usually seen on the trunk or under the breasts. It presents as white spot disease in tanned individuals and a red scaly disease on white skin.

Additional Diagnostic Features Often a very light pink colour and easy to miss. Look especially on the upper back, axillae and under breasts.

For further information on Pityriasis versicolor
See this reference or view additional images in Globalskinatlas.

The M in PMs PET is for Mycosis fungoides. This is an old name for T cell lymphoma of the skin. It is quite rare but important to diagnose because an early lesion can look like psoriasis or low grade eczema. This can be seen in the composite image below.

Additional Diagnostic Features This is a clinical diagnosis early because the initial biopsy may not be diagnostic. Fixed or slow growing plaque, sharp edges sometimes in bizarre shapes. Itchy later when they thicken.

For further information View this reference or see other images in Globalskin Atlas

We have mentioned that the PMs PET is called PETAL. The AL is to remind you of some other red scaly diseases including the Annular erythema called EAC or erythema annulare centrifugum and the L to remind you of Lupus erythematosus and Lichen planus.
EAC will remind you of a fungal tinea infection but it has a trailing or inward looking scale rather than at the edge as in a fungus and the lesions grow quickly and often merge together.
See this example from Globalskin Atlas

The L for lupus erythematosus refers to the discoid type which is usually seen on the face or back. On the face you will mistake it for sun induced skin cancer but if you freeze it then it will just come back around the frozen area! The scale is throughout the lesion, and is very adherent sometimes going into the hair follicles and giving a carpet tack appearance underneath when you try to lift it off!
See these examples from Globalskin Atlas or for more information on Discoid Lupus View this reference.

Other P diagnoses There are several other rarer skin diagnoses that are red and scaly and begin with Pityriasis. These include pityriasis rubra pilaris, pityriasis lichenoides and pityriasis alba. A composite of these conditions is shown below and there are links to global skin atlas and dermnet for further information.

Funny distribution excluding Linear

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Skin rashes can be found in unusual distributions eg
Flexural
Acral
Photo distribution
Periungual
Periocular
Perioral
Glans penis
Vulva
Solitary localised- This suggests a localised contact dermatitis if red and scaly with a broken surface or a fixed drug reaction if red and non scaly with slight hyperpigmentation. If recurrent vesicles preceded by localised pain then herpes simplex is the likeliest diagnosis.

You can try a manequin in the Diseases by Site link opposite to look at skin diseases by site or better still go to Global Skin Atlas Diagnosis and use the Specific Search Site function for a comprehensive list of diseases occuring at a specific site..

Funny Colour

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Excluding red and skin colors we will look at lesions that are an uncommon colour.
Yellow lesions Yellow lesions look yellow because of fat, sebaceous material, carotene, jaundice pigment or drugs. At one stage resolving bruises go through a yellow phase. Yellow papules or nodules are Xanthomas, xanthogranulomas including necrobiotic, Sebaceous hyperplasia and other sebaceous tumours, Gouty tophi around elbows and knees.
Yellow skin consider carotene pigmentation of palms and soles, jaundice from any cause and drugs such as quinacrin.

Purple lesions
Purple lesions are that colour because of altered or venous blood , overgrowth of blood vessels or infiltrates of neutrophils or lymphocytes into the skin. Consider therefor haemangiomas, angiosarcomas, Kaposi's sarcoma, port wine stains or other vascular malformations, vasculitis with leaked blood cells, lymphocytoma, lymphoma, and the plaques and nodules of Sweet's syndrome where the infiltrating cells are lymphocytes. Sarcoidosis can also have a purple colour in the skin. Better also consider drugs giving a lichenoid drug reaction eg Thiazides and of course Lichen planus itself with itchy papules at the wrists and lower legs.
.
Green lesions
There are not many green lesions in the skin. Copper bracelets can cause green staining under them. Thereafter it would need to be exogenous pigmentation from a dye. Do not know any drugs causing a generalised green skin colour.

Blue lesions
Things appear blue in the skin if melanin is in the upper dermis or below. It has something to do with light scattering by the tissues. The same melanin higher in the epidermis shows as black. Venous blood is also blue as is the drug minomycin or clofazimine when they are deposited in the skin. Blue papules are blue nevi , melanoma metastases, leukaemic deposits or some lichen planus papules. Blue nodules in neonates are due to severe viral infections with extramedullary haematopoesis or neuroblastoma metastases!
Cyanosis makes you blue as well!

Grey lesions
Again consider drugs such as Minocycline and post inflammatory hyperpigmentation particularly with fixed drug reactions and lichen planus. It is usually due to melanin in melanophages in the dermis.

Test Cases

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These cases will be analysed using the nemonics previously learned.

This is obviously a red scaly lesion. Note there are two other smaller lesions near it.
The PMs PET nemonic applies. Is it psoriasis, eczema or tinea?
Treated psoriasis can heal in the middle leaving a scaly outer edge. It is not broken on the surface for eczema. It has a peripheral scale for tinea so it could be a fungal infection.
Now what about the PM bit. Pityriasis rosea possible if this was the big herald patch early lesion. Pit rosea often has a trailing scale that points inward. Pit versicolor is less likely with the central clearing.
On balance this could be a tinea( ringworm) infection or the herald patch of pityriasis rosea. The history would help but you should do some scrapings of the lesion's scaly edge for fungal microscopy and culture. Examination of this patient elsewhere showed the typical oval lesions following the lines of the ribs seen in pityriasis rosea. What we have been examining is the herald patch. View DermNet for more information on Pit rosea.

Nail problems

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There are a variety of common conditions that patients bring to their doctor's attention. Longitudinal ridging is a normal feature of aging. Splitting of the ends of the nails is due to trauma and too much water exposure. Separation of the nail from the nailbed is also due to excess wet work where the bonds between the nail and the underlying nail bed are weakened and the nail separates. This is known as onycholysis. Any candida found under the separated nail is a contaminant.Nails are solid keratin which is the food for dermatophyte fungii. Hence most crumbly nails are due to a dermatophyte infection but some may be due to psoriasis. Take nail clippings for culture. Nail distortion is commonly due to ageing and poorly fitting shoes.

Specific conditions involving the nails

Brittle Brittle nails

Curved Koilonychia,Pincer nails

Dystrophic Darier's disease,Lichen striatus,Psoriasis,Onychomycosis,Onychophagia,Proximal subungual onychomycosis,Trachyonychia,

Groove Digital mucous cyst

Malaligned Congenital malalignment

Onycholysis Onycholysis,(numerous causes,water,photodrug etc),Onychomycosis,Psoriasis,Traumatic,Epidermolysis bullosa aquisita,Pemphigus vulgaris,Acropustulosis of Hallopeau

Pigmentation Drug induced(AZT,Bleomycin,Methotrexate),Longitudinal melanonychia

Pits Alopecia areata

Purpura Splinter hemorrhages,Subungual hematoma

Ridges longitudinal Aging,Lichen planus,median nail dystrophy

Ridges transverse Beau's lines,Chronic paronychia,Habit nail tic deformity,

Short Brachyonychia

Thickened Chronic mucocutaneous candidiasis,nail hypertrophy and Onychogryphosis,Onychauxis,Onychomycosis,Pachyonychia congenita,Psoriasis

Coloured Blue(Blue nails drugs or Wilson's disease),Green (Pseudomonas),White(Half and half nails,Leukonychia,Muehrcke's lines,Terry's nails,White superficial onychomycosis),Yellow (Nail discoloration cigarretes,Yellow nail syndrome)

Posterior nail fold Paronychia,Chronic candidiasis,Scleroderma,Dermatomyositis (ragged cuticles)

Modified from Regional Dermatology Gary M White

Hair problems

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Localised hair loss has three common possibilities, alopecia areata, tinea capitis and trichotillomania. In alopecia areata the area of hair loss is complete, the involved area is smoothe and the hairs may be broken at the edges.The bald area is not inflammed. In tinea capitis the area is never completely bare, the hairs are broken, there may be scale on the surface and signs of scalp inflammation. In trichotillomania the hairs may be broken and the pattern of loss is unusual. The scalp may show inflammation around recently traumatically removed hairs.

Management-Take scrapings if scaly for fungal culture and include a few hairs. Do a hairpull test to see if the hairs around the bare area come out from the base with a telogen bulb on the end, a feature typical of alopecia areata.

Rare causes-Incontinentia pigmenti, nevus sebaceous, post tick bite, follicular mucinosis, Ofugi' disease, localised morphea, aplasia cutis, post herpes zoster, meningocoele,

Generalised hair loss The three common causes are androgenetic alopecia, telogen effluvium and generalised alopecia areata. Androgenetic alopecia is by far the commonest cause in both males and females. Females retain the frontal hair line but get considerable thinning behind it. Telogen efluvium follows a fever, general anaesthetic, weght loss or coming off the contraceptive pill or warfarin among other causes. The hair loss is across all the scalp but there are no bald bits! Generalised alopecia areata is a difficult diagnosis as it gives features similar to telogen effluvium but a scalp biopsy will separate the two conditions. Generalised hair loss is an unfortunate side effect of chemotherapy for various cancers.

Specific conditions involving the Hair

Attachments Pediculosis capitis, Piedra

Fragile Hair Menke's disease, Monilethrix, Netherton's syndrome, Pili torti, Trichothiodystrophy

Green hair Copper

Heterochromia Flag sign of Kwashiorkor and Marasmus

Lighter colored hair Phenylketonuria

Loose hair Loose anagen syndrome

Silver Grey hair Chediak-Higashi syndrome

Uncombable Felting, Uncombable hair syndrome

White Forelock Piebaldism, Wardenburg's syndrome

Whitening Canites

Whorled Scalp whorls

Modified from Regional Dermatology Gary M White