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With over 2,000 different skin diseases, it can seem a daunting task trying to fathom out the cause of a patient’s rash. Adopting a systematic approach to both history-taking and examination can help to narrow down the most likely cause...
Learning objectives
After reading this feature you should be able to:
- Identify a skin condition by using a systematic diagnostic approach
- Understand which skin conditions are more likely to occur on a particular part of the body
- Recognise red flags that require referral.
Introduction
The first things to consider are the patient’s age, gender and race. For example, viral rashes are more commonly seen in children, whereas a potential malignancy is more prevalent with advancing age. Certain conditions also tend to be age specific — for example, acne generally affects teenagers whereas rosacea is mostly seen in those between 45-60 years of age.
Some skin conditions have a tendency to be more common in women or men. For instance, melasma is more commonly seen in women, whereas men are more likely to suffer from a fungal infection, particularly in the feet or groin.Finally, consider a patient’s race, as conditions like vitiligo and post-inflammatory hyperpigmentation from acne are generally seen in people of colour.
Patients presenting with a new skin problem
The next step is to ask about the following aspects of their condition:
How long have they had the problem?
The timescale can be important as drug eruptions, allergic contact dermatitis and impetigo all develop over a matter of days, whereas fungal infections and psoriasis develop more slowly.
In addition, ask whether the lesions/rash has changed at all over time because this may help with the diagnosis. For instance, urticarial wheals from an allergic reaction will typically resolve after 24 hours without trace, whereas psoriasis plaques can change over several weeks and become increasingly covered in scale.
Where did it start?
Always ask a patient if they have other areas of their skin affected by the same or a similar problem. For instance, a potential fungal groin rash in a man could be the result of athlete’s foot. Pompholyx eczema, which gives rise to itchy flesh-coloured lumps along the sides of the fingers, can also be present on the soles of the feet but the patient may not make the connection.
Looking at the patient's skin
Three important aspects of a skin problem that should always be considered are site and distribution, shape and border, and colour.
Important aspects:
Skin conditions can typically affect specific areas of the body and knowing which ones are more likely on a particular part of the body can help establish the diagnosis. For example, eczema tends to affect flexor surfaces (i.e. inner creases such as the inner elbow and behind the knees). In contrast, psoriasis affects the opposite side (extensor surfaces) such as the elbow and knees.
Consider whether or not the rash is symmetrical (e.g. affects both arms or hands). A symmetrical rash tends to imply that it is due to an internal cause – atopic eczema will often be seen in both elbow creases. In contrast, an asymmetric rash (e.g. affecting a single arm, hand or leg) is most likely to have an external cause.
Other pointers that can provide clues to the diagnosis include:
- A facial rash that spares the area of skin under the chin and behind the ears is potentially a photo-sensitive reaction
- A problem that follows a path along a dermatome (an area of skin that is supplied by a single spinal nerve), such as along the side of the chest, might be herpes zoster (shingles)
- A linear eruption at a site of skin trauma or scar is termed the Koebner phenomenon and can be seen in patients with psoriasis
- Several itchy papules close together, for example on the lower leg, may indicate insect bites.
Knowledge of the shape and border of a skin condition is important, as many are characteristic.
For example, discoid eczema lesions are normally highly pruritic and coin-shaped on the limbs rather than the trunk. The border of a patch of psoriasis is well-defined, whereas atopic eczema typically fades into the surrounding skin.
A fungal infection with tinea extends outwards in a ring-like fashion with a raised border and central clearing. The lesions of erythema multiforme are described as target lesions and consist of concentric rings (like a dartboard).
Not all skin problems cause redness but when a patient has erythema, it blanches on pressing. Purpura is non-blanching due to extravasation of blood (i.e. leakage into the surrounding tissue).
Parents of young children often worry that if their child’s rash doesn’t blanch after pressing with a glass it is meningitis – but a non-blanching rash can be caused by straining, coughing or vomiting.
Other changes in skin colour can result from yeast infections such as pityriasis versicolor, which causes hypopigmentation (areas of paler skin) and the autoimmune disorder, vitiligo, which causes depigmentation or loss of colour. Increased pigmentation is seen with melasma.
Other questions to ask
Ask about itching (pruritus). Itching is a frequent symptom that becomes intolerable when it flares, often waking patients at night, and can give rise to scratch marks. Pruritus is so common in eczema that if the patient doesn’t complain about itching, you can quickly rule out eczema as a cause of their rash. Pruritus is very commonly seen with:
- Scabies
- Atopic eczema
- Contact dermatitis
- Insect bites
- Lichen planus.
Systemic diseases
If a patient describes pruritus but has no obvious skin symptoms, it might be due to several systemic illnesses such as:
- Renal disease
- Liver failure
- Diabetes
- Hypothyroidism.
Such patients should be referred to their GP for investigation. Other symptoms might include:
- Pain: e.g. herpes zoster (shingles)
- Tenderness: e.g. inflammatory conditions such as eczema
- Bleeding: e.g. may be seen with malignancy
- Discharge: e.g. often seen with infected lesions.
Eczema and psoriasis often run in families so ask about family history. Even the presence of associated atopic diseases, such as asthma and hay fever, raises the possibility of eczema as a cause of the rash. Alternatively, if other family/household members have the rash, think about a contagious cause (e.g. scabies or head lice).
Certain jobs increase the risk of specific skin problems. Typically, hairdressers, mechanics, bricklayers and even gardeners will suffer from irritant hand dermatitis and allergic contact dermatitis due to exposure to solvents, cements, pesticides and even plants. Asking about whether any work colleagues have similar symptoms is a useful indicator of a potential cause.
Don’t forget to ask about hobbies. Patients might keep various pets or fish and develop an allergy to their foods and cleaning solutions.
Another consideration is whether the condition improves while on holiday, as work-related allergic/irritant contact dermatitis is likely to get better when an individual is not exposed to the offending irritant.
When asking about current medication, make sure this includes prescribed, OTC treatments and any new cosmetics. Topical antibiotics can lead to allergic contact dermatitis and systemic agents such as tetracyclines are associated with photoallergic reactions.
Ask about any recently started medicines, either when the rash started or in the preceding two to three weeks before the problem began.
Always check if a patient has tried something as there is little point in suggesting a product they have already used without success. It should also be remembered that some treatments can alter the course of a skin problem.
A classic example is using a topical steroid on a fungal rash, which results in tinea incognita. When first applied, a topical steroid will reduce the inflammatory component of the fungal infection but, over time, the infection becomes less scaly and more pustular.
Don’t forget to ask about aggravating or relieving factors. For example, patients with rosacea will normally find their condition is worsened by exposure to sunlight, cold wind, hot spicy foods and alcohol intake. In contrast, those with eczema, psoriasis and acne will normally describe how their condition is usually improved during warmer, sunny weather, only to worsen again during the colder months.
The presence of stress through e.g. work and family problems can often lead to an exacerbation of eczema, psoriasis and acne – so it is important to ask about potential current stressors.
Always ask patients if they have any symptoms such as pyrexia, malaise, joint pains or any evidence of swelling, which could indicate an underlying malignancy or systemic disease such as systemic lupus erythematosus.
Finally, ask about how the condition affects them and why they are seeking treatment. Don’t be afraid to ask them what they think is the likely cause!
Criteria for referral – red flags
Many cases of mild to moderate skin problems can be managed in community pharmacies but it is important to appreciate which patients require referral to their GP.
Any patients with signs of infection (i.e. inflammation, discharge and tenderness) should be referred. Other conditions that warrant referral include:
- Moderate to severe acne (i.e. acne with many papules/pustules, which is widespread and affecting the chest and back)
- Infected eczema
- Suspected bacterial infection
- Shingles
- Drug-induced skin reactions
- Possible skin cancers/pre-malignant lesions
- Widespread fungal nail and scalp infections
- Psoriasis (note that apart from emollients there are no effective over-the-counter treatments for the condition)
Referral is also required where there is uncertainty about the diagnosis.
Case study – a positive presentation of guttate psoriasis
John is a 22-year-old music student who regularly goes to the gym. He comes to see you about a rash that has developed on both arms over the past three days and then rapidly spread over a few hours to his chest and legs. John is becoming very self-conscious about it and describes it as “a bit itchy”.
John has no other medical conditions or a history of skin problems but mentions that his father had scaly skin on his elbows. He also says, in passing, that about 10 days ago he had a bad cold and lost his voice for a few days.
What is the likely cause of his rash?
On examination, you notice a large number of papules on both his legs, arms and chest, some of which have a slight degree of scaling.
John has already explained how long he has had the rash and says that it is itchy. This suggests that it is unlikely to be eczema, which patients usually describe as very itchy.
What are the possible causes of his rash?
A widespread papular rash can have several causes including a drug-related eruption, but John is not currently taking any medicines. A common cause in young adults is pityriasis rosea, although this normally starts with a single, larger lesion, known as the Herald patch. This didn’t happen in John’s case.
In addition, subsequent lesions in pityriasis rosea develop over one to two weeks whereas, in John’s case, the lesions appeared quickly over a few days. One potential cause is guttate psoriasis – but is there any evidence in his history that might support this diagnosis?
There are at least three pieces of information revealed by John which support a diagnosis of guttate psoriasis:
- John described how the rash spread over a few days, a typical feature of guttate psoriasis. The term ‘guttate’ means drop-like and the condition is characterised by numerous, small, salmon-pink ‘tear-drop’ lesions affecting the trunk and limbs
- Another clue is the possible family history of psoriasis. John mentioned his father had scaly skin on the elbows, which might represent psoriasis. There is a strong link between a family history of psoriasis and the development of guttate psoriasis
- A final clue comes from John’s recent cold and sore throat in which he lost his voice. This could have been a streptococcal throat infection and there is a strong association between streptococcal infections and guttate psoriasis.
What treatment should be offered to John?
Fortunately, most cases of guttate psoriasis clear after a few weeks and no specific treatments are required, but John can use an emollient if the lesions are pruritic. John’s guttate psoriasis cleared after a few weeks and did not leave any scarring. There have been no recurrent episodes.
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