Figure. 7.10 Marked palmar desquamation in the resolving stage of acute generalized pustular psoriasis.

    The pustules are sterile, but secondary infection of preexisting psoriatic plaques should be excluded by culture of pustule contents and blood cultures. Occasionally, tiny areas of macerated keratin within plaques of psoriasis (especially in flexures or under occlusion) may mimic pustular psoriasis, usually in patients without the obvious malaise of true GPP.

    Oral lesions of fissured tongue or geographical tongue (migratory glossitis) occur with increased frequency in patients with generalized pustular, erythrodermic, and other actively increasing psoriasis (Fig.7.11).

Figure. 7.11 A red, fissured tongue may be associated with generalized pustular psoriasis.

    Variants of GPP include subacute forms (Fig.7.12), and an essentially identical eruption can occur de novo in pregnancy, known as impetigo herpetiformis. It recurs in subsequent pregnancies but remits after delivery. GPP can also occur in children, usually resembling a severe seborrheic dermatitis initially. Bullous lesions may occur in pustular and other forms of psoriasis (Fig.7.13).

Figure. 7.12 A less aggressive form of generalized pustular psoriasis can occur and is known as the subacute annular variant (of Lapière), shown here on the thigh; these patients are not usually systemically unwell, and the eruption progresses over several weeks.

Figure. 7.13 Bullae may occur in patients with psoriasis exposed to excessive sun.

Palmoplantar pustulosis

This disorder and acropustulosis of Hallopeau may be grouped together as localized pustular psoriasis. However, it is a matter of debate whether palmoplantar pustulosis (PPP) is a form of psoriasis or a closely related disorder. It has a different age of onset (mostly over 50 years), a female predominance (equal sex incidence in ordinary psoriasis), different HLA associations, and a strong link (about 90%) with cigarette smoking (compared with about 50% in plaque psoriasis). However, about 20–30% of patients have ordinary plaque psoriasis, far more than expected by chance, suggesting that it is a psoriasis variant rather than a separate disorder.

    Palmoplantar pustulosis usually presents as a localized scaling erythematous plaque on one or both feet, or (less commonly) on the palms of the hands, or at both sites. The heel, instep, and central palm are the most common sites. Characteristically, the pustules are large (usually about 5mm), and several stages of evolution of pustules are present concurrently (Fig.7.14). When there is a solitary plaque on one foot, it is important to exclude fungal infection; in symmetric disease, pompholyx eczema (especially with secondary infection) may resemble pustular psoriasis but usually occurs at a younger age and causes much more prominent symptoms.

Figure. 7.14   Palmoplantar pustulosis. The precise relationship of this disorder to psoriasis remains debatable, but the physical signs are typical. ( a ) The pustules are fairly large (often 5 mm in diameter), on a well-defined erythematous scaly background. ( b ) Closer view of pustules, demonstrating the mixture of different colors (yellow, green, and ‘dried up' brown) that are typically present. ( c ) The palm lesions in this patient are very suggestive of ‘ordinary' psoriasis, despite the more pustular appearance on the heel. (Panel b from Lawrence CM, Cox NH. Physical Signs in Dermatology, 2nd edn. London: Mosby, 2002.)

    Unfortunately, PPP is diffIcult to treat; stopping smoking does not reverse the disorder, and it usually runs a chronic course.

Acropustulosis of Hallopeau

This disorder resembles PPP in morphology of pustules but affects one or more (usually several) distal digits. The subungual pustules that occur may result in transient or permanent loss of nails (Fig.7.15). It is most troublesome on the fingers due to the potential for irreversible damage to the nails, and due to the functional deficit that it causes.

Figure. 7.15 Acropustulosis of Hallopeau, showing the nail destruction that may occur. The pulps of the digits may also be affected.

Unstable psoriasis

This is not usually viewed as a specific variant of psoriasis, but it has important implications. The term is used to describe psoriasis that is rapidly increasing in extent; is often very red, inflamed, and pruritic; and often exhibits areas of Koebner reaction (Fig.7.16). Sometimes, there is a trigger such as a recent infection, or Koebnering of psoriasis into a drug rash. The important issue is to avoid potentially irritant treatments such as anthralin or vitamin D analogs, as these may further inflame the lesions. Generally, use of a potent topical corticosteroid with gradual decrease in potency, while concurrently introducing more specific agents, will allow the psoriasis to settle to a less inflamed state, but careful supervision is required, as some patients will progress to subtotal or total erythroderma.

Figure. 7.16 psoriasis on the thigh: widespread, bright-red, inflammatory psoriasis with silvery but rather loose scaling and some fissuring.

 

Figure. 7.17   Erythrodermic psoriasis. ( a ) Erosive areas on the trunk in a patient with a long history of psoriasis, which had rapidly become ‘unstable' and inflamed. ( b ) Erythrodermic psoriasis. The differential diagnosis includes drug eruptions, dermatitis, and cutaneous lymphoma. A previous history of psoriasis or the presence of typical nail changes may be useful, but biopsy may be required.

Erythrodermic and exfoliative psoriasis

Erythrodermic psoriasis (Fig. 7.17) is an uncommon form of psoriasis that usually occurs in patients with known and deteriorating psoriasis, but it can occasionally present de novo. In the latter case, it is important to differentiate other causes of erythroderma (Table 7.4) because, although some of the treatments for erythroderma are common to all causes, certain treatments are more psoriasis-specific. In particular, in erythrodermic psoriasis, systemic corticosteroids are best avoided (due to the risk of pustular psoriasis on withdrawal), but agents such as systemic retinoids or methotrexate, which would not usually be used for most other causes of erythroderma, may be appropriate. It is also important to consider factors that may have provoked erythroderma, such as withdrawal of corticosteroids or other systemic therapies, UV burns, drug reactions, and so on.

    The systemic consequences of erythroderma are listed in Table 7.5. Thermoregulation is impaired due to central effects of interleukins and other cytokines, the huge cutaneous blood flow causing heat loss, and impaired sweating. Shedding of scale, and the malabsorption that occurs in erythrodermic states, leads to loss of protein and of folate, of which body stores are low; ankle edema may occur due to hypoproteinemia or high-output cardiac failure. The high epidermal turnover causes hyperuricemia.

    Exfoliative psoriasis (Fig.7.18; see also Fig.18.52) is considered with erythroderma, as it usually arises in similar situations. However, some patients have profound shedding of scale without a prominent erythrodermic component; they are less unwell systemically but may still have the metabolic consequences of erythroderma (see Table 7.5), especially hypoproteinemia and folate deficiency.

Figure. 7.18 Exfoliative psoriasis in a patient taking lithium, a known trigger for psoriasis.

Specific sites of psoriasis

The main sites affected by psoriasis, with their features and differential diagnosis, are summarized in Tables 7.1 and 7.3. A few site-specific differential diagnostic and therapeutic issues are discussed here; more general aspects of both issues are discussed and summarized later.

Hands and feet

The hands and feet can be affected by ordinary plaque psoriasis or by PPP (discussed earlier). Plaque psoriasis of the palms or soles (Fig.7.19) can occur without psoriasis at other sites, in which case it can be extremely difficult to distinguish from a chronic dermatitis in some individuals. A frictional dermatitis known as hyperkeratotic dermatitis of the palms is particularly difficult to differentiate with confidence, and even histologic examination of a biopsy specimen may show mixed features (see Fig.6.34).

Figure. 7.19 Psoriasis of the hands. ( a ) This patient had nail changes, which may be helpful in the differential diagnosis from dermatitis. ( b ) Discrete, well-demarcated silvery plaques on the hands, a pattern typical of psoriasis. This type of palmar involvement may be very limiting, despite the small proportion of body area involved. ( c ) The Caro–Senear ridge, a pattern of psoriasis that is useful to distinguish this from dermatitis. The lesions are usually on the sides of the fingers or border of the hand, and have central umbilication. ( d ) Psoriasis of hands with a predominantly acral distribution, marked nail dystrophy, and terminal interphalangeal joint arthritis.

Features that may help to establish a diagnosis include the following.

    Where the diagnosis of psoriasis can be made with confidence, it allows a large number of topical and systemic treatment options that would not typically be used for dermatitis (e.g. strong tars, anthralin, vitamin D analogs, systemic retinoids, and methotrexate). Palm and sole psoriasis tends to be more keratotic than typical psoriatic plaques, and a keratolytic agent is an important component of the therapeutic regimen. It is also disproportionately disabling for the surface area involved, and may be a reason to use systemic therapies. Finally, a clear distinction from dermatitis may be important for the patient's occupation.

    Palmoplantar pustulosis can likewise be difficult to distinguish from fungal infection, especially if only one palm or sole is affected.

Scalp

The scalp is commonly affected by psoriasis (Fig.7.20). The typical pattern is one of well-demarcated erythematous plaques with thick silvery scaling, affecting the area above the ears or the occipital region. Involvement of the frontal scalp margin is also common but usually less scaly. The differential diagnosis is predominantly from seborrheic dermatitis (especially frontal scalp margin and around the ears, see Ch.6) and from lichen simplex (occipital).

Figure. 7.20   Scalp psoriasis. ( a ) Demonstrating the typical silvery scaling and the commonly associated involvement of the ears. Scalp involvement in isolation causes diagnostic confusion. ( b ) Involvement of scalp psoriasis: involvement of the lateral scalp margin, extending on to the nape, is a common pattern. ( c ) Alopecia may occur due to severe inflammation in scalp psoriasis. If treated promptly with keratolytics and antiinflammatory measures such as corticosteroids, there is usually full regrowth. In this example, intact follicles capable of regrowth are clearly visible.

    Treatment is affected by the presence of hair, which limits use of ointments and may be stained by some treatments.

Flexures and genital psoriasis

Flexural psoriasis (Fig.7.21) may occur in isolation (sometimes termed inverse psoriasis), especially in children. Umbilical involvement is common, usually in those with psoriasis at other sites but occasionally as the presenting feature.

Figure. 7.21   ( a ) Flexural psoriasis affecting the umbilicus is common. It has a more moist erythematous appearance, with less scaling, compared with plaques at non-flexural areas. ( b ) Flexural psoriasis affecting the groin of an adult man. Treatment options may be limited by irritancy. ( c ) Flexural psoriasis of the sacrum and perianal skin. The sacrum in particular is a common area for psoriasis, but symptomatic lesions and diagnostic problems are more likely with the perianal lesion. ( d ) Flexural psoriasis, in this case in the groin area, is a common variant of psoriasis in children. ( e ) Flexural psoriasis of the inframammary area is common, and may require treatments with an antiseptic or anticandidal component to deal with secondary infection. ( f ) Flexural psoriasis in the axilla of an adult may be difficult to distinguish from seborrheic dermatitis.

    Flexural psoriasis differs from psoriasis found at more typical sites, as the moist, warm, thin skin of the flexures tends to be erythematous but not particularly scaly. It is frequently misdiagnosed as candidiasis or erythrasma. Napkin psoriasis in infants resembles seborrheic dermatitis. Treatments for the flexures should be relatively mild compared with those for other body sites.

    Genital psoriasis also causes diagnostic and therapeutic difficulty (Fig.7.22). In women, vulval skin may require short-term potent steroid applications to achieve symptomatic control of psoriasis; distinction from candidiasis is important. In men, genital involvement not infrequently occurs in isolation and causes diagnostic difficulty, but rarely causes significant symptoms other than anxiety about the diagnosis.

Figure 7.22   Genital psoriasis. ( a ) Vulval psoriasis in an adult, which had been treated unsuccessfully as candidiasis. The confluent erythema might occur in either disorder, but the degree of inflammation, lack of satellite lesions, and lack of therapeutic response make candidal infection unlikely. ( b ) Penile psoriasis, with typically psoriatic morphology of discrete plaques. Genital lesions of psoriasis are often less distinct than in this case. ( c ) Penile psoriasis with an annular morphology, known as a circinate balanitis. This patient also had HLA-B27-positive ankylosing spondylitis. A similar morphology occurs in Reiter syndrome (Fig. 7.30). ( d ) Scrotal lesions of psoriasis. The glans penis is also affected.

Nails

The nails are frequently involved in patients with psoriasis, and can provide useful support for an uncertain diagnosis (Fig.7.23). In patients with psoriatic arthropathy, nail involvement occurs in about 80%, and is almost inevitable in distal interphalangeal joint arthropathy. However, psoriatic nail dystrophy can also occur in isolation, in which case it is typically confused with fungal infection; note also that fungal infection may occur in the already damaged psoriatic nail, especially thickened toenails.

    The features of nail psoriasis are often striking and specific, allowing a confident diagnosis even in the absence of psoriasis at other sites. They include well-defined pits of the nail plate, onycholysis, subungual hyperkeratosis, oil drop appearance, and salmon-pink subungual patches.

Other sites and variants

The face is said to be uncommonly affected by psoriasis, although this is not true of the patients with psoriasis of severity requiring hospital referral (Fig.7.24). Linear variants of psoriasis may occur in isolation or with psoriasis at other sites, and are relatively resistant to treatment (Fig.7.25).

Figure. 7.24 Linear psoriasis, associated with ordinary chronic plaque psoriasis at other sites. There remains debate about whether some such cases represent a Koebner reaction into an inflammatory linear verrucous nevus. An identical distribution pattern may occur in lichen planus (Fig. 8.16).

Figure. 7.25 Linear psoriasis, associated with ordinary chronic plaque psoriasis at other sites. There remains debate about whether some such cases represent a Koebner reaction into an inflammatory linear verrucous nevus. An identical distribution pattern may occur in lichen planus (Fig. 8.16).

Differential diagnosis of psoriasiform disorders

Ordinary plaque psoriasis is usually clinically characteristic, but atypical lesions, less common patterns, or psoriasis at less common sites (especially if in isolation) may cause diagnostic difficulty. The differential diagnosis depends on the pattern and site; the main considerations are listed in Tables 7.2 and 7.3. Some specific issues that cause problems have been discussed in the earlier sections.

In general, the disorders most likely to be confused with psoriasis, and their chapter cross-references, are as follows.

PRACTICE POINTS   

Psoriatic arthropathy

On average, psoriatic arthropathy occurs in about 5% of patients with psoriasis (Fig.7.27), but it is more common (about 35%) in those with more severe psoriasis. It has been divided into five subtypes (Moll and Wright classification), but other forms can probably be added (Table 7.6). However, patterns may co-exist, evolve to include others, and vary in different countries, so a simpler three-pattern classification has recently been proposed, consisting of asymmetric oligoarthritis, symmetric polyarthritis, and spondyloarthropathy, and a even a two-pattern classification of peripheral arthritis and axial (with or without peripheral) arthritis. The axial patterns are more common in men, and the acral type in women. Skin involvement precedes arthropathy in two-thirds of patients; the remainder are equally divided into those with concurrent onset and those in whom the joint disease occurs first. The most common pattern is the oligoarticular asymmetric pattern, in which the digits are often affected by a symmetric polyarthropathy of medium-sized joints.

Figure. 7.27 Psoriatic arthropathy. ( a ) Affecting the terminal interphalangeal joint of the thumb and the proximal interphalangeal joint of the ring finger, in this case obviously associated with extensive psoriasis of the skin also. ( b ) Psoriatic arthropathy of the terminal interphalangeal joint. Adjacent psoriasis is not seen, but the nail changes are suggestive of psoriasis. ( c ) Arthritis mutilans, a typically psoriatic pattern of arthritis, which is associated with a characteristic ‘pencil in cup' radiographic appearance of the digits.

    Nail psoriasis is common in patients with psoriatic arthropathy, especially of acral pattern (associated with HLA-B38), and ocular symptoms such as uveitis are associated with the axial pattern (often in patients who are HLA-B27-positive). The most difficult differential diagnosis, especially in the absence of skin lesions, is rheumatoid arthritis (Table 7.7).

White/Cox: Diseases of the Skin, 2ed.(c) 2006, Elsevier Inc. All rights reserved.