# Internal Medicine/Papulosquamous Disorders
## Psoriasis

Psoriasis stands out as one of the most prevalent skin conditions globally, affecting up to 2% of the world's population. It is an immune-related disorder that exhibits distinct clinical features, including reddish, well-defined papules and circular plaques covered by a silvery, scaly layer. Psoriasis skin lesions can be itchy to varying degrees. Injured skin areas can also develop psoriasis lesions, known as the Koebner phenomenon. Additionally, external factors such as infections, stress, and certain medications like lithium, beta blockers, and antimalarial drugs can worsen psoriasis. The most common form is plaque-type psoriasis, characterized by stable, slow-growing plaques that remain relatively unchanged for long periods. Typically, these plaques appear on the elbows, knees, buttock creases, and the scalp, and they tend to appear symmetrically. Plaque psoriasis usually develops gradually and follows a mild course, seldom remitting spontaneously. Inverse psoriasis, on the other hand, affects skinfold regions like the armpits, groin, under the breasts, and navel, and it can also involve the scalp, palms, and soles. These lesions are well-defined plaques but may lack scaling due to their location.
Guttate psoriasis, or eruptive psoriasis, primarily affects children and young adults. It suddenly emerges in individuals without a prior history of psoriasis or in those with chronic plaque psoriasis. Patients with this type exhibit numerous small, reddish, scaly papules, often following upper respiratory tract infections caused by beta-hemolytic streptococci. The differential diagnosis should consider conditions like pityriasis rosea and secondary syphilis.
In pustular psoriasis, patients can have the disease localized to the palms and soles or a more generalized form. In both cases, the skin appears red, with pustules and varying degrees of scaling. When it's localized to the palms and soles, it can be easily confused with dyshidrotic eczema. In its generalized form, patients experience episodes characterized by fever, the eruption of sterile pustules, and intense redness. These episodes may even lead to erythroderma. Fever and pustular outbreaks tend to recur. Factors like local irritants, pregnancy, medications, infections, and discontinuation of systemic glucocorticoids can trigger this form of psoriasis. In nonpregnant patients, oral retinoids are the preferred treatment.
Fingernail involvement, which can manifest as small pits, separation of the nail from the nail bed, thickening, or excessive nail tissue underneath, can provide a clue to the diagnosis of psoriasis, particularly when the clinical presentation is atypical.
According to the National Psoriasis Foundation, approximately 30% of psoriasis patients develop psoriatic arthritis (PsA), which is most common between ages 30 and 50. PsA presents in various subtypes, including symmetric PsA, asymmetric PsA, distal PsA, spondylitis, and arthritis mutilans. Symmetric PsA often resembles rheumatoid arthritis and accounts for roughly 50% of PsA cases. Asymmetric arthritis, comprising about 35% of cases, can affect any joint and may cause "sausage fingers." Distal PsA, occurring in about 5% of PsA patients, involves fingers and toes and is often accompanied by nail issues like pitting. Spondylitis is observed in approximately 5% of PsA cases, while arthritis mutilans, the most severe form, primarily affects the small joints of the hands and feet and accounts for fewer than 5% of cases.
Psoriasis patients face an increased risk of metabolic syndrome, which includes a higher likelihood of cardiovascular events with increased morbidity and mortality. Therefore, appropriate screening tests are recommended. While the exact cause of psoriasis remains unclear, there is a definite genetic component, with 30–50% of patients reporting a family history of the condition. Psoriatic lesions contain infiltrates of activated T cells that produce cytokines responsible for the excessive growth of skin cells, leading to the characteristic clinical features. Treatments for severe psoriasis often involve agents that inhibit T-cell activation, clonal expansion, or proinflammatory cytokine release.
The treatment approach for psoriasis depends on the type, location, and extent of the disease. All patients should be advised to avoid excessive skin dryness or irritation and maintain adequate skin hydration. In most cases of localized plaque-type psoriasis, mid-potency topical glucocorticoids are effective, though their long-term use may lead to reduced effectiveness and skin thinning. Topical vitamin D analogs like calcipotriene and retinoids such as tazarotene are also suitable treatments for limited psoriasis and have largely replaced older topical agents like coal tar, salicylic acid, and anthralin.
Ultraviolet (UV) light, whether from natural or artificial sources, proves effective for many patients with widespread psoriasis. UVB, narrowband UVB, and UVA light with oral or topical psoralens (PUVA) are used therapeutically. UV light's immune-suppressing properties contribute to its therapeutic effect, but it is also mutagenic, potentially increasing the risk of nonmelanoma and melanoma skin cancers. Thus, UV-light therapy is contraindicated for patients taking cyclosporine and should be used cautiously in immunocompromised patients due to the elevated skin cancer risk.
For severe and extensive psoriasis, various systemic agents are available. Oral glucocorticoids are discouraged due to the risk of life-threatening pustular psoriasis upon discontinuation of treatment. Methotrexate is an effective option, especially for patients with PsA. The synthetic retinoid acitretin is useful when immunosuppression must be avoided but is limited by its teratogenic effects. Apremilast, a phosphodiesterase type 4 inhibitor, is approved for both psoriasis and PsA but should be used cautiously in the presence of renal failure or depression.
The understanding of psoriasis as a T-cell–mediated disorder has driven therapeutic research toward immunoregulation. Immunosuppressive agents like cyclosporine have shown efficacy in psoriasis treatment, and biologic agents with more selective immunosuppressive properties and improved safety profiles are being developed (see Table 57-4). These biologics have demonstrated effectiveness and good tolerability in treating psoriasis, though caution is necessary for certain patient comorbidities. Tumor necrosis factor-α (TNF-α) inhibitors, for example, may exacerbate congestive heart failure (CHF) and should be used carefully in patients with CHF or at risk for it. Moreover, none of the immunosuppressive agents used for psoriasis treatment should be initiated if the patient has a severe infection (including tuberculosis, HIV, hepatitis B or C), and patients on such therapy should be regularly screened for tuberculosis. There have been reports of progressive multifocal leukoencephalopathy and lupus erythematosus linked to TNF-α inhibitors. These systemic agents also carry an increased risk of skin cancer, necessitating close monitoring for its development.

## Lichen Planus

Lichen planus (LP) is a papulosquamous disorder that can affect the skin, scalp, nails, and mucous membranes. The primary cutaneous lesions are itchy, polygonal, flat-topped, violet-colored papules. Examination of these papules often reveals a network of gray lines called Wickham's striae. LP can occur anywhere on the skin but has a preference for areas like the wrists, shins, lower back, and genitalia. It can also affect the scalp, leading to scarring hair loss, and the nails, potentially causing permanent deformities or nail loss. LP is often associated with mucous membrane involvement, particularly in the buccal mucosa, where it can range from a mild, white, reticulated rash to severe, erosive stomatitis. Erosive stomatitis may persist for extended periods and may elevate the risk of oral squamous cell carcinoma. Drug-induced LP-like eruptions have been observed with medications such as thiazide diuretics, gold, antimalarials, penicillamine, phenothiazines, and in patients with chronic graft-versus-host disease. LP may also be linked to hepatitis C infection. The course of LP varies, but most patients experience spontaneous remission within 6 months to 2 years after the disease onset. Topical glucocorticoids are the primary treatment.

## Pityriasis Rosea

Pityriasis rosea (PR) is a papulosquamous rash of unknown origin that often occurs in the spring and fall. It typically starts with the development of a 2- to 6-cm circular lesion called the "herald patch." Over the following days to weeks, numerous smaller circular or papular lesions emerge, mostly on the trunk. These lesions are usually oval, with their long axis aligned with the lines of skin folds. Individual lesions can range in color from red to brown and often have a trailing scale. PR shares several clinical features with secondary syphilis, but palm and sole lesions are extremely rare in PR and common in secondary syphilis. The rash in PR is usually moderately itchy and persists for 3–8 weeks. Treatment aims at relieving itching and typically involves oral antihistamines, mid-potency topical glucocorticoids, and, in some cases, UVB phototherapy.