Sarcoidosis is an immune-mediated systemic disorder typified by granuloma formation in involved organs, most commonly of the lung parenchyma and the skin. The inciting immune activating agent remains unknown (hypotheses include autoimmune, infectious, and environmental triggers). There are genetic predispositions related to components of the inflammatory response. Family history of the disease increases the risk. The disease affects all ages and races and ethnicities, with peak incidence at age 30-50 in men and 50-60 in women. It is slightly more common in women. Cutaneous disease is often the first sign of systemic sarcoidosis.

Sarcoidosis commonly presents with abnormalities identified incidentally on chest radiography. Although the disease can affect different organs, systemic symptoms such as fever, night sweats, and weight loss are common. Sarcoidosis can affect the lungs, with symptoms such as chest pain, shortness of breath, cough, and most commonly, pronounced fatigue. It can also affect the peripheral lymph nodes, heart, kidneys, gastrointestinal tract, central nervous system (CNS), liver, spleen, bone, muscle, and endocrine glands. Venous thromboembolism and pulmonary hypertension are potential complications of sarcoidosis. Approximately 90% of patients will have lung involvement. Pulmonary fibrosis and bronchiolectasis result in "honeycombing" of the lung and represent end-stage lung disease due to chronic granulomatous inflammation. Hilar lymphadenopathy is asymptomatic and affects 90% of patients. Approximately 10% of patients have hypercalcemia.

Neurosarcoidosis commonly manifests as cranial-nerve deficits. Any part of the neurologic system can be impacted by sarcoidosis; a vast range of neurologic signs and symptoms may be present. Delays in diagnosis and treatment can lead to permanent disability. The diagnostic workup for neurosarcoidosis should include an MRI of the head, cerebrospinal fluid analysis, and detection of sarcoidosis outside the nervous system.

Approximately 25% of patients will have cutaneous involvement, and some patients may have skin-limited disease. Asymptomatic red-brown dermal papules and/or plaques that favor the face, neck, upper extremities, and upper trunk are the most common specific cutaneous findings. Less common manifestations include sarcoid lesions with epidermal change such as scale (ichthyosiform sarcoid), hypopigmentation, subcutaneous nodules, and ulceration. Sarcoidosis has a predilection for scars and may be seen within tattoos.

Sarcoidosis of the scalp is a rarely reported cutaneous finding. Both nonscarring and scarring alopecia may occur in association. Scarring plaques are usually localized, and nonscarring alopecia may be localized or generalized. Underlying plaques may be red-brown, violaceous, and/or hypopigmented. They may be raised or depressed and atrophic. Scaling can be seen, and scalp involvement can resemble seborrheic dermatitis. An annular morphology has been reported. Most reported cases have been seen in African American women. Scalp sarcoid has been correlated with the presence of systemic involvement. Scalp sarcoid can be recalcitrant to treatment.

The pathogenesis of sarcoidosis is poorly understood. However, it is characterized by noncaseating epithelioid granulomas made up mostly of CD4+ helper T-cells, a predominantly Th1 type immune response, and elevated levels of interferon (IFN)-gamma and interleukin (IL)-2.