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What is Pityriasis Lichenoides?
Pityriasis lichenoides is a rare inflammatory skin disease characterized by the development of papules, i.e. small, raised, reddened (erythematous) skin lesions less than 1 cm in diameter. While relatively uncommon, it affects patients of all ages and can significantly impact quality of life due to its recurrent and often unpredictable course.
The disease presents in two main clinical forms depending on the speed of onset and the nature of the lesions:
- Pityriasis Lichenoides et Varioliformis Acuta (PLEVA): the acute form, characterized by a rapid onset with more inflammatory lesions.
- Pityriasis Lichenoides Chronica (PLC): the chronic form, with a slower, more indolent course.
An additional, rare, and more severe variant exists known as ulceronecrotic PLEVA (also called febrile ulceronecrotic Mucha-Habermann disease), which is distinguished by the presence of ulcerated and necrotic lesions accompanied by systemic symptoms. Intermediate forms between PLEVA and PLC have also been documented, where a single patient may simultaneously exhibit lesions characteristic of both the acute and chronic phases.
The disease predominantly affects individuals in the first two decades of life and shows a predilection for males. The prognosis is generally favorable: both PLEVA and PLC tend to resolve spontaneously over the course of months to years, and serious complications are rare.
Currently, no preventive measures exist to reduce the risk of developing pityriasis lichenoides.
Causes
The exact cause of pityriasis lichenoides remains unknown. However, researchers have proposed several hypotheses, most of which point toward an immune-mediated mechanism.
Hypersensitivity Reaction Theory
The most widely accepted hypothesis is that pityriasis lichenoides represents a hypersensitivity reaction to an infectious agent or, less commonly, to a drug. This theory is supported by clinical evidence linking the onset of the disease (particularly its acute form) to recent infections.
In some acute and familial cases, the following pathogens have been identified:
- Staphylococcal bacteria, detected in throat swabs
- IgM antibodies against Mycoplasma pneumoniae, indicating a recent infection with this atypical bacterium
- Antibodies against:
- Epstein-Barr Virus (EBV), the causative agent of infectious mononucleosis
- Human Immunodeficiency Virus (HIV)
- Toxoplasma gondii, a protozoan responsible for toxoplasmosis
Drug-Induced Cases
Certain medications have also been implicated as potential triggers of PLEVA. Documented cases have been associated with drugs such as estrogen-progestin combinations and astemizole (an antihistamine no longer widely used).
Contagiousness
It is important to note that during the acute infectious phase, a patient with PLEVA may be contagious. The mode of transmission depends on the specific infectious agent involved. Patients and caregivers should be informed of this possibility, particularly when a bacterial or viral cause has been identified.
Lymphocytic Dyscrasia Hypothesis
Some researchers have proposed that pityriasis lichenoides may represent a benign clonal T-cell proliferative disorder, sitting on a spectrum between an inflammatory reaction and true cutaneous lymphoma. This remains a subject of ongoing scientific debate, and regular follow-up is recommended to monitor for any atypical evolution.
Symptoms
The clinical presentation of pityriasis lichenoides varies significantly depending on the subtype.
PLEVA (Acute Form)
In PLEVA, lesions appear predominantly on the trunk and proximal extremities, although they can spread to cover the entire body surface. Key features include:
- Erythematous papules (raised red bumps) and, less commonly, vesicles (fluid-filled blisters) that rupture easily
- Formation of hemorrhagic crusts after the vesicles break open, which is the origin of the term varioliformis, given its resemblance to smallpox scars
- Lesions are typically asymptomatic, though some patients experience mild itching (pruritus)
- New lesions appear in repeated crops over several weeks
- Resolution may leave behind depressed or raised scars
PLC (Chronic Form)
PLC is characterized by a milder but prolonged clinical course:
- Appearance of red-brown papules that gradually flatten and become covered by a central silvery scale that detaches easily
- Multiple lesions in different stages of evolution are present simultaneously
- Lesions are asymptomatic or mildly pruritic
- The condition may persist for weeks, months, or even 1–2 years, with repeated episodes
- Resolution of individual lesions occurs over 3–4 weeks and often leaves hypopigmented macules (lighter patches compared to surrounding skin), which may persist for months
Ulceronecrotic PLEVA (Rare, Severe Variant)
This is the most severe form of pityriasis lichenoides and primarily affects children and adolescents. It is characterized by:
- Sudden onset of papular lesions that rapidly ulcerate and coalesce, forming large, painful ulcers with raised erythematous borders
- Systemic symptoms are always present, including:
- Fever
- Abdominal pain
- Muscle pain (myalgia)
- Enlarged lymph nodes (diffuse lymphadenopathy)
After several hyperacute episodes, this form either evolves into classic PLEVA or undergoes complete remission. Due to the risk of systemic involvement and potential complications, this variant requires prompt medical evaluation and management.
Diagnosis
The diagnosis of pityriasis lichenoides is primarily clinical, based on careful observation of the skin lesions, their distribution, and their evolution over time. However, confirmation through a skin biopsy (histological examination) is generally required, as the clinical picture can overlap with several other dermatological conditions.
Histological Findings
Under the microscope, PLEVA typically shows:
- A lymphocytic infiltrate (predominately CD8+ T-cells) surrounding blood vessels
- Epidermal necrosis and spongiosis
- Extravasation of red blood cells into the skin
PLC shows similar but less pronounced changes, with a predominantly CD4+ T-cell infiltrate.
Differential Diagnosis
Pityriasis lichenoides must be distinguished from the following conditions:
-
Varicella (Chickenpox): Caused by the varicella-zoster virus (VZV). Lesions also appear in different stages simultaneously and progress to crusts, but typically begin on the face and scalp before spreading downward: a pattern not typical of PLEVA. Varicella is also associated with fever and resolves within 1–3 weeks.
-
Guttate Psoriasis: Presents with small, erythematous, scaly, and pruritic macules, often triggered by an acute infection (especially streptococcal pharyngitis). While it can look similar to PLC, the scales in psoriasis are thicker and more silvery, and lesions do not typically develop hemorrhagic crusts.
-
Lymphomatoid Papulosis (LyP): A rare, recurrent, self-healing cutaneous lymphoproliferative disorder. The papules in LyP tend to be larger than those in pityriasis lichenoides and are histologically characterized by atypical lymphocyte infiltrates (large CD30+ cells). Distinguishing LyP from pityriasis lichenoides can be particularly challenging and may require repeated biopsies.
Treatment
Before initiating any treatment, it is essential to identify and address potential triggering factors, such as underlying infections or causative medications.
The overall management of pityriasis lichenoides is symptomatic, as there is no definitive cure. The choice of treatment depends on the clinical subtype, severity, and the patient’s age and overall health.
First-Line Treatment: Phototherapy
Phototherapy is the mainstay of treatment for both PLEVA and PLC. It involves controlled exposure of the skin to ultraviolet (UV) radiation at specific wavelengths, administered in a clinical or specialized outpatient setting.
- Narrowband UVB (NB-UVB) phototherapy is most commonly used and has demonstrated good efficacy and an acceptable safety profile.
- Sessions are typically scheduled three times per week, with gradual increases in UV dose based on patient tolerance.
- Phototherapy works by modulating the immune response in the skin, reducing inflammation and suppressing the abnormal lymphocytic activity underlying the disease.
Methotrexate
Methotrexate is an antimetabolite drug that acts by:
- Reducing excessive proliferation of skin cells (epidermopoiesis)
- Exerting immunosuppressive effects on T-lymphocytes
It is particularly useful in more severe or refractory cases and in ulceronecrotic PLEVA. It is administered at low doses, typically once a week, and requires regular blood monitoring for potential side effects (e.g., liver toxicity, bone marrow suppression).
Other Therapeutic Options
The following treatments have been described in the literature with variable success rates:
- Topical corticosteroids (creams, lotions, ointments): useful for symptom relief, particularly to reduce inflammation and itching in localized lesions
- Natural sun exposure: moderate sun exposure may have a beneficial effect similar to phototherapy, though it is less controlled and not a substitute for clinical treatment
- Oral antibiotics (e.g., erythromycin, tetracycline): particularly relevant when an infectious trigger is identified. They may also exert anti-inflammatory properties independent of their antimicrobial effects. Tetracyclines are generally avoided in children under 8 years of age.
Monitoring and Follow-Up
Given the uncertain link between pityriasis lichenoides and lymphoproliferative disorders, regular dermatological follow-up is recommended. Patients should be monitored for:
- Disease progression or transformation
- Development of systemic symptoms
- Treatment response and tolerability
Sources and Bibliography
- Wolff K., Johnson R., Saavedra A. Fitzpatrick’s Color Atlas and Synopsis of Clinical Dermatology. Italian edition based on the seventh English-language edition, edited by Mauro Alaibac. Piccin, 2015.
- Saurat J., Grosshans E., Laugier P., Lachapelle J. Dermatology and Sexually Transmitted Diseases. Italian edition edited by Girolomoni G. and Giannetti A. Third edition, 2006. Masson.
- Bowers S., Warshaw EM. Pityriasis lichenoides and its subtypes. Journal of the American Academy of Dermatology. 2006;55(4):557-572.
- Ersoy-Evans S. Commentary: Pityriasis lichenoides in childhood. Journal of the American Academy of Dermatology. 2007;56(2):205-210.
- Khachemoune A., Blyumin ML. Pityriasis lichenoides: pathophysiology, classification, and treatment. American Journal of Clinical Dermatology. 2007;8(1):29-36.
- National Organization for Rare Disorders (NORD). Pityriasis Lichenoides.
