Langerhans Cell Histiocytosis (Eosinophilic Granuloma)

Demographics

Pathogenesis

Modern Classification (Histiocyte Society):

1. Single-System LCH:

   • Single-site (solitary eosinophilic granuloma)
   • Multiple-site (polyostotic, pulmonary)

2. Multisystem LCH:

   • Without risk organ involvement
   • With risk organ involvement (liver, spleen, bone marrow, lung)

Skeletal Manifestations

Systemic Manifestations (Multisystem Disease)

Risk Stratification

Treatment by Disease Extent

Surgical Considerations

Complications

Prognosis

• What it is. Neurodegenerative LCH (LCH-ND) is a progressive, non-mass CNS syndrome that can appear months to many years after the systemic disease has apparently resolved, and is a leading cause of long-term morbidity in survivors. It is distinct from a space-occupying CNS LCH lesion - there is no tumour to resect.
• Who is at risk. The strongest associations are craniofacial/"CNS-risk" lesions (orbit, mastoid, temporal, sphenoid, maxilla/zygoma), diabetes insipidus, multisystem disease, and BRAF V600E positivity - the same lesions that predict diabetes insipidus predict LCH-ND, and the two often coexist as part of the pituitary-hypothalamic/CNS-risk picture.
• How it presents and looks. Insidious cerebellar signs (ataxia, dysarthria, tremor, dysmetria), pyramidal signs, behavioural/learning decline and eventually cognitive impairment. MRI shows symmetrical T2/FLAIR high signal in the dentate nuclei/cerebellar white matter and basal ganglia, sometimes with cerebellar atrophy - changes that can precede symptoms, so at-risk children are surveilled with MRI.
• Why it matters / management. It is thought to be driven by MAPK-activated myeloid cells seeding the CNS (BRAF V600E has been found in the marrow/blood of affected patients), i.e. a neuro-inflammatory/degenerative process rather than direct tumour. Treatment is difficult and not standardised - options include chemotherapy (cytarabine), and increasingly MAPK-pathway (BRAF/MEK) inhibitors, with the hope (unproven) that early targeted therapy may prevent or halt progression. Established neurodegeneration is often irreversible, which is why recognition and surveillance matter.

• A different disease in adults. Isolated pulmonary LCH (PLCH) is largely a disease of young-to-middle-aged adult smokers (typically 20-40 years, roughly equal sex incidence). Unlike multisystem childhood LCH it is often a reactive, polyclonal response to cigarette smoke in many cases (though clonal BRAF V600E-positive forms also occur), which is why it behaves differently.
• Presentation. Cough and dyspnoea, or an incidental finding; a classic acute presentation is a spontaneous pneumothorax (from cyst rupture), which can be recurrent. Constitutional symptoms and extrapulmonary disease are usually absent in the isolated pulmonary form.
• Imaging. HRCT is characteristic: combined nodules and thin-walled cysts with an UPPER- and MID-zone predominance and sparing of the costophrenic angles/lung bases - the reverse of the basal predominance of many other cystic/fibrotic lung diseases. The nodules cavitate into cysts over time.
• Management - stop smoking first. Smoking cessation is first-line and can stabilise or regress disease; it is the single most important intervention. Systemic therapy (e.g. cladribine, or MAPK-directed therapy in clonal disease) is reserved for progressive disease, and lung transplantation for end-stage fibrosis (with a risk of recurrence, especially if smoking resumes). Recurrent pneumothorax may need pleurodesis.

Global Epidemiology

• Incidence approximately 4-9 per million children per year; rarer in adults (1-2 per million)
• Peak 1-3 years for multisystem disease; 5-10 years for single-system bone disease
• Slight male predominance (around 1.2-2:1); reported across all populations worldwide
• Bone is the most common organ involved overall (around 80% of paediatric cases)
• Pulmonary LCH in adults is strongly smoking-associated and follows a distinct natural history

Side-by-Side Guidance

Registry & Trial Networks

• Outcome data are driven by international cooperative trials (LCH-I, LCH-II, LCH-III, LCH-IV) rather than implant/arthroplasty registries
• The Histiocyte Society coordinates global protocol enrolment; regional groups (e.g. NACHO in North America, national paediatric oncology groups in Europe) feed shared datasets
• BRAF/MAPK genotyping is increasingly captured prospectively to stratify targeted-therapy use

High- vs Limited-Resource Practice

• High-resource: routine BRAF/MAP2K1 genotyping, PET-CT staging, access to BRAF/MEK inhibitors and cladribine/cytarabine salvage, long-term endocrine/neurocognitive surveillance
• Limited-resource: diagnosis may rest on morphology plus CD1a/S100 where Langerin and molecular testing are unavailable; skeletal survey substitutes for PET-CT; vinblastine + prednisone remains the affordable, effective backbone; targeted agents often inaccessible
• Universal principles: confirm tissue diagnosis, stage for risk organs, avoid radiotherapy in children, and screen lifelong for diabetes insipidus

• Reactive vs neoplastic: the discovery of clonal MAPK mutations (BRAF V600E, MAP2K1) reframed LCH as an inflammatory myeloid neoplasm, yet the prominent inflammatory infiltrate keeps the "neoplasia vs immune dysregulation" debate alive.
• Optimal duration of targeted therapy: BRAF/MEK inhibitors produce rapid responses but disease frequently rebounds on cessation, and the ideal duration, role of combination with chemotherapy, and long-term toxicity in children remain undefined.
• Vertebra plana management: the balance between observation, intralesional steroid and bracing is not standardised; most lesions reconstitute, but predicting the minority that deform is imprecise.
• Neurodegenerative LCH: progressive cerebellar/brainstem neurodegeneration can appear years after apparent cure; its pathogenesis, screening interval, and whether early MAPK-directed therapy prevents it are unresolved.
• Role of PET-CT vs skeletal survey: PET-CT is more sensitive for occult and extraskeletal disease, but cost, radiation and access limit universal use, so staging strategy varies by setting.

Langerhans cell histiocytosis (LCH) is a clonal proliferation of CD1a+ and Langerin+ dendritic cells with a spectrum ranging from solitary eosinophilic granuloma to disseminated multisystem disease (Letterer-Siwe). It predominantly affects children aged 5-10 years with male predominance (2:1), with the skull (50%) and femur (20%) being the most common skeletal sites. Characteristic findings include "punched-out" lytic skull lesions, vertebra plana in the spine, and histology showing Langerhans cells with coffee-bean nuclei and Birbeck granules. BRAF V600E mutation is present in 50-60% and is a therapeutic target. Treatment ranges from observation/curettage for unifocal bone disease to chemotherapy for multisystem involvement.