Erdheim–Chester disease (ECD)  is an extremely rare disease characterized by the abnormal multiplication of a specific type of white blood cells called histiocytes, or tissue macrophages (technically, this disease is termed a non-Langerhans-cell histiocytosis). It was declared a histiocytic neoplasm by the World Health Organization in 2016. Onset typically is in middle age. The disease involves an infiltration of lipid-laden macrophages, multinucleated giant cells, an inflammatory infiltrate of lymphocytes and histiocytes in the bone marrow, and a generalized sclerosis of the long bones. Signs and symptoms
Long bone involvement is almost universal in ECD patients and is bilateral and symmetrical in nature. More than 50% of cases have some sort of extraskeletal involvement. This can include kidney, skin, brain and lung involvement, and less frequently retroorbital tissue, pituitary gland and heart involvement is observed.Bone pain is the most frequent of all symptoms associated with ECD and mainly affects the lower limbs, knees and ankles. The pain is often described as mild but permanent, and juxtaarticular in nature. Exophthalmos occurs in some patients and is usually bilateral, symmetric and painless, and in most cases it occurs several years before the final diagnosis. Recurrent pericardial effusion can be a manifestation, as can morphological changes in adrenal size and infiltration.A review of 59 case studies by Veyssier-Belot et al. in 1996 reported the following symptoms in order of frequency of occurrence:
Bone pain
Retroperitoneal fibrosis
Diabetes insipidus
Exophthalmos
Xanthomas
Neurological signs (including ataxia)
Dyspnea caused by interlobular septal and pleural thickening. Kidney failure
Hypopituitarism
Liver failure

Diagnosis
Radiologic osteosclerosis and histology are the main diagnostic features.
Rothmund–Thomson syndrome (RTS) is a rare autosomal recessive skin condition. There have been several reported cases associated with osteosarcoma. A hereditary basis, mutations in the DNA helicase RECQL4 gene, causing problems during initiation of DNA replication has been implicated in the syndrome. Signs and symptoms
Sun-sensitive rash with prominent poikiloderma and telangiectasias
Juvenile cataracts
Saddle nose
Congenital bone defects, including short stature and radial ray anomalies such as absent thumbs
Hair growth problems (absent eyelashes, eyebrows and/or hair)
Hypogonadism has not been well documented
Hypodontia
Calcium problems (not documented in journals)
Ear problems (not documented in journals but identified by patients in support groups)
Produces osteosarcomaThe skin is normal at birth. Between 3 and 6 months of age, the affected carrier develops poikiloderma on the cheeks. This characteristic "rash" that all RTS carriers have can develop on the arms, legs and buttocks. "Poikiloderma consists of areas of increased and decreased pigmentation, prominent blood vessels, and thinning of the skin." Accelerated aging
In humans, individuals with RTS, and carrying the RECQL4 germline mutation, can have several clinical features of accelerated aging. These features include atrophic skin and pigment changes, alopecia, osteopenia, cataracts and an increased incidence of cancer. Also in mice, RECQL4 mutants show features of accelerated aging. Causes
RTS is caused by a mutation of the RECQL4 gene, located at chromosome 8q24.3. The disorder is inherited in an autosomal recessive manner.