The terms ‘orphan diseases’ and ‘rare diseases’ are not synonymous.

Orphan diseases

Orphan diseases are those which are not widely researched, where specific treatment is not available, and which may only be of limited interest to scientists and doctors. Consequently, patients feel abandoned and ‘orphaned’ in the world of healthcare. Orphan diseases may be either common or rare.

The more common orphan diseases are exemplified by the so-called neglected infectious diseases, which are endemic to areas ravaged by poverty in Africa, Asia and the Americas. These disorders affect 1 billion people worldwide and can cause disfigurement, lifelong disabilities and morbidity, and eventually lead to the death of 1 million people annually. The neglected tropical infectious diseases comprise lymphatic filariasis, African trypanosomiasis, schistosomiasis, trachoma, onchocerciasis, leishmaniasis, Chagas disease, etc . Coexistence with AIDS or malaria is common. Access to drugs is limited by financial cost. However, in recent years, several pharmaceutical companies have donated drugs to treat neglected tropical disorders (e.g . albendazole for lymphatic filariasis).

The orphan lung diseases comprise many disorders and are described in more detail in the recent Orphan Lung Diseases issue of the European Respiratory Monograph .

Rare diseases

Rare diseases are defined numerically – they are diseases that affect fewer than one person in 2000 in Europe. There are about 6000 such disorders, including well-characterised diseases as well as syndromes and anomalies (table 1). Most (about 80%) are of genetic origin. Many rare diseases are also orphan diseases; however, some rare diseases have received significant attention, leading to comprehensive research and ensuing treatments such that they may no longer be considered orphan (a good example is idiopathic pulmonary arterial hypertension: see chapter 21).

Granulomatosis with polyangiitis (Wegener’s)
Microscopic polyangiitis
Eosinophilic granulomatosis with polyangiitis (Churg–Strauss)
Behçet’s disease
Takayasu’s arteritis
Autoimmune diseases  
Anti-basement membrane syndrome
Pulmonary alveolar proteinosis
Disorders of genetic origin  
Lymphangioleiomyomatosis associated with tuberous sclerosis
Multiple cystic lung disease in Birt–Hogg–Dubé syndrome
Primary ciliary dyskinesia
Other idiopathic disorders (lung limited)  
Idiopathic eosinophilic pneumonias
Tracheobronchopathia osteochondroplastica
Tracheobronchomegaly (Mounier–Kuhn syndrome)
Idiopathic bronchiolitis
Other rare diseases  
Thoracic endometriosis
Langerhans’ cell histiocytosis
Table 1 – The main rare lung diseases.

This chapter does not include neoplastic disorders (those causing benign or malignant tumours). However, rare chest tumours or unusual manifestations of malignancies may need to be considered in differential diagnosis (e.g. pulmonary artery sarcoma, metastatic cavitary nodules).

See the entire Rare and orphan lung diseases Chapter