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Rare Diseases

What is a rare disease?

The European commission describes a rare disease as a ‘Life-threatening or chronically debilitating disease – mostly inherited...affecting fewer than 5 people in 10,000’.

More than 50% of rare diseases occur in children under the age of 5 years and 80% of these are congenital or hereditary. There are between 6,000 and 8,000 rare diseases that are known about. With advances in genetics research, information about more diseases is emerging all the time.


CARIS was established in 1998 as a register of congenital anomalies in Wales, and has always included some rare diseases of genetic / hereditary origin. These include conditions detected by the Newborn Bloodspot Screening Programme.

In recent years the remit of CARIS has extended to include increased registration of rare diseases  that may fall outside the original scope of CARIS, in that they may be diagnosed some time after a child has achieved their first birthday.

Information is only held by CARIS about a person with a rare disease / congenital anomaly until they have reached their 18th birthday, (or would have reached their 18th birthday if they were not born alive, or died before this). Consideration is being given to extending rare diseases registration to include adults, and we would welcome feedback on this.

UK and international context

Since inception the CARIS team have worked with colleagues across the UK, Europe and also internationally to share information in order to improve knowledge about congenital anomalies, and more recently about rare diseases.

Currently, work is being carried out across Europe pooling scarce resources to work together to improve information about rare diseases; to support research and national disease registries; and to encourage the development of rare disease plans in each country. CARIS plays an active role in this European-wide work.  Rare diseases are also sometimes referred to as “orphan” diseases and a European information portal called Orphanet is available as a reference portal for information on both rare diseases and orphan drugs. There is also a European network of patient organisations active in the field of rare diseases.

Within the UK there is a  UK Strategy for Rare Diseases published in 2013, and each of the four UK countries has developed their own implementation plans for that strategy, supported by a UK Rare Disease Policy Board, Advisory Group and Forum. The aim of the UK strategy is to improve the lives of all those affected by rare diseases. Additionally there has been investment in building genome sequencing capacity which (over time) will improve diagnosis of, and knowledge about, rare diseases. And there is a UK umbrella forum for people and families affected by rare diseases – Rare Disease UK

Within Wales, Welsh Government has developed a Rare Diseases Implementation Plan. The plan estimates that about 150,000 people in Wales are affected by a rare disease, very few of whom are recorded on the CARIS database. About 17,600 individuals with a rare disease are recorded on the CARIS database. The reasons for the difference in numbers are multiple and include:

  • Some cases will not be reported to CARIS, either because they are very mild and do not require intervention, or because clinicians don’t report them.

  • Rare childhood cancers are recorded by the Welsh Cancer Intelligence and Surveillance Unit (WCISU) and not held by CARIS, so not currently included in our numbers. This may change in future.

  • CARIS does not hold information on individuals over the age of 18 at this time. This may change in future.

  • While CARIS was established in 1998 CARIS rare disease register was only established in 2013. A very large number of congenital anomalies that have been recorded on the database since 1998 are also rare diseases, but not all.


Each year we will publish a table updating the information held in CARIS on rare diseases. This is published as an Official Statistics release. Because of the potential of identification of individuals in a small country like Wales, only diseases with a minimum of five occurrences have been published in our table of rare disease prevalence.

Structural anomalies

Please note that many of the structural anomalies on which we report prevalence rates elsewhere (e.g. diaphragmatic hernia or congenital cataract) can also be classified as rare diseases, and are not duplicated on this table but can be found here.

Please also note that simple subtraction from earlier versions of this table will not give the number of children with these conditions born in 2015. Many of these conditions are diagnosed in late infancy and childhood.

These figures are the latest reported to CARIS for children born  and diagnosed within the whole 21 year period (1998-2018).

Because of the large number of conditions, this year the table is being split under the following headings :

  • Genetic

  • Immunodeficiency

  • Neoplasms

  • Chromosomal

  • Syndromes

  • Other

If you are not sure which heading a rare disease comes under, there are clear instructions in the download on how to find a condition on the main report pages.



CARIS:  Rare diseases (1998 to 2022)

CARIS: Rare diseases (1998 to 2021)

CARIS: Rare diseases (1998 to 2020)

Welsh Implementation Plan for Rare Diseases Annual Update 2018 can be found here.


Special thanks are due to:

The Medical Geneticists and the Shire database

Dr Stuart Moat – Newborn Bloodspot Screening

Dr Sian Morgan- Cytogenetics Laboratory

Professor John Gregory and Dr Justin Warner – Paediatric Endocrinologists

Dr Eulee Seow – Paediatric Ophthalmologist