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If a disease affects fewer than 1 in 2 000, the European Union defines it as a rare disease. Globally, about 300 million people live with a rare disease, and in Sweden about 400 000. There are approximately 7 000 different rare diseases. The clinical picture varies from a single symptom to complex patterns with multiple organs affected, often combined with cognitive and motor impairment. At least 72 % of all rare diseases are genetic and 70% have childhood onset. Many patients are undiagnosed and do not receive optimal treatment. Today, only 5% of rare diseases have an approved treatment option. With modern genetic high throughput techniques, many disease-causing mutations are identified, increasing the possibility of personalized treatment and prevention strategies, designed by the individual's genetic conditions, i.e. precision medicine.

Inherited metabolic diseases usually present in the neonatal period or before school age. A growing portion of the disorders can be treated successfully, and an increasing number of patients are now treated in adult medicine. Several of the disorders also exist as attenuated variants without distinct symptoms in childhood. They can present as an acute onset event during metabolic stress in adulthood. We describe three patients with acute clinical decompensation in adulthood with severe sequelae and propose investigations to help diagnose such patients. One patient was unconscious from ammonia intoxication and developed severe neurological sequelae because of a defect in the urea cycle. A second patient had seizures caused by a defect in the conversion of homocysteine to methionine, resulting in lack of S-adenosylmethionine. A third patient had a stroke and grossly elevated homocysteine caused by undiagnosed homocystinuria. Recently there have been dramatic improvements in diagnostic methods, and new therapies are continuously being developed. Knowledge of these disorders is therefore of increasing importance also in adult medicine.

A growing number of factors have been recognized as crucial in sexual development, and many clinical conditions have become understandable as mutations in relevant genes have been identified. In some cases this has led to the development of DNA diagnostics, which can be of some aid in the workup of this type of patient. So far it is mainly more serious disorders which have been elucidated, thanks to the identification of extensive mutations in the key genes for sexual development. Exactly to what extent less severe damage in these genes underlies more subtle disorders of sexual development, such as disturbances in pubertal development and diminished fertility, is with few exceptions unknown. We are still aware of only a fraction of the information hidden in our genetic heritage. Developments in this field are nonetheless rapid, and molecular analyses will probably become more and more part and parcel of the workup of patients with a wide variety of disturbances in sexual development.

Congenital adrenal hyperplasia (CAH) is the common name of a constellation of diseases that impair cortisol synthesis in the adrenal cortex. As defects in each of three steroidogenic enzymes, 21-hydroxylase, 11 beta-hydroxylase, and 3 beta-hydroxysteroid dehydrogenase, promote overproduction of adrenal androgens, affected female fetuses may be virilised. The major cause of CAH is 21-hydroxylase deficiency, the incidence of which is 1:10,000 live births in the Swedish population. Of the 10-15 children born in Sweden each year with 21-hydroxylase deficiency, 3-5 will be virilised girls who must undergo traumatic surgery of the external genitalia. This can be prevented by administration of dexamethasone to the gravida during pregnancy. Prenatal treatment was introduced in Sweden in 1985, prenatal diagnosis being based in most cases on direct mutational analysis using allele-specific PCR on DNA from chorionic villus samples. In our experience, genotype corresponds well to phenotype, and we recommend that all children with 21-hydroxylase deficiency be genotyped in order to prepare the family for rapid and reliable prenatal diagnosis and possible treatment when the next child is awaited. Since 1985, 35 women have received prenatal treatment in Sweden, six of the 35 fetuses being found to be affected females and treated until term. As compared with their older sisters, all of these six girls were characterised by no signs, or only minor signs, of virilisation, and only one required surgery because of labial fusion and recurrent urinary tract infections. As a group, the 35 infants were characterised by normal birth weight and length, and normal growth during the first year of life. Passage of developmental milestones was normal though several adverse events, both in treated mothers and infants, have been reported. Approximately one fourth of the women treated throughout pregnancy reported some side-effect (e.g., excessive weight gain, severe cutaneous striae, mood fluctuations and irritability, acne and hirsutism, or oedema). One unaffected boy, treated for seven weeks, was born with severe hydrocephalus and agenesis of the corpus callosum; two affected sisters and one unaffected girl were characterised by failure to thrive during the first year of life, but later recovered (one of the affected sisters was later diagnosed as suffering from mitochondrial disease). Although in our experience prenatal treatment with dexamethasone is effective in preventing virilisation of girls with 21-hydroxylase deficiency, some adverse events have been noted in treated infants. As it remains unknown whether these events were attributable to the treatment, it must still be regarded as experimental, and its use should be centralised and meticulously monitored until more experience has been gained.