Hyperphenylalaninemia is a term encompassing various clinical entities that arise due to phenylalanine accumulation. Deficiency of phenylalanine hydroxylase and tetrahydrobiopterin (in a smaller number of cases) due to genetic mutations transferred by an autosomal recessive pattern is the underlying cause. Genetic testing and determination of phenylalanine levels in the blood are necessary for diagnosis. Therapy consists of dietary restriction of this amino acid and tetrahydrobiopterin administration.
The classical form of PKU presents with a diverse brain pathology, including microcephaly, epilepsy, profound intellectual changes, but also behavioral problems . Tremor and motor system abnormalities such as hemiplegia and paraplegia may also be encountered , while excessive phenylalanine can also cause changes on the skin (eczema, decreased skin and hair pigmentation and a musty body odor) . Psychiatric symptoms in the form of depression, anxiety, and development of phobias may appear in later life .
Severe neurological damage in early childhood must consider HPA in the differential diagnosis and a meticulous physical examination is necessary to assess clinical signs and symptoms. Patient history may be an equally important diagnostic tool, with a focus on family history and onset of symptoms. To confirm HPA, however, levels of phenylalanine in blood should be measured and on the basis of its values, the diagnosis of PKU (> 1000 µmol/L) or non-PKU HPA (120-1000 µmol/L) can be made  . Additionally, a BH4 loading test may be used to confirm BH4-associated HPA . Subsequent genetic testing to determine the exact mutation responsible for the disease may be performed as well .
Restriction of dietary intake of phenylalanine was considered as the mainstay of therapy in patients across all subtypes, but individuals with milder forms may be monitored for a period of time and assess whether a need for a dietary restriction is necessary . The introduction of BH4 into therapy has shown marked improvements in treatment and is now recommended for patients in whom BH4 deficiency is reduced  . It is important to emphasize that treatment must be life-long in order to maintain circulating levels of phenylalanine within physiological limits, but the issue of adherence to therapy has been documented and presents a significant challenge for the patient and for the physician .
The prognosis significantly varies on the subtype. Timely and adequate life-long therapy does have a significant effect on the quality of the patient's life. The introduction of screening during a neonatal period in many countries has led to early recognition and therapy, thus reducing the burden of the disease even in those with severe deficiencies, but if patients are left untreated, severe brain damage and intellectual disability may occur .
HPA is considered to be an autosomal recessive disease and two main genetic events have emerged as underlying causes  :
It is important to mention that HPA may be considered as a multifactorial disease, as isolated PAH and BH4 deficiencies may not be as dangerous without a constant dietary intake of phenylalanine, which further increases the circulating concentrations of this amino acid .
Significant variations of HPA prevalence and incidence have been observed across different parts of the world. Incidence rates range from 1 in 2,600, 1 in 4,500 and 1 in 15,000 live births in Turkey, Ireland, and France, respectively  , to 1 in 143,000 and 1 in 200,000 live births in Japan and Finland . Consequently, estimated carrier rates are much higher in countries of high vs low incidence (1 in 26-33 individuals vs 1 in 50-200) . Additionally, marked variations in BH4 deficient cases were detected, as studies show that only 1-2% of Caucasians have this genetic defect, compared to 15% and 19% in Turkey and Taiwan, while 66% of patients from Saudi Arabia were diagnosed with BH4 deficiency in the setting of elevated phenylalanine levels . It is still unknown why such differences across the world exist.
HPA starts with genetic mutations that impair normal conversion of phenylalanine to tyrosine. Under physiological conditions, phenylalanine is an essential amino acid used for a synthesis of proteins and excess levels are converted to tyrosine (in the presence of oxygen) through the activity of PAH and by a support of BH4 , a cofactor necessary for this chemical reaction. In the setting of genetic mutations that either cause deficiency of PAH or BH4, phenylalanine is not converted to tyrosine at a desirable rate, causing this amino acid to accumulate and induce toxic effects to the brain and other parts of the central nervous system . The exact mechanism for brain damage is not known, but it is suspected that the large amino acid transport affects protein synthesis, neurotransmitter function and the activity of dopamine  .
Fortunately, many countries have introduced mandatory screening for PAH deficiency in regular practice and testing during newborn period is now becoming a part of regular practice  . This strategy is, by far, the most important method to prevent brain damage and symptoms associated with this condition, while screening of relatives at risk for HPA can also be a valid preventive strategy .
Hyperphenylalaninemia (HPA) is an autosomal recessive metabolic disease that primarily arises due to deficiency of phenylalanine hydroxylase (PAH) , a hepatic enzyme that normally converts phenylalanine to tyrosine. As a result, accumulation of phenylalanine in the circulation occurs . So far, more than 500 different mutations have been identified in the PAH gene located on chromosome 12 , while deficiency of tetrahydrobiopterin (BH4), a cofactor essential for proper functioning of PAH, has also been proposed as a cause of HPA . The overall incidence rates are estimated at 1 in 15,000 individuals, but significant differences exist across the world . In Turkey and Ireland, HPA is diagnosed in 1 in 2,600 and 4,500 live births, respectively, whereas a rate of 1 in 143,000 live births was established in Japan . HPA has three distinct phenotypes, which are formed on the basis of severity of HPA and tolerance of phenylalanine from food :
To make the diagnosis, levels of phenylalanine in blood, as well as an activity of BH4 cofactor through a procedure known as BH4 loading test are evaluated in order to assess the severity of a deficiency, while genetic testing to determine the exact mutation on chromosome 12 may be performed to confirm the diagnosis . At this moment, therapy consists of restriction of daily phenylalanine intake through food and BH4 supplementation, which has emerged as a very important treatment strategy . It may be used only in individuals who develop HPA due to BH4 deficiency, in addition to PAH deficiency, with numbers varying from 66% in Saudi Arabia, 15% in Turkey and only 1-2% of Caucasian patients . Despite the fact that mandatory screening for HPA and PKU has been implemented in various countries, these conditions pose a significant burden, partly because of the need for a life-long dietary regimen in many patients, but also because unrecognized patients may develop severely debilitating neurological symptoms that have a detrimental impact on their daily life .
Hyperphenylalaninemia (HPA) is a genetic disorder characterized by excess levels of phenylalanine in the body. Phenylalanine is an essential amino acid, meaning that it must be supplied from a diet (the body is not able to synthesize it internally) and it is used for formation of various proteins. Once the necessary amounts are used up, it is normally converted to tyrosine through the activity of an enzyme called phenylalanine hydroxylase (PAH), with the help of a cofactor, tetrahydrobiopterin (BT4) and several other enzymes. In the setting of various genetic mutations that cause insufficient production of PAH and/or BT4, accumulation of phenylalanine in the circulation occurs, resulting in HPA. Based on the amount of this amino acid in the blood, HPA may be further classified into phenylketonuria (PKU), non-PKU HPA and variant HPA. PKU is the most severe form of HPA that is characterized by severe brain damage and symptoms such as intellectual disability, seizures, tremor and motor abnormalities, while non-PKU HPA has a somewhat milder clinical presentation. The reason why neurological symptoms are the hallmark of HPA is because phenylalanine is considered to be toxic for the brain when present in high concentrations, but the exact mechanism of damage remains unknown. To make the diagnosis, it is necessary to evaluate phenylalanine levels in the blood, while genetic testing to confirm the exact mutation may be performed as well. PKU is established when levels are > 1000 µmol/L, while non-PKU HPA is diagnosed when a concentration between 120-1000 µmol/L is observed. Dietary restriction of phenylalanine is the most important therapeutic measure, which implies a reduction of food high in proteins (meat, fish, eggs, cheese), but the introduction of BT4 supplementation has shown to be of significant benefit for patients in whom BT4 deficiency is the cause of HPA. Fortunately, numerous countries over the world have implemented screening of newborn babies for HPA, so that early initiation of treatment can prevent the occurrence of symptoms that may be severely debilitating for the patient and the family. Having in mind the fact that HPA is transmitted through an autosomal recessive pattern of inheritance, meaning that each parent is a carrier of a single mutated gene, but is not ill and if both genes are transferred to their child, their presence will cause a disease. For this reason, screening of families in whom cases of HPA have been confirmed may also be an effective preventive strategy.