This is a form of dysautonomia but differentiated from familial dysautonomia by a lack of familial dysautonomic symptoms such as loss of sense of pain and smell. While L-threo-DOPS has been described as being "very effective for restoring noradrenergic tone and correcting postural hypotension, response to treatment is variable and the long-term and functional outcome is unknown."

Researchers have put together retrospective data collections in order to better under the progression of this orphan disease. Most studies show a perinatal period marked by inadequacy of the ANS to control blood pressure, blood sugar, and body temperature. The experiences of orthostatic hypotension, exercise intolerance, and "traumatic morbidity related to falls and syncope" have been documented later in lives of people with this condition. To provide a basis for improving the understanding of the epidemiology, genotype/phenotype correlation, outcome of these diseases, their impact on the quality of life of patients, and for evaluating diagnostic and therapeutic strategies, a patient registry was established by the non-commercial International Working Group on Neurotransmitter Related Disorders (iNTD).

Since the conversion of dihydroxyphenylserine (Droxidopa; trade name: Northera; also known as L-DOPS, L-threo-dihydroxyphenylserine, L-threo-DOPS and SM-5688), to norepinephrine bypasses the dopamine beta-hydroxylation step of catecholamine synthesis, L-Threo-DOPS is the ideal therapeutic agent. In humans with DβH deficiency, L-Threo-DOPS, a synthetic precursor of noradrenaline, administration has proven effective in dramatic increase of blood pressure and subsequent relief of postural symptoms.

L-DOPS continues to be studied pharmacologically and pharmacokinetically and shows an ability to increase the levels of central nervous system norepinephrine by a significant amount. This is despite the fact that L-DOPS has a relative difficulty crossing the blood-brain barrier when compared to other medications such as L-DOPA. When used concurrently, there is evidence to show that there is increased efficacy as they are both intimately involved and connected to the pathway in becoming norepinephrine.

There is hope and evidence that L-DOPS can be used much more widely to help other conditions or symptoms such as pain, chronic stroke symptoms, and progressive supranuclear palsy, amongst others. Clinically, L-DOPS has been already shown to be helpful in treating a variety of other conditions related to hypotension including the following:

- Diabetes induced orthostatic hypotension

- Dialysis-induced hypotension

- Orthostatic intolerance

- Familial amyloidotic polyneuropathy

- Spinal Cord Injury related hypotension

Empirical evidence of mild effectiveness has been reported using mineralocorticoids or adrenergic receptor agonists as therapies.

Other medications that can bring relief to symptoms include:

- phenylpropanolamine- due to pressor response to vascular α-adrenoceptors

- indomethacin

Vitamin C (ascorbic acid) is also a required cofactor for the Dopamine beta hydroxylase enzyme. Recent research has shown that vitamin C rapidly catalyzes the conversion of dopamine to norepinephrine through stimulation of the dopamine beta hydroxylase enzyme.

In GRA, the hypersecretion of aldosterone and the accompanying hypertension are remedied when ACTH secretion is suppressed by administering glucocorticoids.

Dexamethasone, spironolactone and eplerenone have been used in treatment.

Morphine has been found to be effective in aborting episodes; sometimes it is the only medication that can combat the sympathetic response. Morphine helps lower respiration rates and hypertension. It is given in doses of two milligrams to eight milligrams but can be administered up to twenty milligrams. Nausea and vomiting are common side effects. Withdrawal is sometimes seen in patients.

People affected by adipsia lack the ability to feel thirst, thus they often must be directed to drink. Adipsic persons may undergo training to learn when it is necessary that they drink water. Currently, there is no medicine available to treat adipsia. For people with adipsia because of hypothalamic damage, there is no surgical or medicinal option to fix the damage. In some cases where adipsia was caused by growths on thirst centers in the brain, surgical removal of the growths was successful in treating adipsia. Although adipsic persons must maintain a strict water intake schedule, their diets and participation in physical activities are not limited.

People affected by diabetes insipidus have the option of using the intranasal or oral hormone desmopressin acetate (DDAVP), which is molecularly similar enough to vasopressin to perform its function. In this case, desmopressin helps the kidneys to promote reabsorption of water. Some doctors have reported success in treating psychogenic adipsic patients with electroconvulsive therapy, although the results are mixed and the reason for its success is still unknown. Additionally, some patients who do not successfully complete behavioral therapy may require a nasogastric tube in order to maintain healthy levels of fluids.

The two most common medications used in the treatment of paroxysmal sympathetic hyperactivity are morphine sulfate and beta-blockers. Morphine is useful in helping halt episodes that have started to occur. Beta-blockers are helpful in preventing the occurrence of 'sympathetic storms'. Other drugs that have been used and have in some cases been helpful are dopamine agonists, other various opiates, benzodiazepines, clonidine, and baclofen. Chlorpromazine and haloperidol, both dopamine antagonists, in some cases have worsened PSH symptoms. These drugs are in use currently for treatment; exact pathways are not known and wide-range helpfulness is speculative.

SR deficiency is currently being treated using a combination therapy of levodopa and carbidopa. These treatments are also used for individuals suffering from Parkinson's. The treatment is noninvasive and only requires the patient to take oral tablets 3 or 4 times a day, where the dosage of levodopa and carbidopa is determined by the severity of the symptoms. Levodopa is in a class of medications called central nervous system agents where its main function is to become dopamine in the brain. Carbidopa is in a class of medications called decarboxylase inhibitors and it works by preventing levodopa from being broken down before it reaches the brain. This treatment is effective in mitigating motor symptoms, but it does not totally eradicate them and it is not as effective on cognitive problems. Patients who have been diagnosed with SR deficiency and have undergone this treatment have shown improvements with most motor impairments including oculogyric crises, dystonia, balance, and coordination.

Glucocorticoid remediable aldosteronism (GRA), also describable as "aldosterone synthase hyperactivity", is an autosomal dominant disorder in which the increase in aldosterone secretion produced by ACTH is no longer transient.

It is a cause of primary hyperaldosteronism.

If there is evidence of overdose or it is suspected, the patient should be given gastric lavage, activated charcoal, or both; this could make the difference between life and death in a close situation. It can however aggravate the patient which should be taken into account.

The first line treatments are diazepam and a non-selective beta blocker; other antihypertensive drugs may also be used. It is important to note that not all benzodiazepines and beta blockers are safe to use in an adrenergic storm; for instance, alprazolam and propranolol; alprazolam weakly agonizes dopamine receptors and causes catecholamine release while propranolol mildly promotes some catecholamine release - each worsening the condition.

Adrenergic storms are often idiopathic in nature; however if there is an underlying condition, then that must be addressed after bringing the heart rate and blood pressure down.

Mice that have a deficiency in the SPR gene display altered pterin profiles and diminished levels of dopamine, norepinephrine, and serotonin. These disturbances indicate that SPR is important in maintaining homeostasis of BH and the functions of BH dependent enzymes. The researchers investigated the role of SPR in the regulation of BH in mice by using the gene targeting technique and generating a mouse strain deficient in the SPR gene. Gene targeting is a technique used to delete a gene, add a gene, remove exons, and introduce point mutations and can be either permanent or temporary. The mice that were SPR deficient showed similar symptoms to those observed in patients with SR deficiency including impaired body movement. These similarities show that mice are useful models for furthering knowledge on significant issues concerning SR and BH deficiencies.

Treatment of THB deficiencies consists of THB supplementation (2–20 mg/kg per day) or diet to control blood phenylalanine concentration and replacement therapy with neurotransmitters precursors (L-DOPA and 5-HTP) and supplements of folinic acid in DHPR deficiency.

Tetrahydrobiopterin is available as a tablet for oral administration in the form of "tetrahydrobiopterin dihydrochloride" (BH4*2HCL). BH4*2HCL is FDA approved under the trade name Kuvan. The typical cost of treating a patient with Kuvan is $100,000 per year. BioMarin holds the patent for Kuvan until at least 2024, but Par Pharmaceutical has a right to produce a generic version by 2020. BH4*2HCL is indicated at least in tetrahydrobiopterin deficiency caused by GTPCH deficiency or PTPS deficiency.

Adipsia, also known as hypodipsia, is a symptom of inappropriately decreased or absent feelings of thirst. It involves an increased osmolality or concentration of solute in the urine, which stimulates secretion of antidiuretic hormone (ADH) from the hypothalamus to the kidneys. This causes the person to retain water and ultimately become unable to feel thirst. Due to its rarity, the disorder has not been the subject of many research studies.

Adipsia may be seen in conditions such as diabetes insipidus and may result in hypernatremia. It can occur as the result of abnormalities in the hypothalamus, pituitary and corpus callosum, as well as following pituitary/hypothalamic surgery.

It is possible for hypothalamic dysfunction, which may result in adipsia, to be present without physical lesions in the hypothalamus, although there are only four reported cases of this. There are also some cases of patients experiencing adipsia due to a psychiatric disease. In these rare psychogenic cases, the patients have normal levels of urine osmolality as well as typical ADH activity.

For those whose RLS disrupts or prevents sleep or regular daily activities, medication may be useful. Evidence supports the use of dopamine agonists including: pramipexole, ropinirole, rotigotine, and cabergoline. They reduce symptoms, improve sleep quality and quality of life. Levodopa is also effective. One review found pramipexole to be better than ropinirole.

There are, however, issues with the use of dopamine agonists including augmentation. This is a medical condition where the drug itself causes symptoms to increase in severity and/or occur earlier in the day. Dopamine agonists may also cause rebound when symptoms increase as the drug wears off. In many cases, the longer dopamine agonists have been used the higher the risk of augmentation and rebound as well as the severity of the symptoms. Also, a recent study indicated that dopamine agonists used in restless leg syndrome can lead to an increase in compulsive gambling.

- Gabapentin or pregabalin, a non-dopaminergic treatment for moderate to severe primary RLS

- Opioids are only indicated in severe cases that do not respond to other measures due to their high rate of side effects.

Benzodiazepines, such as diazepam or clonazepam, are not generally recommended, and their effectiveness is unknown. They however are sometimes still used as a second line, as add on agents. Quinine is not recommended due to its risk of serious side effects involving the blood.

This condition is very rare; approximately 600 cases have been reported worldwide. In most parts of the world, only 1% to 2% of all infants with high phenylalanine levels have this disorder. In Taiwan, about 30% of newborns with elevated levels of phenylalanine have a deficiency of THB.

According to some guidelines, all people with RLS should have their serum ferritin level tested. The ferritin level, a measure of the body's iron stores, should be at least 50 µg/L (or ng/mL, an equivalent unit) for those with RLS. Oral iron supplements can increase ferritin levels. For some people, increasing ferritin will eliminate or reduce RLS symptoms; a ferritin level of 50 µg/L is not sufficient for some and increasing the level to 80 µg/L may further reduce symptoms. However, at least 40% of people will not notice any improvement. It is not advised to take oral iron supplements without first having ferritin levels tested, as many people with RLS do not have low ferritin and taking iron when it is not called for is unlikely to offer any therapeutic benefit whilst still able to cause adverse events. All parenteral iron treatments require diagnosis with laboratory tests to avoid iron overload.

For women with phenylketonuria, it is important for the health of their children to maintain low Phe levels before and during pregnancy. Though the developing fetus may only be a carrier of the PKU gene, the intrauterine environment can have very high levels of phenylalanine, which can cross the placenta. The child may develop congenital heart disease, growth retardation, microcephaly and intellectual disability as a result. PKU-affected women themselves are not at risk of additional complications during pregnancy.

In most countries, women with PKU who wish to have children are advised to lower their blood Phe levels (typically to between 2 and 6 mg/dL) before they become pregnant, and carefully control their levels throughout the pregnancy. This is achieved by performing regular blood tests and adhering very strictly to a diet, in general monitored on a day-to-day basis by a specialist metabolic dietitian. In many cases, as the fetus' liver begins to develop and produce PAH normally, the mother's blood Phe levels will drop, requiring an increased intake to remain within the safe range of 2–6 mg/dL. The mother's daily Phe intake may double or even triple by the end of the pregnancy, as a result. When maternal blood Phe levels fall below 2 mg/dL, anecdotal reports indicate that the mothers may suffer adverse effects, including headaches, nausea, hair loss, and general malaise. When low phenylalanine levels are maintained for the duration of pregnancy, there are no elevated levels of risk of birth defects compared with a baby born to a non-PKU mother.

Supplementary "protein substitute" formulas are typically prescribed for people PKU (starting in infancy) to provide the amino acids and other necessary nutrients that would otherwise be lacking in a low-phenylalanine diet. Tyrosine, which is normally derived from phenylalanine and which is necessary for normal brain function, is usually supplemented. Consumption of the protein substitute formulas can actually reduce phenylalanine levels, probably because it stops the process of protein catabolism from releasing Phe stored in the muscles and other tissues into the blood. Many PKU patients have their highest Phe levels after a period of fasting (such as overnight), because fasting triggers catabolism. A diet that is low in phenylalanine but does not include protein substitutes may also fail to lower blood Phe levels, since a nutritionally insufficient diet may also trigger catabolism. For all these reasons, the prescription formula is an important part of the treatment for patients with classic PKU.

The oral administration of tetrahydrobiopterin (or BH4) (a cofactor for the oxidation of phenylalanine) can reduce blood levels of this amino acid in some people. Most people, however, have little or no benefit.

Tentative evidence supports dietary supplementation with large neutral amino acids (LNAAs). The LNAAs (e.g. leu, tyr, trp, met, his, ile, val, thr) may compete with phe for specific carrier proteins that transport LNAAs across the intestinal mucosa into the blood and across the blood–brain barrier into the brain. It use is really only indicated in adults who will not follow an appropriate diet.

Another interesting treatment strategy for is casein glycomacropeptide (CGMP), which is a milk peptide naturally free of Phe in its pure form CGMP can substitute the main part of the free amino acids in the PKU diet and provides several beneficial nutritional effects compared to free amino acids. The fact that CGMP is a peptide ensures that the absorption rate of its amino acids is prolonged compared to free amino acids and thereby results in improved protein retention and increased satiety compared to free amino acids. Another important benefit of CGMP is that the taste is significantly improved when CGMP substitutes part of the free amino acids and this may help ensure improved compliance to the PKU diet.

Furthermore, CGMP contains a high amount of the phe lowering LNAAs, which constitutes about 41 g per 100 g protein and will therefore help maintain plasma phe levels in the target range.

No curative treatment is available for prolidase deficiency at this time, although palliative treatment is possible to some extent.

The latter mainly focuses on treating the skin lesions through standard methods and stalling collagen degradation (or boosting prolidase performance, where possible), so as to keep the intracellular dipeptide levels low and give the cells time to resynthesise or absorb what proline they cannot recycle so as to be able to rebuild what collagen "does" degrade. Patients can be treated orally with ascorbate (a.k.a. vitamin C, a cofactor of prolyl hydroxylase, an enzyme that hydroxylates proline, increasing collagen stability), manganese (a cofactor of prolidase), suppression of collagenase (a collagen degrading enzyme), and local applications of ointments that contain L-glycine and L-proline. The response to the treatment is inconsistent between affected individuals.

A therapeutic approach based on enzyme replacement (administering functional prolidase) is under consideration.

Due to the weakened immune response in PD cases, it is also of paramount importance to keep any infections under control, often with heavy antibiotics.

There are several known causes of adrenergic storms; in the United States, cocaine overdose is the leading cause. Any stimulant drug has the capacity to cause this syndrome if taken in sufficient doses, but even non-psychotropic drugs can very rarely provoke a reaction.

Monoamine oxidase inhibitors (MAOIs) are a class of drugs that inhibit the enzyme monoamine oxidase. This enzyme is responsible for breaking down many compounds; basically, anything with a primary amine moiety is likely to be oxidized by monoamine oxidase. An important substrate of the enzyme MAO is tyramine. MAOIs inhibit the enzyme either reversibly, in which MAO is inhibited only until the drug is cleared from the system, or irreversibly, in which the substrate binds permanently to the enzyme, rendering it inactive and effectively destroying it. Irreversible MAOIs are potentially more dangerous, because the body takes about two weeks to regenerate MAO enzymes to functional levels. Two subtypes of MAO exist: MAO-A and MAO-B; this is relevant to adrenergic storms, as there are significant differences between the two types, such as their differential expression throughout the body, and range of substrates. While both MAO-A and MAO-B metabolize tyramine, only MAO-A is present in the gastrointestinal tract and singularly metabolizes the majority of consumed tyramine. (The small portion normally passing into circulation is mostly degraded in the liver where both MAO types act.) Consequently, MAOIs that irreversibly inhibit MAO-A will permit high levels of circulating tyramine able to cause tyramine-induced hypertensive crisis. Aged cheese, beer, red wine, some mushrooms, and fermented products such as pickles are foods containing high levels of tyramine that passed into circulation can such a hypertensive crisis.

Adrenergic storms are not provoked often from MAOI-tyramine interactions; hypertensive crisis alone does not diagnose adrenergic storm, although there will always be hypertension in an adrenergic storm, along with tachycardia and rapid, shallow breathing. However, if a patient on MAOIs uses recreational quantities of any drug with stimulant effects on the CNS, it can provoke an adrenergic crisis (along with the inevitable hypertensive crisis). Deaths have occurred from individuals attempting to combine MAOIs with various entheogens to attain a stronger psychedelic experience, both from adrenergic storms and serotonin syndrome. Combining drugs like MDMA, 2C-B, mescaline, 2C-T-7, etc. with even small quantities of MAOIs - small quantities of both drugs - is still extremely risky. Nevertheless, some users claim to use certain combinations successfully.

Subarachnoid hemorrhage is an extremely serious condition in which a neural membrane is breached and the brain itself is compromised. The onset is sudden, described as "the worst headache of one's life," and many grave symptoms follow. Adrenergic storm is often present among these symptoms, and is responsible for some of the dangers, both long-term and short, of subarachnoid hemhorrhage adrenergic storm, through a complex cascade of processes starting with the movement of subarachnoid blood into the brain. Apparently, as the intracranial pressure increases, the brain is squeezed and catecholamines are forced out of their vesicles into the synapses and extracellular space. An alternative explanation that has been proposed is that this increased in intracranial pressure transduces through the brain parenchyma through to the blood vessels producing a loss in effective cerebral perfusion. This triggers the sympathetic nervous system to secrete more norepinepherine and epinepherine increasing blood pressure and heart rate to dangerous levels to maintain cerebral perfusion.

Rarely, a pheochromocytoma (tumor of the medullar tissue of the adrenal glands, which are located anterior to the kidney), may result in an adrenergic storm. This type of tumor is not common to begin with, and furthermore, the subtype that can cause massive adrenaline release is rarer still. Patients with pheochromocytoma can unexpectedly fly into a rage or sink into trembling fear, possibly dangerous to themselves and others as their judgment is impaired, their senses and pain threshold are heightened, and the level of the adrenalin in their bloodstream is more than most people ever experience; pheochromocytoma can, very rarely, kill by internal adrenaline overdose. But overall, adrenergic storm is an uncommon but certainly not rare phenomenon associated with the also uncommon condition of pheochromocytoma.

Porphyria

Acute hypoglycemia is reversed by raising the blood glucose. Glucagon should be injected intramuscularly or intravenously, or dextrose can be infused intravenously to raise the blood glucose. Oral administration of glucose can worsen the outcome, as more insulin is eventually produced. Most people recover fully even from severe hypoglycemia after the blood glucose is restored to normal. Recovery time varies from minutes to hours depending on the severity and duration of the hypoglycemia. Death or permanent brain damage resembling stroke can occur rarely as a result of severe hypoglycemia. See hypoglycemia for more on effects, recovery, and risks.

Further therapy and prevention depends upon the specific cause.

Most hypoglycemia due to excessive insulin occurs in people who take insulin for type 1 diabetes. Management of this hypoglycemia is sugar or starch by mouth (or in severe cases, an injection of glucagon or intravenous dextrose). When the glucose has been restored, recovery is usually complete. Prevention of further episodes consists of maintaining balance between insulin, food, and exercise. Management of hypoglycemia due to treatment of type 2 diabetes is similar, and the dose of the oral hypoglycemic agent may need to be reduced. Reversal and prevention of hypoglycemia is a major aspect of the management of type 1 diabetes.

Hypoglycemia due to drug overdose or effect is supported with extra glucose until the drugs have been metabolized. The drug doses or combination often needs to be altered.

Hypoglycemia due to a tumor of the pancreas or elsewhere is usually curable by surgical removal. Most of these tumors are benign. Streptozotocin is a specific beta cell toxin and has been used to treat insulin-producing pancreatic carcinoma.

Hyperinsulinism due to diffuse overactivity of beta cells, such as in many of the forms of congenital hyperinsulinism, and more rarely in adults, can often be treated with diazoxide or a somatostatin analog called octreotide. Diazoxide is given by mouth, octreotide by injection or continuous subcutaneous pump infusion. When congenital hyperinsulinism is due to focal defects of the insulin-secretion mechanism, surgical removal of that part of the pancreas may cure the problem. In more severe cases of persistent congenital hyperinsulinism unresponsive to drugs, a near-total pancreatectomy may be needed to prevent continuing hypoglycemia. Even after pancreatectomy, continuous glucose may be needed in the form of gastric infusion of formula or dextrose.

High dose glucocorticoid is an older treatment used for presumptive transient hyperinsulinism but incurs side effects with prolonged use.

Response to treatment is variable and the long-term and functional outcome is unknown. To provide a basis for improving the understanding of the epidemiology, genotype/phenotype correlation and outcome of these diseases their impact on the quality of life of patients, and for evaluating diagnostic and therapeutic strategies a patient registry was established by the noncommercial International Working Group on Neurotransmitter Related Disorders (iNTD).

Certain medications, including NSAIDs (Motrin/Ibuprofen) and steroids can cause hypertension. Other medications include extrogens (such as those found in oral contraceptives with high estrogenic activity), certain antidepressants (such as venlafaxine), buspirone, carbamazepine, bromocriptine, clozapine, and cyclosporine.

High blood pressure that is associated with the sudden withdrawal of various antihypertensive medications is called rebound hypertension. The increases in blood pressure may result in blood pressures greater than when the medication was initiated. Depending on the severity of the increase in blood pressure, rebound hypertension may result in a hypertensive emergency. Rebound hypertension is avoided by gradually reducing the dose (also known as "dose tapering"), thereby giving the body enough time to adjust to reduction in dose. Medications commonly associated with rebound hypertension include centrally-acting antihypertensive agents, such as clonidine and methyl-dopa.

Other herbal or "natural products" which have been associated with hypertension include ma huang, St John's wort, and licorice.

Paroxysmal hypertension is episodic and volatile high blood pressure, which may be due to stress of any sort, or from a pheochromocytoma, a type of tumor involving the adrenal medulla. Patients with paroxysmal hypertension who test negative for pheochromocytoma are said to be suffering from a clinical entity called "pseudopheochromocytoma." This disorder is due to episodic dopamine discharge and has been observed predominantly in hypertensive women, that had episodes that shared similar characteristics of pheochromocytoma but testing proved negative and had ruled out the tumor. In patients with pseudopheochromocytoma dopamine was found to be significantly increased post-paroxysm. The paroxysm is said to be similar to the hypertensive episodes described by Page. These episodes commonly occur after diencephalic stimulation. Therefore, pseudopheochromocytoma, shares many characteristics of "Page's syndrome."

Pseudopheochromocytoma, colloquially known as page's syndrome, is caused predominantly by episodic dopamine discharge, stressors including pain or anxiety, or possibly repressed emotions caused by prior emotional trauma and commonly, a repressive way of coping emotionally. Therefore, treatment of pseudopheochromocytoma is aimed at psychological support and intervention with antidepressants, but also treatment with alpha and then beta blockers in resistant cases.

In the United States, hyperkalemia is induced by lethal injection in capital punishment cases. Potassium chloride is the last of the three drugs administered and actually causes death. Injecting potassium chloride into the heart muscle disrupts the signal that causes the heart to beat. This same amount of potassium chloride would do no harm if taken orally and not injected directly into the blood.

Persons with the genotype for PKU are unaffected in utero, because maternal circulation prevents buildup of [phe]. After birth, PKU in newborns is treated by a special diet with highly restricted phenylalanine content. Persons with genetic predisposition to PKU have normal mental development on this diet. Previously, it was thought safe to withdraw from the diet in the late teens or early twenties, after the central nervous system was fully developed; recent studies suggest some degree of relapse, and a continued phenylalanine-restricted diet is now recommended.

PKU or hyperphenylalaninemia may also occur in persons without the PKU genotype. If the mother has the PKU genotype but has been treated so as to be asymptomatic, high levels of [phe] in the maternal blood circulation may affect the non-PKU fetus during gestation. Mothers successfully treated for PKU are advised to return to the [phe]-restricted diet during pregnancy.

A small subset of patients with hyperphenylalaninemia shows an appropriate reduction in plasma phenylalanine levels with dietary restriction of this amino acid; however, these patients still develop progressive neurologic symptoms and seizures and usually die within the first 2 years of life ("malignant" hyperphenylalaninemia). These infants exhibit normal phenylalanine hydroxylase (PAH) enzymatic activity but have a deficiency in dihydropteridine reductase (DHPR), an enzyme required for the regeneration of tetrahydrobiopterin (THB or BH), a cofactor of PAH.

Less frequently, DHPR activity is normal but a defect in the biosynthesis of THB exists. In either case, dietary therapy corrects the hyperphenylalaninemia. However, THB is also a cofactor for two other hydroxylation reactions required in the syntheses of neurotransmitters in the brain: the hydroxylation of tryptophan to 5-hydroxytryptophan and of tyrosine to L-dopa. It has been suggested that the resulting deficit in the CNS neurotransmitter activity is, at least in part, responsible for the neurologic manifestations and eventual death of these patients.