Disorders In Depth
Dopamine-Beta-Hydroxylase Deficiency (DBH)
Norepinephrine and epinephrine are crucial determinants of minute-to-minute neural regulation of blood pressure and are also present at central nervous system sites likely to be involved in a variety of behaviors. Norepinephrine and epinephrine thus seem so important to human beings that it seemed unlikely for many years that subjects without these catecholamines would survive the perinatal period and develop to adulthood.
This view changed with recognition of a congenital syndrome of severe orthostatic hypotension, noradrenergic failure, and ptosis of the eyelids in two young adults. The syndrome differs from familial dysautonomia and various autonomic disorders seen in adults in that the defect can be localized to the noradrenergic and adrenergic tissues. There is virtual absence of norepinephrine, epinephrine, and their metabolites. However, there is greatly increased dopamine in plasma, cerebrospinal fluid, and urine.
- As children, DBH deficient patients have had a markedly reduced ability to exercise, perhaps because of hypotension engendered by the physical exertion. Because of occasional syncope, anticonvulsive medications have been given in some patients, even though no abnormality was seen on the electroencephalogram.
- Symptoms have generally worsened in late adolescence and by early adulthood, patients complain of profound orthostatic hypotension, especially early in the day and during hot weather or after alcohol ingestion.
- In addition to ptosis of the eyelids, there is a tendency for nasal stuffiness to occur, especially in the supine posture.
- A male patient had appropriate erectile function, but retrograde rather than antegrade ejaculation.
- Presyncopal symptoms in these patients have included dizziness, blurred vision, dyspnea, nuchal discomfort, and occasionally chest pain.
- The physical examination usually includes a normal or low normal supine blood pressure and a normal heart rate but a standing blood pressure that is less than 80 mmHg systolic.
- Heart rate rises on standing but appears to have an attenuated elevation given the very low blood pressure with upright posture.
- Pupils are somewhat small but respond to light and accommodation. Parasympatholytics dilate the eye appropriately.
- Many specialized tests differentiate these patients from those with familial dysautonomia. Cholinergic sensitivity as assessed by conjunctival methacholine is normal, and intradermal histamine evokes a typical flare reaction in DBH deficiency, whereas this does not occur in familial dysautonomia. These patients are further distinguished from familial dysautonomia in having normal tearing, intact corneal and deep tendon reflexes, normal sensory function and normal senses of taste and smell. Also, subjects thus far recognized have not been of Ashkenazi Jewish extraction.
- Atrial fibrillation, occasionally occurs in adults.
- These patients have no response even to high doses of tyramine, which normally increases blood pressure by releasing neuronal norepinephrine. Central autonomic regulation and catecholamine release mechanisms are intact, but dopamine, rather than norepinephrine, levels increase on assumption of the upright posture, during sustained handgrip, and after tyramine administration, while they decrease after clonidine administration.
- Muscle sympathetic nerve traffic, as measured by direct intraneuronal recordings, is present in excess under basal conditions but is otherwise normally modulated by baroreflex mechanisms in these patients. Therefore, primary autonomic neuronal pathways are intact and respond to appropriate stimuli, but dopamine instead of norepinephrine is present in noradrenergic nerve terminals. The excessive microneurographic sympathetic nerve traffic appears to be due to insufficient central alpha-2 adrenoreceptor stimulation.
- Sympathetic cholinergic function is intact, as assessed by normal sweating.
- Parasympathetic function is also preserved, as assessed by intact sinus arrhythmia, normal heart rate increase during Valsalva, and tachycardia after atropine.
- A diagnosis of DBH deficiency is based on the characteristic plasma catecholamine pattern of absent norepinephrine and epinephrine and elevated dopamine.
The DBH gene is located at 9q34. In 2000, the genomic basis of DBH deficiency was elucidated. Kim et al. identified seven novel variants, including four potentially pathogenic mutations, in the human DBH gene of two unrelated DBH deficient patients and their families. Both patients had an intronic mutation that can lead to aberrant splicing. However, this mutation generates both aberrant and properly spliced DBH messages, and patients with DBH deficiency have no DBH gene product, even when a polyclonal antibody against DBH is used to measure it. It is possible that another variant at DBH is necessary to produce the DBH deficiency phenotype.
- Fludrocortisone at relatively high doses has successfully raised blood pressure with some benefit. Indomethacin has also been of modest benefit in raising blood pressure, but one patient had aggressive ideation while receiving this drug.
- The monoamine oxidase inhibitor tranylcypromine also produced paranoid thinking in one patient.
- There has been a reasonable pressor response to phenylpropanolamine (25 and 50 mg), perhaps because of the hypersensitive alpha-adrenoreceptors in these patients.
- Once the specific enzymatic defect had been elucidated, investigators knew that a better treatment for DBH deficiency could be devised. DBH deficient patients possessed all the machinery to handle neurotransmitters effectively, but simply had a 'false neurotransmitter', dopamine, instead of the physiological neurotransmitter, norepinephrine. Their alternative therapeutic approaches were to reduce the former (with metyrosine) or increase the latter (with dihydroxyphenylserine).
- Metyrosine is an inhibitor of tyrosine hydroxylase and reduces the synthesis of catecholamines, both in the periphery and in the central nervous system. Metyrosine significantly reduces urinary and plasma dopamine levels in DBH deficiency.
- A more favorable long-term result has been achieved with L-dihydroxyphenylserine (droxidopa or L-DOPS). This agent is a prodrug acted upon by endogenous dopa decarboxylase to yield norepinephrine. DBH is not required for the conversion of DOPS to norepinephrine and, thus, this defective enzyme can be bypassed. The administration of DOPS to these patients results in dramatic increases in blood pressure and in the restoration of plasma and urinary levels of norepinephrine toward normal. Long-term experience with this drug indicates continued efficacy at the 250 mg or 500 mg tid regimen.
DBH deficiency and its successful treatment by DOPS has provided valuable lessons in catecholamine pharmacology and encourages us to hope that other autonomic disorders may one day also yield to genuinely effective therapeutic intervention. Indeed, other newly recognized genetic autonomic and catecholamine disorders are now being recognized, including tetrahydrobiopterin deficiency, dopa decarboxylase deficiency. Menkes disease, monoamine oxidase deficiency and other disorders of dopamine metabolism.