If one is prepared to accept that there are different types of PD, we might wish to distinguish between those subtypes which involve the death of doperminergic neurons and those involving alternative reasons for a lack of dopamine. One such reason is tyrosine hydroxylase deficiency.

The main, but not only, feature of PD is a shortage of dopamine in those parts of the brain dealing with movement. Dopamine is the product of a chain of biochemical reactions, including the synthesis of levodopa from tyrosine. Tyrosine is found in adequate amounts in many foods, so a shortage of it is not usually a problem. Therefore, adding more tyrosine to the diet doesn't usually increase levodopa levels markedly. What limits the amount of dopamine produced is the enzyme tyrosine hydroxylase, which acts as a catalyst in the conversion of tyrosine to levodopa. [1]

The amount of tyrosine hydroxylase (TH) released is controlled by a gene. Defects in this gene result in rare diseases, such as infantile Parkinson's and dopa- responsive dystonia [2].

One can speculate that there are people with a lifetime shortage of TH and, hence, dopamine. In their early years this would, apart from acute cases, be asymptomatic. But, as they age, normal levels of neuronal death would cause TH levels to fall until, in later life, they cross a threshold where the shortage becomes a problem. I suspect that some of the variation in the observed age of onset of IPD could be caused by variations due to the TH gene. Or, put another way, some people who are not diagnosed as mild cases of DRD in childhood are misdiagnosed in later life as having IPD. Such people are likely to have an early onset of symptoms, but a slow progression.

Has anyone here been tested for TH deficiency?

References

[1] http://en.wikipedia.org/wiki/Tyrosine_hydroxylase

[2] http://www.pndassoc.org/diseases/th.html

John