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a question of genetics / heredity......

  Is this disease a genetic defect that is hereditary or is it a mutation?  If a parent has PSP are the children at risk of developing the condition?  Are there any tests that can detect the presence of this disease years before the person becomes symptomatic?
  Thank you for your time!!
  John Leiska
By "this disease" I assume you mean PSP, because you asked a question about a parent with it. The term "palsy" is an unfortunate one, because there is really no single disease called "palsy" and that is a vague term in the late 20th century.
Progressive Supranuclear Palsy (PSP) is a disorder which looks superficially like Parkinson's Disease, and was only really discovered to be different in the early 1960s. In both PD and PSP, there are groups of brain cells which are degenerating (dying slowly).
In PD, the main group of dying cells is the ones that make dopamine in the substantia nigra (a region in the midbrain, which is in the northern brainstem). In PSP, there is a more widespread loss of cells in the midbrain, many of which regulate eye movements. That's the original basis on which the disease was found to be different from PD, and where the term PSP came from. Actually, though, we find the early presence of postural disturbance (especially falling) to be a much more reliable indicator of PSP versus PD.
The microscopic appearance of PD and PSP brains are different. Not only are there different groups of cells dying, but the cells accumulate different sorts of debris as they degenerate. We now understand PSP to involve abnormal "tau proteins" but still really don't know what that's all about.
In both PD and PSP, there are recognized genetic and environmental components. Clearly, neither disease is purely genetic. For example, in Huntington's disease (picked for an example, not related), there is a dominant pattern of inheritance in which 50% of offspring of an affected parent may develop the disease. That inheritance was recognized long ago. There are other diseases which have been recognized as "recessive" inheritance because of the way the disease clusters in the family.
Neither PD nor PSP have a "dominant" or "recessive" pattern. But in PSP there does seem to be growing evidence that there is a genetic susceptibility. Susceptibility to what? Presumably, some environmental factor about which we have no idea yet. The relative risk is unknown. So, if a relative of yours has PSP, there is perhaps an elevated chance that you could have it, but it isn't very high. I see many patients with PSP and have not yet identified anyone whose parent had it, nor anyone whose sibling had it, nor anyone whose offspring have it.
In both PD and PSP, cell loss occurs before symptoms occur. In PD (better understood and more common disease), estimates vary but they say you could lose 50-80 percent of the original population of cells (dopamine producers in the substantia nigra, a region the size of your fingernail) BEFORE having significant symptoms. If you expect progression of PD over 10-30 years, it is conceivable that the loss of cells could have begun over 10 years in advance of early symptoms. But we really don't know.
In PSP, progression is more rapid (2-5 years to significant disability), and I suppose the time prior to the onset of first symptom might be shorter as well. I am unaware of any estimates suggesting how much cell loss is necessary to produce the first symptoms.
There are no tests which detect PSP or PD reliably even when symptoms are present, let alone prior to the symptomatic stage. Diagnosis is entirely based on clinical experience of the examiner and matching to described diseases. Sometimes the pathologist (at autopsy) reveals that we've been fooled and what we thought was PSP was PD and vice versa. The same goes for other parkinson look-alike diseases such as MSA, DLBD, and CBGD.
I hope this is helpful. CCF MD mdf.

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