Recently, there have been many posts relating to the cause of PSP, so I would like to give you some food for thought. I will have to leave out extensive information, but hopefully some of it will make sense.
All the neurological diseases (some discussed on this forum) have a common theme of one or two normal functioning brain proteins "going wrong", and not being eliminated by the usual mechanisms. Eventually they form "toxic" proteins (most likely responsible for neuron death, and it travels along neural pathways and turns normal tau into "toxic" tau). This damaged tau then ends up as (neurofibrillary) tangles. Each neurological disease is generally classified by the type of protein (or proteins) found in (responsible for) these tangles at autopsy, along with their shape and location both in different cells and areas of the brain.
When we ask the question about the cause for PSP, we should probably think about the other diseases that have the same protein as the most dominant feature (tau) "going wrong" (called pure tauopathies). There are at least 9 of these diseases (to name 3 others - CBD, FTD linked to chromosome 17 and Dementia Pugilistica. This latter one I will mention later).
Idiopathic Parkinson's (where PSP shares some symptoms) is associated with another protein called alpha synuclein (it forms Pick's bodies in the brain cells). So, perhaps in asking about the cause of PSP, we might find some answers in the causes for Parkinson's or any other of these neurological diseases. For example, Alzheimer's has two proteins that "go wrong" (tau and beta amyloid).
Of course, at this present time, medical science has not found any single cause for this host of neurological diseases (even in diseases like Multiple Sclerosis that is thought to be an autoimmune disease, the initiating reasons are unknown).
Having said all this, there are risk factors to many of these diseases, but there is often different findings among scientists concerning such factors e.g. do pesticides (like paraquat) induce Parkinson's.
With PSP there are some "named" risk factors such as genetics, environment and diet (no virus or similar has been found).
We have talked about genetics in a previous post and noted that although it does occur, it accounts for less than 1% of PSP conditions.
Consistent head trauma can cause a "tauopathy" similar to PSP/Parkinson's' (found as Dementia Pugilistica or "punch drunk" in sports people from recurring concussions - now called Chronic Traumatic Encephalopathy). Those who have posted about head or neck trauma being a cause of the PSP in their loved one may have some grounds for their concerns. Yet, this type of tauopathy has some characteristic symptoms not found in PSP. (It has been shown that Parkinson's' can occasionally be linked later in life to head trauma, affecting the dopaminergic nigro-striatal pathways).
Certain (neuro) toxins have been found in the diet of some communities that cause a PSP-like condition (annoniacin found in soursop and sweetsop fruits - it affects the mitochondria).
Even low education has been attributed to a factor in PSP that influences one's diet, occupation and the number of brain synapses developed!!! (papers by Dr Golbe - see his most recent 2013 review of PSP on YouTube or lecture notes: "Google" …PSP: A Medical Update -Society for Progressive Supranuclear Palsy).
There have been innumerable papers written on the possible effects of farming and industry with respect to finding a cause(s) for these neurological diseases. Nothing conclusive has emerged. This brings me to my "food for thought"……
Are we thinking PSP and associated diseases are "modern" and thus looking for a recent generational answer? (Idiopathic) Parkinson's' was so named in 1817, and was probably the disease known as "shaking palsy" in AD175. We have talked already on this forum about Charles Dickens in 1857 describing a man who had similar symptoms to PSP. Neurofibrilliary tangles were first "seen" in the beginning of the 1900's (in Alzheimer's). Multiple Sclerosis was desribed over 150 years ago.It seems plausible that most of the neurological diseases we see today have been around since time immemorial (like other diseases). In times when there were no pesticides or immunisations to blame etc etc.
While medical science is coming closer to understanding the mechanisms involved in these damaged brain proteins (and designing specific therapeutics), its unlikely these diseases will be attributable to a "cause". It is more likely to be multifaceted, and a combination of some yet unknown complex genetic factors, along with exacerbating inflammatory processes. In other words, factors that have also been around since the beginning of the human race.