A very
simple overview of PD pathology:
Parkinson’s disease is a complex disorder, but one of the
most important aspects of the pathology is the death of neurons in a region of the brain called the substantia nigra. These
neurons project to the striatum, where they release a chemical called dopamine.
In PD, the death of these neurons leads to reduced levels of dopamine in the
striatum, which in turn results in symptoms such as slowness in movement and
rigidity.
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Parkinson's Disease involves death of dopamine releasing neurons which project from the substantia nigra to the caudate nucleus and putamen (collectively called the striatum) |
The need
for new ‘neuroprotective’ treatments
Current treatments for PD (such as
levodopa) revolve around restoring striatal dopamine to normal levels. However,
these drugs may cause intolerable side-effects and their effectiveness declines
over time. Furthermore, these ‘dopaminergic’ therapies cannot help with
symptoms which are not caused by deficits in dopamine.
The real problem is that all our current treatments are symptomatic
– they address only the shortfall in dopamine levels, rather than actually providing
neuroprotection (i.e. halting or reversing the process of cell death which
occurs).
Why have
neuroproective treatments been so hard to develop?
One of the most important reasons that neuroprotective
treatments have been so hard to develop is that 70-80% of striatal dopamine is
already lost by the time diagnosis is made (Fearnley & Lees, 1991; Stoessl, 2007).
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Loss of dopaminergic (DA) neurones in the substantia nigra occurs perhaps 5-6 years before the onset of PD (solid line). If we can predict PD early, perhaps we can slow this cell loss and delay the onset of symptoms (dashed line). |
This means that once we have identified patients with PD the damage has already been done and it would
be too late for neuroprotective treatments to prevent the development of
parkinsonian symptoms (Streffer et al, 2012).
Why are
projects such as PREDICT-PD so important?
Projects like PREDICT-PD are important because being able to
identify people likely to go on to develop PD will allow trials of
neuroprotective therapies to be started earlier in the disease process (Postuma et al, 2010; Postuma & Montplaisir, 2009; Streffer et al, 2012). These neuroprotective treatments would then stand a much
better chance of preventing or slowing the onset of PD.
How might we be able to predict PD?
Fortunately, there may be some clues as to an individual’s
risk of PD which emerge before the onset of sympotms. PD is believed to have a ‘prodromal’ phase where
non-motor symptoms (such as smell loss and depression) or even very subtle
motor symptoms may emerge, before the full onset of the disease (Hawkes, 2008; Postuma et al, 2012a). These prodromal symptoms include:
- Non-motor symptoms
- Loss of smell (Shah et al., 2009)
- Depression (Reijnders et al, 2008)
- 'Autonomic' nervous system dysfunction
- Constipation (Abott et al, 2001; Savica et al, 2009)
- Variability in heart rate and other cardiac autonomic abnormalities (Palma et al, 2013)
- Sleep problems (Boeve, 2013; Videnovic & Golombek, 2013)
- Cognitive impairment (Aarsland et al, 2010; Williams-Gray et al 2013)
- Problems with vision (Postuma et al, 2012a)
- Subtle motor symptoms - slowed movment, tremor etc. (Postuma et al 2012b)
There are also environmental and
genetic risk factors for PD – such as exposure to pesiticides (which increases
risk of PD) smoking or coffee drinking (which decrease risk of PD), family
history of PD (genetic risk factors) or previous head injury (Noyce et al, 2012).
Combining information on risk of PD
across all these different areas may allow the disease to be accurately
predicted and for disease-modifying treatments to be started much earlier in
the disease course, where they are more likely to provide beneficial effects.
For more information on how the PREDICT-PD project is going about achieving this, click
here.
Jargon buster:
Substantia nigra: a region of the midbrain containing neurons
which project to the striatum, where they release dopamine
Striatum: a region of the brain involved in control of
movement (including starting and stoping movements) in addition to other
functions
Dopamine: a neurotramsitter (a signalling chemical used by
neurons to communicate with one another) whose levels are reduced in PD
Prodrome: an early symptom or group of symptoms which may
indicate the start of a disease before the classically defined onset of that
disease e.g. a rash which may occur before the onset of an infectious disease
or the loss of smell which may occur prior to developing PD
Autonomic nervouss ystem: the division of the nervous system which controls bodily functions
such as heart rate, breathing rate, digestion, urination and sexual functions.
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