Parkinson’s disease (PD) is a neurodegenerative and chronic disorder of the central nervous system that progresses slowly and progressively incapacitates the person who suffers it. PD is often classified as a movement disorder, but it also produces some alterations in certain mental functions such as memory, learning and the expression of emotions.
This disease was first described in 1817 by the British doctor James Parkinson in his publication An essay on the shaking palsy, where he exposed the main symptoms of PD. Anyway, it was the French neurologist Jean-Martin Charcot who first used the term maladie de Parkinson (which means PD in French), naming this disease after James Parkinson.
Causes of Parkinson’s disease
To explain how PD is caused it is important to know what dopamine is. Dopamine is an organic chemical which functions both as a hormone and a neurotransmitter and it plays several significant roles in the control of the movements of the body.
Keeping this in mind, PD is produced by the progressive death of the neurons in charge of producing dopamine, called dopaminergic neurons, which can be found in a region of the brain called basal ganglia, specifically in a heterogeneous region of the midbrain known as Substantia nigra pars compacta.
Consequently, there is a decrease of the levels of dopamine in the Nigrostriatal pathway, a very important dopamine pathway that projects itself from the dopaminergic cell bodies of the Substantia nigra pars compacta in the brainstem to the corpus striatum, connecting through axons the midbrain with the forebrain. This decrease leads into a marked reduction in dopamine function in this pathway and to less control in the movements of our body.
Most patients affected by PD lose between 60% and 80% of dopaminergic neurons.
Who does Parkinson’s disease affect to?
PD is a universal disease which can trouble equally both men and women of all ethnic groups. It is more frequent in people between 50 and 70 years old and it affects one in a thousand people, being the second most common neurodegenerative disease (after Alzheimer’s disease, which is the first one).
Well known people such as the Spanish painter Salvador Dalí, the actors Michael J. Fox and Robin Williams, the singer Neil Diamond, the boxer Mohamed Ali, and the dictators Franco and maybe Hitler suffered this disease.
Risk and protective factors
Though the main cause of PD is the progressive death of dopaminergic neurons, the origin of the illness (the reason why these neurons start dying) isn’t clear enough. Anyway, there are some risk factors that can contribute to develop the pathology. Some of them are:
Age: It is the principal risk factor. Therefore, PD is more common in countries with a greater life expectancy.
Family: In the last years, certain heritable genes that could cause this disease, have been discovered and it is believed that about 10-15% of PD cases are hereditary.
Traumatic head injuries: This kind of injuries can produce brain inflammation that can lead into neuronal death, causing a kind of PD called “Posttraumatic parkinsonism”.
Pesticide exposure: Specially the long exposure to insecticides.
On the other hand, there exist some protective factors that can reduce the risk of developing this sickness such as drinking coffee and tea, having normal cholesterol levels and high levels of vitamin D (eating fish, cheese, yogurt or mushrooms, drinking milk, etc) and doing regular physical exercise to improve our physical and mental state.
Main symptoms
The first symptoms of the disease are mild, but they get worse over time interfering with daily activities. The four primary symptoms of PD are:
Trembling, which usually starts in a hand, in a foot or in the jaw.
Rigidity: Muscles remain tense and contracted, causing pain and/or weakness (stiffness).
Bradykinesia, defined as the delay and loss of spontaneous and automatic movements.
Postural instability, which makes sick people lose their balance.
Trembling, rigidity, bradykinesia and postural instability are the main symptoms of PD because they appear in almost every person affected by it.
Secondary symptoms
In addition, there are many other symptoms that can change depending on the patient. Some of them are:
Depression: It is probably the most common secondary symptom but, fortunately, it can be treated using antidepressants.
Emotional changes: People affected can become insecure, fearful and pessimist. They also tend to socialize less, in order to avoid that others notice their symptoms.
Voice changes: People affected usually speak softly and with a monotonous voice and they even stutter in some occasions.
Difficulty swallowing and chewing: The muscles used in the swallowing and chewing processes may work with less efficiency, leading into nutritional problems in the patient.
Sexual dysfunction: Pain, poor blood circulation and the antidepressants used to relieve depression can cause sexual dysfunction.
Fatigue: Usually caused by pain, movement difficulties and muscular stress.
Dementia: Some patients may develop dementia, which affects to memory, reasoning speed and other mental skills.
Sleeping problems: Caused by nightmares, trembling and rigidity.
Gastrointestinal problems such as constipation or urinary urgencies.
There are many other secondary symptoms of PD such as anxiety, sense of smell’s disturbances, sweating or an increased risk of melanoma, which appearance vary depending on the patient.
Treatment
Currently, there is no treatment capable of protecting dopaminergic neurons from death, but there are some useful therapies that can be used to relieve PD symptoms. The most commonly used treatment for PD is levodopa (L-DOPA), a drug that can transform itself into dopamine in the human body. There also exist some surgical treatments but they might be dangerous in some occasions.
In addition, secondary symptoms can be relieved by antidepressants for depression or sleeping pills to relieve the sleeping problems, and with the help of specialists such a speech therapist for the voice changes, a psychologist for the emotional changes, a nutritionist for the possible nutritional problems…
Furthermore, patients affected by PD are told to do physical exercise daily in order to maintain muscle tone and the motor functions.
Conclusion
To sum up, PD is a neurodegenerative and chronic disorder of the central nervous system that progresses slowly and progressively incapacitates the person who suffers from it. It is caused by the death of dopaminergic neurons and, though its origin isn’t clear enough, some risk and protective factors are well-known.
Presently, there isn’t any treatment capable of protecting dopaminergic neurons from death so it is necessary to invest in research in order to find new drugs that can minimize or reverse PD.
Bibliography and references
Standaert D, Saint-Hilaire M, Thomas C, Collard J and Collard R. Parkinson’s Disease Handbook. [Internet]. New York: Medtronic; 2017 [cited 1st oct 2019]. Available from: https://www.apdaparkinson.org/wp-content/uploads/2017/02/APDA1703_Basic-Handbook-D5V4-4web.pdf
Gil C and Martínez A. El Parkinson. Madrid: CSIC; 2015
Parkinson J. An Essay on the Shaking Palsy. London; 1817
Parkinson’s Foundation. Frequently Asked Questions, a guide to Parkinson’s disease [Internet]. Miami [cited 2nd oct 2019]. Available from: https://www.parkinson.org/sites/default/files/attachments/Parkinsons-Disease-Frequently-Asked-Questions.pdf
Schultz W. Activity of pars reticulata neurons of monkey substantia nigra in relation to motor, sensory and complex events. Journal of Neurophysiology. 1986; 55 (4): 660–677
Ojo JO, Rezaie P, Gabbott PL, Stewart MG. Impact of age-related neuroglial cell responses on hippocampal deterioration. Frontiers in Aging Neuroscience. 2015; 7: 57.
Spielman L et al. Physical activity and exercise attenuate neuroinflammation in neurological diseases. Brain Research Bulletin. 2016; 125: 19-29.
NHS. Parkinson’s disease [Internet]. [Page last reviewed 30th apr 2019; cited 5th oct 2019]. Available from: https://www.nhs.uk/conditions/parkinsons-disease/causes/