Peripheral neuropathy and Parkinson’s disease

Today I will address the potential link between Parkinson’s disease and a common neurologic condition called peripheral neuropathy. This topic was submitted via the Suggest a Topic portal. I am grateful for your suggestions so please continue to let us know what you’d like to learn more about!
In order to understand what peripheral neuropathy is and what symptoms it can cause, we will briefly review the components of the nervous system.
Central nervous system vs. peripheral nervous system
Neurologic control of the body is very broadly divided into two systems – the central nervous system which consists of the brain and the spinal cord – and the peripheral nervous system which consists of the network of nerves that are outside the brain and spinal cord, and innervate the limbs and the organs of the body.
The peripheral nervous system is composed of three types of nerves: autonomic nerves, sensory nerves and motor nerves. Different types of nerves have varying diameters and are generally divided into those that are small and those that are large.

Autonomic nerves exert control over functions that are not under conscious direction such as respiration, heart function, blood pressure, digestion, urination, sexual function, pupillary response, and much more. This information is conveyed on small fibers.
Motor nerves carry information on limb movement from the brain and spinal cord to the limbs. This information is conveyed on large fibers.
Sensory nerves carry information on the various sensations felt by the limb to the brain and spinal cord. There are two main types of sensory nerves:

Pain and temperature fibers which are small fibers
Vibration and joint position sense fibers which are large fibers

The peripheral nervous system and Parkinson’s disease

It is well-established that the autonomic nervous system can be significantly affected in PD causing symptoms such as constipation, urinary dysfunction and orthostatic hypotension. The autonomic nerves that bring signals to the gut for example, can be directly affected by Lewy body-like accumulations and neurodegeneration. (This is not the only way that automatic functions of the body are affected in PD however. There can also be Lewy bodies and neurodegeneration in the parts of the brain that control these functions.)
What remains unclear is if motor and sensory nerves are also affected in PD.
What is peripheral neuropathy?
Peripheral neuropathy (PN) is a condition in which there is damage to peripheral nerves. Symptoms depend on which type of nerves are affected and can result in:

Weakness
Imbalance with walking
Numbness
Pain or paresthesias (sensations such as burning or tingling) in the limbs

The legs are more commonly affected than the arms because the nerves to the legs are longer than the arms and therefore more prone to damage.
Symptoms of peripheral neuropathy
The symptoms of PN can be non-specific, and a person therefore may not be able to distinguish on their own whether his/her symptoms are due to PN or another condition. PN, however, often results in specific findings on a neurologic exam, such as decreased sensation to pin prick or vibration or the lack of ability to discern which way a toe is being pointed without looking.

Content provided by the American Parkinson Disease Association

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