Central vs. Peripheral
When healthcare professionals conduct a neurological assessment, they may encounter issues within either the central or peripheral aspects of the nervous system. For those specializing in musculoskeletal conditions, peripheral problems are often more familiar than central. However, if you encounter a patient with undetected and significant central nervous pathology, it’s important to identify it. Identifying this is best done during the patient’s history, but it’s crucial to be aware of what certain findings in the neurological examination mean. So, how do you differentiate central and peripheral damage?
Central injury
Central nervous system injuries refer to damage within the central nervous system, often referred to as “upper motor neurone syndrome” (1).
During neurological assessments, signs of central injury may include muscular hypertonia, rigidity, spasticity, hyperreflexia, clonus, or the Babinski sign. Fine motor skills tend to be more affected than gross strength (1,2).
In a clinical setting, diminished strength during muscle testing may be observed. Atrophy is not always present. Increased reflex responses and the possibility of clonus during reflex testing are indicators. During plantar reflex testing, the toes may extend and fan out (known as the Babinski sign) instead of flexing. Rigidity and spasticity can be assessed by passively moving joints like wrists (flexion-extension), elbows (flexion-extension), and ankles (dorsi-plantarflexion). Slow, passive movements assess rigidity, while fast, passive movements assess spasticity.
Common examples of central nervous system injury include conditions such as stroke, head/spinal trauma, or parkinsonism. When such signs are detected, further investigation through a comprehensive neurological examination and possible referral to the appropriate specialist is warranted.
Peripheral injury
The peripheral nervous system comprises 12 pairs of cranial nerves and 31 pairs of spinal nerves (1,2). Despite being traditionally seen as outside the brain and spinal cord, motor neurons have their origins within the spinal cord, specifically distal to anterior horn cells (2,3). Therefore, local spinal cord injuries can lead to peripheral symptoms if they affect motor anterior horn cells, often referred to as “lower motor neurone syndrome” (1).
When examining patients with peripheral nervous system injuries or diseases, clinicians may find signs such as muscle atrophy, reduced muscle tone (though this can occur initially after central injury as well), decreased reflexes, or muscle weakness/paralysis (2,4). Patients with central nervous system injuries may also have weakness, but in these cases, the spinal reflex arc remains intact, meaning reflexes can still be elicited (4).
In clinical practice, reduced strength during muscle testing, decreased reflex responses, or the absence of reflexes may be evident. Damage to motor peripheral neurons can result in a 20-30% loss of muscle substance (atrophy) within three months (4). For instance, in cases of median nerve damage, muscle atrophy, particularly in the thumb, is typical. While cramps are common among healthy individuals, they can also occur in peripheral conditions such as neuropathies, ALS, and vascular diseases (4).
Common examples of peripheral nervous system injuries or diseases include Guillain-Barre syndrome, Charcot-Marie-Tooth disease, disc herniation with nerve root involvement, polio, peripheral polyneuropathy (associated with alcohol or diabetes), myasthenia gravis, and trauma. ALS affects both the central and peripheral nervous systems.
Summing it up
To summarize, here’s a quick overview of the differences between central and peripheral nervous system injuries (5):
Here are affected areas, with regards to where the lesion is
While this discussion has primarily focused on motor neuron testing, it’s important to remember that sensory deficits can present differently in cases of central and peripheral injuries. Here’s an overview (6):
References
1. Helseth E, Harbo HF, Rootwelt T. Nevrologi og nevrokirurgi. Bergen: Fagbokforlaget; 2019.
2. Reiman MP. Orthopedic clinical examination. 2016.
3. Holck P. forhornceller. I: Store medisinske leksikon [Internett]. 2020 [sitert 14. januar 2021]. Tilgjengelig på: http://sml.snl.no/forhornceller
4. Brodal P. Sentralnervesystemet. Oslo: Universitetsforlaget; 2013.
5. Jan M, Al-Buhairi A, Baeesa S. Concise outline of the nervous system examination for the generalist. Neurosci Riyadh Saudi Arab. 1. januar 2001;6:16–22.
6. Howlett WP. Neurology in Africa: clinical skills and neurological disorders [Internett]. Bergen: University of Bergen; 2012 [sitert 20. januar 2021]. Tilgjengelig på: http://www.uib.no/cih/en/resources/neurology-in-africa
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