Definition of Paresis vs Plegia
Paresis refers to partial or incomplete loss of voluntary motor function (weakness), while plegia refers to complete loss of voluntary motor function (total paralysis). 1
Core Definitions
Paresis is defined as:
- A partial movement impairment of neural or muscular origin 2
- Weakening of a muscle or group of muscles due to nerve damage or disease 2
- Incomplete ability to move the affected body part 1
- Preservation of some motor strength below the injury level 3
Plegia is defined as:
- Complete loss of the function of a body part or parts 2
- Total paralysis with complete inability to move the affected area 1
- Loss or complete impairment of motor function due to a lesion of the neural or muscular mechanism 2
Clinical Implications and Assessment
Recognition in Clinical Practice
In advanced stages, the distinction between paresis and plegia is obvious on visual assessment, but in earlier stages, specific functional tests are needed to determine the degree of motor impairment 2:
- Clasping test: When lifting by the tail, normal animals spread legs away from the abdomen; retracting limbs toward the abdomen indicates paresis/paralysis 2
- Grip test: The patient should grip bars when placed on a grid and pulled slightly back; inability indicates weakness 2
- Kyphosis test: Placing on a flat surface and observing gait; inability to straighten the spine indicates impairment 2
Terminology Precision
The terms "complete" and "incomplete" should characterize the completeness of the lesion itself, not be used as qualifiers with "plegia." 3 Patients with any preservation of motor strength should be described as "paretic" rather than having "incomplete plegia," as this creates terminological confusion 3.
Anatomical Prefixes and Clinical Patterns
The prefix system clarifies the distribution of motor deficit 4:
- "Hemi-": One half of the body
- "Mono-": One limb
- "Para-": Lower limbs (both)
- "Di-": Two symmetrical segments on both sides
- "Tetra-": Four limbs
The suffix determines severity: "-plegia" for total paralysis versus "-paresis" for partial paralysis 4.
Common Clinical Pitfall
The most critical error is using "incomplete paraplegia" when "paraparesis" is the correct term. 3 This misuse has become widespread in spinal cord injury literature but contradicts classical neurological terminology 3. Any patient with preserved motor function below the injury level has paresis, not "incomplete plegia" 3.
Example in Facial Nerve Pathology
In vocal fold pathology, paralysis refers to loss or impairment of motor function, while paresis is a partial movement impairment, both of neural or muscular origin 2. However, EMG findings in suspected paresis cases may be similar to paralysis, as there are no different diagnostic criteria 2, making clinical assessment of movement the primary distinguishing factor.