What does “paresis” mean?

A paresis (from the Greek: “weakness”) is an incomplete paralysis. In cases of mild paresis, only slight limitations in mobility are noticeable, whereas in cases of severe paresis, only minimal movements are detectable—despite full muscle contraction. Complete paralysis, in which no muscle movement is detectable despite full muscle contraction, is referred to as “paralysis” or “plegia.”

There are two types of paresis: “Peripheral paresis” results from damage to the motor nerve pathways, which extend from the spinal cord through the nerve roots and peripheral nerves to the muscles (second motor neuron). These forms of paresis are characterized by a loss of muscle strength (“flaccid paralysis” or “flaccid paresis”). In contrast, there is “central paresis” (or “central paralysis”). It results from damage to the motor nerve pathways that run from the motor neurons in the brain to the spinal cord (first motor neuron).

These pareses result from uncontrolled muscle activity, known as spasticity. The motor deficit in central pareses is not caused by a loss of strength, but by stiffness that impedes normal movement. In ALS, peripheral and central paresis can occur together. The slow progression of paresis is a key characteristic of ALS.