What is primary lateral sclerosis (PLS)?

PLS is a special form of ALS in which only uncontrolled muscle activity in the form of muscle stiffness (spasticity) is detectable. PLS is caused by a degeneration of motor neurons in the brain (first motor neuron), while the motor neurons in the spinal cord (second motor neuron) remain largely intact. PLS begins with spasticity of one limb (or the tongue) and spreads – starting from the first affected region – to other muscle regions of the body over a longer period of time. A characteristic feature of PLS is that muscle weakness and muscle wasting (both symptoms of degeneration of the second motor neuron) are not detectable. PLS is a very rare variant of ALS that affects less than 5% of all ALS patients.