English: Illustration showing the most common types of ALS and how the upper and lower motor neurons may be affected in each area. On the top row, most classical ALS is either "spinal onset" in which case the arms or legs are affected first, or "bulbar onset" in which case the mouth and throat region is affected first. Degeneration of the upper motor neuron, lower motor neuron, or both, cause a different set of symptoms in each region, such as weakness, atrophy, spasticity, and/or cramps. On the bottom row, three rarer subtypes of ALS are shown which start in isolated regions (such as the feet or lower leg in the "flail leg" variant) and tend to stay isolated to that region either for the duration of the illness or for a long time. Primary lateral sclerosis (PLS) resembles ALS except that only the upper motor neurons are affected, causing symptoms like spasticity and/or cramps but not weakness, atrophy, or fasciculations. One challenge for diagnosis is that while ALS may initially present in one way, over the course of years it can change, so some clinicians prefer not to state categorically that an individual patient has ALS as opposed to a rarer variant. The rarer variants of flail leg, flail leg, PLS, and PMA are generally associated with a longer survival period than classical ALS.