Which phenotypes of the site of symptom onset of ALS can be distinguished?

The onset of symptoms in ALS is an important criterion for the classification of phenotypes and influences both the prognosis and the course of the disease. It is defined according to the body region of the first symptoms.

With onset in the head (O1), also known as “bulbar onset”, the first symptoms are speech and/or swallowing disorders. The onset on the arm is further differentiated: If the weakness or movement disorder (slowed and uncoordinated movements) first occurs in the hand, it is referred to as a distal arm onset (O2d), while in a proximal arm onset (O2p), the muscles in the shoulder area are affected first.

A rarer onset affects the trunk muscles. A distinction is made here between onset with respiratory weakness (O3r) and onset with trunk instability (O3a), in which the weakness of the trunk muscles initially occurs without breathing problems. More frequently, onset occurs in the legs. Distal leg onset (O4d) is characterized by weakness or coordination problems in the foot, while proximal leg onset (O4p) affects the hip or thigh muscles first.