What symptom eventually causes als to be fatal?

Progressive weakness: Amyotrophic lateral sclerosis, als, or lou gehrig's disease is characterized by progressive weakness, muscle wasting, involuntary twitching (usually of many muscles), and difficulty speaking, swallowing, and breathing. Most people with ALS die of respiratory failure or pneumonia.

Bulbar involvement: Once the unfortunate patient develops problems with swallowing and breathing, there is only supportive care available, and eventual additional complications such as infection become challenges.