Fasciculations (twitching) in early ALS can be the first symptom. Motor neuron death is the hallmark of ALS, and the loss of axons is what causes fasciculations in ALS, so they are a red flag and most competent neurologists would ask you to return in 6 months if fasciculations and fibrillations are detected in an EMG. Fasciculations and fibrilations are idicative of motor neuron death, or de-innervation in ALS. Follow-up would be looking for increased MUP (motor unit potential) in the EMG which would be signs of re-innervation. Re-innervation is the body's response to motor neuron loss, so the neurons from the surrounding muscles try to grow into the muscle with the denervation. This cycle can be very very fast, as in weeks, to very very slow, which is my situation.
I began getting fasciculations in the summer of 2015 in my lower left extremity (LLE). After 7 EMGs, the fasciculations had spread to the lower right, upper left, and torso (to include my diaphragm) as well as my tongue (early 2018). It was a very slow progression, so they did a muscle and nerve biopsy, as well as a brain MRI. The biopsy showed atrophy and the MRI showed white matter issues in my corticospinal tract (the part of the brain that controls motor neurons/movement). Finally, with mild to moderate weakness in my LLE and RLE, they provided me with a proper diagnosis in October 2019. I am an outlier in the statistics, which is a blessing.
I have spent years learning about ALS etiology and it is a fact that a good percentage of people get fasciculations/twitching well before any other symptom in ALS. All fasciculations are not necessarily ALS though, and many other causes can trigger them. Fasciculations can be benign, or symptoms of many other conditions including radiculopathy.