"A speech disorder in which the flow of speech is disrupted by involuntary repetitions and prolongations of sounds, syllables, words or phrases, and involuntary silent pauses or blocks in which the stutterer is unable to produce sounds". This is as I have always understood stuttering or stammering as the Brits have it.
With great surprise I learned that stuttering also occurs in sign languages. I learned this from a mailing list that deals with sign languages and SignWriting. The implications are quite profound. It means that stuttering is not necessarily a speech disorder and consequently when it is not, speech therapy does not work.
This similarity in the problems between signed and spoken languages indicate that the format of communication is incidental. The same mechanisms are at play and therefore one is as good as the other. To me this seems obvious many people rate their own method of communication as superior. The spoken language is superior for when communication with me as I am dumb when it comes to signed languages...