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Clinically pure PLS can be defined by isolated UMN signs 4 years after symptom onset, and is a syndrome of slow progression with high levels of function. Prior to the fourth year, the diagnosis of PLS cannot be made with certainty because many patients develop LMN signs, UMN-dominant ALS, defined by predominantly UMN disease with minor LMN signs, has disability similar to ALS, but slower progression. |
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