hsp is pure umn dysfunction that mainly effects just the legs,pls gradually over years/decades effects upper body and bulbar areas as well.
at the start it's hard to say which one it is,as like myself it started in my legs first.
your husbands neuro really needs to follow his progression for 3-5yrs to see if the umn symptoms spread.
i presume if hsp is suspected he has no lmn signs/symptoms as yet to suspect
als.
hang in there,we are all here to help.
take good care.
caroline