A couple more answers have come in - some say it is breed specific. What breed is your dog? The following answer gives a website for this problem:
This problem affects all breeds but some do seem to suffer more than others
I've heard of several Beardies, greyhounds GSDs, Labs and even mongrels
that have been affected.
The posh name for it is Symmetrical Lupoid Onychodystrophy and vets here
have very, very little experience of the problem. Steroids should only be
used as a very last resort, certainly not as the first treatment option.
Most specialists find the results with steroids are not worth the risks. The
first line of defence seems to be with EFA and EPAs, other dogs respond best
to a combination of the antibiotic tetracycline, EFAs and niacinamide. NOT
STEROIDS!!!
There is a yahoo group to help owners with affected dogs at
http://www
bloodaxe.com/SLO.html and their files include treatment protocols etc.
A dog may have one episode in its life or suffer repeated attacks....but
with the correct treatment (NOT STEROIDS) the owner should be able to keep
the problem under control.