In the paper "Cryptanalysis of the Hash Functions MD4 and RIPEMD" the authors introduce the following notation (paragraph 4.1):

$\Delta$$H_0$ = 0 $\xrightarrow{(M_,M')}$ $\Delta$$H$ = 0

What exactly does this mean? I understand that $\Delta$$H$ is probably the difference between hash outputs, but what about $\Delta$$H_0$?

Disclaimer: I have zero knowledge about differential cryptanalysis, so my apologies if the formula above is some kind of standard notation there.