# Detect highest order of derivative in expression?

Is it possible to detect the highest order (or generally all orders) of derivatives in an expression?

Consider

eqn = D[a[x,t],t] == D[a[x,t],x,x]


How would I go about detecting that the highest order of the partial derivative in x is 2 in eqn?

I was thinking of using MatchQ, however this

MatchQ[Derivative[2, 0][a][x, t], eqn]


just returns False.

Thank you!

-

Cases[eqn, Derivative[orders__] -> {orders}, Infinity, Heads -> True]