I want to simplify the following expression
$$ \sum_{x=0}^{D+m} \frac {x!} {(n+1)_x} (\frac {\theta_{eff}} {\mu})^x {D+m \choose x} $$
The parameters $D, m, n, \theta_{eff}, \mu, $ are constants. Can anybody help me in approximating the expression?
Regards