  Cu(C$_8$H$_6$N$_2$)Cl$_2$, a strong-rung spin-1/2 Heisenberg ladder compound,
is probed by means of electron spin resonance (ESR) spectroscopy in the
field-induced gapless phase above $H_{c1}$. The temperature dependence of the
ESR linewidth is analyzed in the quantum field theory framework, suggesting
that the anisotropy of magnetic interactions plays a crucial role, determining
the peculiar low-temperature ESR linewidth behavior. In particular, it is
argued that the uniform Dzyaloshinskii-Moriya interaction (which is allowed on
the bonds along the ladder legs) can be the source of this behavior in
Cu(C$_8$H$_6$N$_2$)Cl$_2$.
