Pupil fails to dilate by giving cocaine in –
In periphery cocaine blocks the uptake of Adrenaline and Noradrenaline in Adrenergic nerve ending resulting in higher concentration of sympathetic neurotransmitters (Sympathomimetic effect) :
The peripheral sympathomimetic action of cocaine produces following effect :


Mydriasis
Rise in B.P.
Local vasoconstriction
Tachycardia


Therefore if the peripheral sympathetic system is paralysed it will lead to reverse of the above mentioned actions.