Contraction of the external sphincter ani muscle in response to a scratch stimulus in the perianal region, testing the integrity of the S4/S5 roots, forms the anal or wink reflex. This reflex may be absent in some normal elderly individuals, and absence does not necessarily correlate with urinary incontinence. External anal responses to coughing and sniffing are part of a highly consistent and easily elicited polysynaptic reflex, whose characteristics resemble those of the conventional scratch-induced anal reflex.
Swash M, Chan CLH, Ponsford S. The anal reflex can be elicited by cough and sniff – validation of a clinical sign. Journal of Neurology, Neurosurgery and Psychiatry 2004; 75: 521 (abstract 027)