The ciliospinal reflex (pupillary-skin reflex) consists of dilation of the ipsilateral

pupil The pupil is a black hole located in the center of the iris of the eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company. It appears black ...

in response to pain applied to the neck, face, and upper trunk. If the right side of the neck is subjected to a painful stimulus, the right pupil dilates (increases in size 1-2mm from baseline). This reflex is absent in

Horner's syndrome Horner's syndrome, also known as oculosympathetic paresis, is a combination of symptoms that arises when a group of nerves known as the sympathetic trunk is damaged. The signs and symptoms occur on the same side (ipsilateral) as it is a lesion ...

and lesions involving the cervical sympathetic fibers. The enhanced ciliospinal reflex in asymptomatic patients with

cluster headache Cluster headache (CH) is a neurological disorder characterized by recurrent severe headaches on one side of the head, typically around the eye(s). There is often accompanying eye watering, nasal congestion, or swelling around the eye on the a ...

is due to preganglionic sympathetic mechanisms.

References

*http://medical-dictionary.thefreedictionary.com/ciliospinal+reflex *http://cancerweb.ncl.ac.uk/cgi-bin/omd?ciliospinal+reflex *http://jnsonline.org/jns/issues/v90n4/pdf/n0900644.pdf{{dead link, date=August 2017 , bot=InternetArchiveBot , fix-attempted=yes Reflexes