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 of the sympathetic trunk. It is characterized by miosis (a constricted pupil), partial ptosis (a weak, droopy eyelid), apparent anhidrosis (decreased sweating), with apparent enophthalmos (inset eyeball). The nerves of the sympathetic trunk arise from the spinal cord in the chest, and from there ascend to the neck and face. The nerves are part of the sympathetic nervous system, a division of the autonomic (or involuntary) nervous system. Once the syndrome has been recognized, medical imaging and response to particular eye drops may be required to identify the location of the problem and the underlying cause. Signs and symptoms Signs that are found in people with Horner's syndrome on the affected side of the face include the following: ptosis (drooping of the upper eyelid) anhidrosis (decreased sweating) miosis (constriction of the pupil) Enophthalmos (sinking of the eyeball into the face) inability to completely close or open the eyelid facial flushing headaches loss of ciliospinal reflex bloodshot conjunctiva, depending on the site of lesion. unilateral straight hair (in congenital Horner's syndrome); the hair on the affected side may be straight in some cases. heterochromia iridum (in congenital Horner's syndrome) Interruption of sympathetic pathways leads to several implications. It inactivates the dilator muscle and thereby produces miosis. It inactivates the superior tarsal muscle which produces ptosis. It reduces sweat secretion in the face. Patients may have apparent enophthalmos (affected eye looks to be slightly sunken in) but this is not always the case. The ptosis from inactivation of the superior tarsal muscle causes the eye to appear sunken in, but when actually measured, enophthalmos is not present. The phenomenon of enophthalmos is seen in Horner's syndrome in cats, rats, and dogs. Sometimes there is flushing on the affected side of the face due to dilation of blood vessels under the skin. The pupil's light reflex is maintained as this is controlled via the parasympathetic nervous system. In children, Horner's syndrome sometimes leads to heterochromia, a difference in eye color between the two eyes. This happens because a lack of sympathetic stimulation in childhood interferes with melanin pigmentation of the melanocytes in the superficial stroma of the iris. In veterinary medicine, signs