Horner’s syndrome causes a droopy lid and smaller pupil on one side of the face. The cause of Horner’s syndrome is an interruption of the sympathetic nerve supply to the eye and lid.
What are the sympathetic nerves?
The autonomic nervous system (the nervous system that unconsciously controls your body’s blood pressure, breathing, heart rate etc.) consists of two arms: the sympathetic and the parasympathetic. The sympathetic controls the “fight or flight” response. In terms of the eye, the sympathetic nerves open the eye wider and dilate the pupil. In contrast, the parasympathetic arm is dominant during relaxation, which lowers the lid and shrinks the pupil. When the sympathetic arm is interrupted, the parasympathetic arm dominates, causing the findings.
What can cause a Horner’s syndrome?
Any disease that interrupts the sympathetic nerves can cause this. The sympathetic nerves to the eye take a long route. They originate in the brain, descend along the spinal column to the chest, then ascend along the carotid arteries to the back of the eye. Therefore, a wide variety of conditions can cause a Horner’s syndrome. The most concerning are tumors or other growths. Also quite serious is a carotid artery dissection, which is associated with neck, jaw, and head pain on that side.
How is Horner’s syndrome diagnosed?
Eye drops are often used to diagnose this condition. The two findings of a Horner’s syndrome, a droopy lid and asymmetric pupils, can occur normally in some individuals, so accurate diagnosis is important. The classic diagnostic drop is one made from cocaine, which will dilate a normal pupil, but not a Horner’s pupil. Because of the difficulty obtaining cocaine, apraclonidine drops are often used, which will constrict a normal pupil, but dilate a Horner’s pupil, causing a reversal of the pupil asymmetry.
What other tests will be performed?
To rule out serious underlying causes, MRI imaging will be performed. Horner’s syndrome in adults is often “idiopathic,” meaning no cause is ever found.
The North American Neuro-Ophthalmology Society website has good patient information on this condition.