Evaluation of Horner's syndrome begins with a history and physical examination.

Physical findings in someone with may include:

• Drooping eyelid on one side of the face
• Constriction of the pupil on the same side of the face
• Absence of sweating on the same side of the face and neck

The diagnosis of Horner's syndrome is made during physical exam. Tests may required in order to find the cause of Horner's syndrome.

Tests that may be used to evaluate Horner's syndrome include:

• CT scan of the brain
• MRI scan of the brain
• CT scan of the spinal cord
• MRI scan of the spinal cord
• Chest x-ray
• CT scan of the chest
• MRI scan of the chest

The instillation of a 4% solution of cocaine into the eye can help differentiate between central and peripheral sympathetic pathway abnormalities. The diagnosis, but not the cause, is usually obvious after a physical examination.