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This is an external photograph of the patient’s face in primary position.  My attention is immediately drawn to the ptosis of the left upper eyelid as well as the anisocoria with the left pupil being smaller than the right pupil.

I would ask the patient how long he has experienced the droopy eyelid and the difference in pupil size.  I would ask him if these findings were getting worse with time.  I would ask if there were any changes in his vision and if he was experiencing double vision.  I would ask if there was any associated eye pain.  I would ask if there was any previous intraocular surgery or eye trauma.  I would ask if he was taking any eyedrops including miotics like pilocarpine.  I would ask if there was any antecedent head trauma and whether he has other known medical problems such as lung cancer or previous stroke.  I would ask if he has any past surgical history specifically of the neck or chest.  On review of systems, I would specifically ask about new headaches, neck pain, new cough, shortness of breath, hemoptysis, numbness, weakness, ataxia, and neck swelling.

I would perform a comprehensive exam of both eyes.  In particular, I would perform a pupillary exam and determine if the anisocoria was worse in bright or dark-lit conditions.  I would look for any abnormalities of the pupillary border and/or iris in either eye.  I would look for signs of current or previous intraocular inflammation in the form of anterior or posterior synechiae.  I would look for any neovascularization of the iris and/or angle that could cause pupillary abnormalities.  I would perform a dilated fundus exam only after my adjunctive testing for the anisocoria.

The pupillary measurements show anisocoria that is greater in dark-lit compared to bright-lit conditions.

My differential diagnosis of anisocoria that is greater in dim lit compared to bright lit conditions is:  

Given the ptosis, the most likely diagnosis is Horner’s syndrome.

Since the anisocoria is greater in dim light, the left pupil is the abnormal one.  If there was no mechanical restriction to dilation, then I would perform either cocaine testing or apraclonidine testing.

Since the anisocoria is still greater than 1 mm after cocaine testing, this is diagnostic of Horner’s syndrome.  I would then proceed with determining which level Horner syndrome this was by performing 1.0% hydroxyamphetamine testing.  This testing involves instilling 1.0% hydroxyamphetamine in both eyes.  If both pupils dilate, then this is either a 1st- or 2nd-order Horner’s syndrome.  If the pupil does not dilate with hydroxyamphetamine, then this is a 3rd-order Horner’s syndrome.

OD:  4.0 mm

OS:  2.4 mm

Since the small pupil still does not dilate after instillation of hydroxyamphetamine, this is a third-order Horner’s syndrome.  Causes of 3rd-order Horner’s syndrome include:  carotid artery dissection, tumors or infections near the base of the skull, migraine, and cluster headache.  Given these potentially very serious causes, I would obtain an emergent MRI/MRA or CT-angiography of the head, neck, and chest.

I would immediately have this patient evaluated by either a vascular surgeon, neurosurgeon, or interventional radiologist since this can be a life-threatening condition.  These physicians will likely initiate anticoagulation or antiplatelet therapy or even consider intravascular stenting.

I would counsel the patient that a carotid artery dissection is an emergency that could lead to stroke with permanent neurologic deficits or even death.