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Stereopsis (three dimensional vision) is important in our every day lives. This ability is dependent on aligning both foveae on the same object with slightly dissimilar perspectives to give a perception of depth. The loss of ability to align and co-ordinate our eyes result in the loss of stereopsis, and can lead to diplopia (double vision), especially when loss of co-ordination is acute in onset.
Hold a torch an arm’s length away from the patient and shine it equally on both eyes. Look at the position of the corneal light reflection (not the red light reflex). This is usually at the centre of each pupil. If one side or the other is towards the outer edge, this indicates an inward deviation of the globe (esotropia) and if there is a reflex more towards the inner edge of the pupil, there is an outward deviation of the globe (exotropia).
- Sit the patient directly opposite you at one arm’s length distance
- Advise the patient you will help keep his/her head still during the examination
- Advise the patient to fixate on a target (red pin or finger) and follow the object with their eyes only
- Gently but firmly place a hand on their forehead and with the other, test all the positions of gaze in that hemifield. Swap hands and do the same in the other hemifield. Look for limitation of globe movement, presence of nystagmus and ask for diplopia, blurring or loss of the image. By the end you will have drawn a union jack with your hand movements
An abnormality in ocular alignment in the primary position is called a squint. After identification of a squint, they can further classified as concomitant or incomitant on testing the extraocular movement. A concomitant squint is one where the angle of deviation between the two eyes remains the same at all directions of gaze. The underlying cause is usually a co-ordination problem between the two eyes and is the most common form of squint. These patients usually do not have diplopia as the brain has been given time to suppress one of the two images. An incomitant squint is one where the angle of deviation between the two eyes changes according to the direction of gaze. The underlying cause is usually a muscle paralysis (ophthalmopegia) or restriction. Paralytic conditions include cranial nerve palsies (mono-neuritis), neuromuscular junction disease (myasthenia gravis), muscle weakness (mitochondrial diseases – Kearns-Sayre syndrome). Restrictive conditions include orbital fractures with herniation/entrapment of extra-ocular muscles, or inflammatory diseases of the extraocular muscles (thyroid eye disease).
Approach to a patient with diplopia
When coming across a patient reporting diplopia, the most important question to ask is whether the double vision is still present when either eye is closed.
Still present on covering either eye – Monocular diplopia
- Reversed with pinhole, indicates local ocular diseases such as refractive errors, cataracts, eccentric intraocular lens implants, misaligned spectacles, non-organic causes
- There is one main image and one ghost image (less clear), they are always touching
Absent when covering either eye – Binocular diplopia
- Indicates the likely presence of an incomitant squint
- There is no ghost image. Both images are as sharp as each other
- Examine extraocular movement, pupils and eyelid (for mass)
- Always remember to rule out a dangerous orbital condition when multiple cranial nerves are involved: orbital cellulitis, local invasion of the orbital apex by a tumor (most commonly nasopharyngeal carcinoma in Hong Kong), cavernous sinus thrombosis. If the superior orbital fissure or cavernous sinus are suspected to be involved, always check ipsilateral corneal sensation (V1) and cheek sensation (V2)
- Four orbital conditions you must not miss: orbital lymphoma, lacrimal gland epithelial tumors, mucormycosis, giant cell arteritis
- To investigate for orbital involvement consider neuroimaging:
o Computed tomography scan of the orbit with contrast: better for pre-operative planning as the extent of bone erosion can be assessed
o Magnetic resonance imaging of the orbit with contrast: better for determining the extent of soft tissue lesions
Diagnostic considerations for isolated cranial nerve palsies affecting extraocular movement
Whilst most acquired isolated oculomotor and abducent nerve palsies are due to mononeuritis, it is important to note than 16-19% of oculomotor nerve palsies are caused by aneurysms. This is a potentially life-threatening condition and an oculomotor nerve palsy may be the only prodrome to a ruptured aneurysm. For a ruptured aneurysm 45% of patients die before reaching the hospital. Of the remaining 55%, only 20% are alive at 10 years. Furthermore 65% of survivors suffer permanent cognitive impairment. The most common location of the aneurysm is at the junction of the internal carotid artery and the posterior communicating artery. If an aneurysm is suspected, an angiogram is indicated.
The most common cause of acquired trochlear nerve palsy is head trauma.
Magnetic resonance angiogram
- Detects 98% of all aneurysms
- High sensitivity for lesions 3 mm or larger
- However a magnetic resonance angiogram is difficult to obtain on an emergency basis
Computed tomography angiogram
- 100% sensitive for large aneurysms
- 65% sensitive for small aneurysms
- However, no patient with a negative computed tomography angiogram scan has ever had ruptured aneurysms in reported literature
Important differential diagnosis of isolated cranial nerve palsies affecting extraocular movement – Myasthenia gravis
Always consider myasthenia gravis as a differential diagnosis for a patient presenting with ophthalmoplegia. Signs to look for include ptosis and ophthalmoplegia, especially with associated fatigability. Ocular myasthenia gravis is common in Asia and there may be no other systemic signs. This condition is able to mimic any disorder causing binocular diplopia except for pupil involving oculomotor nerve palsy. The type of ophthalmoplegia may vary at different times of day and different clinic visits. Myasthenia gravis can occasionally mimic inter-nuclear ophthalmoplegia, however saccadic movement is not reduced.
Specific tests for ocular myasthenia gravis
- Cogan’s lid twitch: Advise the patient to look down for 5-15 seconds. This frees up as many acetylcholine receptors as possible. Then ask the patient to look straight ahead. Patients with myasthenia gravis may show a twitch on looking straight ahead due to hypersensitivity of the neuromuscular junction.
- Enhancement of ptosis: This is based on Hering’s law of equal innervation of both eyelids. If you manually elevate one eyelid, there is decreased muscle tone on the ipsilateral and contralateral eyelids, causing the contralateral side to come down. This is exacerbated in patients with myasthenia gravis.
- Osher’s peek sign: Ask the patient to close their eyes tightly. Myasthenic patients have weak orbicularis oculi and thus one or both eyes may appear to be peeking.
- Sleep test: Asking the patient to sleep in a quiet room for 30-45 minutes may show marked enhancement of eyelid and extraocular muscle function.
- Ice pack test: Reduction of temperature reduces anti-AChR antibody activity. Place an icepack on both closed eyes for 2 minutes and then reassess eyelid and extraocular muscle function. Myasthenic patients will show improvement.
For generalized myasthenia gravis, cholinesterase inhibitors are the mainstay for treatment. However this has been shown to be of limited effectiveness in ocular myasthenia gravis. Instead systemic steroids or other immunosuppressant agents are 2-3 times more likely to improve ocular systems than cholinesterase inhibitors. There is also a role for systemic steroids in preventing secondary development of generalized myasthenia gravis.
Common mistakes in examinations
- Performed too quickly, preventing patients from properly reaching the limits of their extra-ocular movement
- Failing to check for diplopia at extremes of gaze