Testing for diplopia (double vision) involves determining if the double vision is present with one eye closed (monocular) or only when both eyes are open (binocular), and then identifying the underlying cause.
Initial Assessment
- Assess Visual Acuity: Check visual acuity with distance glasses or contact lenses, if normally worn, in each eye separately.
- Identify Monocular vs. Binocular Diplopia:
- Have the patient describe the double vision.
- Ask the patient to cover one eye, then the other.
- If the double vision persists when one eye is covered, it's monocular diplopia. This is usually due to problems within the eye itself (e.g., astigmatism, corneal irregularities, cataracts, dislocated lens).
- If the double vision disappears when either eye is covered, it's binocular diplopia. This indicates misalignment of the eyes and requires further investigation.
Testing for Binocular Diplopia
If binocular diplopia is suspected, the following tests can help determine the cause:
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Corneal Reflex Test (Hirschberg Test): Shine a penlight from about 33 cm and observe the reflection of the light on each cornea. Normally, the light reflex should be slightly nasal to the center of the pupil in each eye. Asymmetry in the location of the reflexes suggests misalignment.
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Cover Test:
- Uncover Test: Ask the patient to fixate on a target. Cover one eye and observe the uncovered eye. If the uncovered eye moves to fixate, it indicates a phoria (tendency for the eyes to misalign).
- Cover-Uncover Test: Cover one eye and observe the other eye. Then, quickly uncover the first eye and observe it for any movement as it takes up fixation. This test can detect both phorias and tropias (manifest misalignments).
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Ocular Motility Assessment: Evaluate the full range of eye movements (up, down, left, right, and diagonally). Look for any restrictions or weakness in eye movements, which can indicate cranial nerve palsies or other neuromuscular problems. Document which direction of gaze worsens the diplopia.
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Red Lens Test: Place a red lens in front of one eye. Ask the patient to look at a white light. The patient will see a red light and a white light (or a pinkish light if the images are close together). The relative positions of the red and white lights can help determine which muscle is affected.
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Prism Neutralization: Place prisms in front of one or both eyes to realign the images. The amount of prism needed to eliminate the double vision can quantify the degree of misalignment.
Further Investigations
Based on the findings from the above tests, further investigations may be necessary to determine the underlying cause of the diplopia. These may include:
- Neuroimaging (MRI or CT scan): To rule out brain tumors, aneurysms, or other neurological conditions.
- Blood Tests: To evaluate for conditions like myasthenia gravis or thyroid eye disease.
Examples
- Example of Monocular Diplopia: A patient with a significant uncorrected astigmatism might experience monocular diplopia due to the distorted image being projected onto the retina.
- Example of Binocular Diplopia: A patient with a third cranial nerve palsy might experience binocular diplopia because the affected nerve controls muscles responsible for eye movement, leading to misalignment.
