Diplopia, or double vision, is a symptom rather than a diagnosis, and the single most useful first step in evaluating it is a question many exams skip: does the double vision persist when one eye is covered, or does it resolve.
That one distinction, monocular versus binocular diplopia, splits the entire differential into two largely separate pathways, and getting it wrong at the outset can send a workup in a genuinely unproductive direction from the very first step.
Many of the specific causes of binocular diplopia are discussed in their own dedicated articles on this site, but the value of a structured, symptom-first approach is in organizing that differential logically rather than working through an unstructured list of possible diagnoses.
Step One: Monocular or Binocular
Simply covering one eye at a time and asking whether the double vision persists is the fastest, highest-yield question in the entire evaluation, and it should be asked before any further testing is pursued.
Monocular diplopia, persisting even with the fellow eye covered, localizes the problem to the affected eye itself, most commonly from an optical or ocular surface cause rather than any misalignment or neurological process.
Binocular diplopia, resolving completely when either eye is covered, means the two eyes are simply not pointing at the same target, and the differential shifts toward ocular misalignment from a muscle, nerve, or neuromuscular junction cause.
Causes of Monocular Diplopia
- Uncorrected or irregular refractive error, including significant astigmatism, one of the most common and most easily corrected causes
- Cataract, particularly early nuclear or posterior subcapsular change, distorting light passing through the lens
- Corneal surface irregularity, including significant dry eye disease or corneal scarring, disrupting the normally smooth refracting surface
- Dislocated intraocular lens or a decentered lens implant, producing edge effects or a doubled image as light passes partly through and partly around the lens
A careful refraction, pinhole testing, and slit-lamp exam of the cornea and lens generally identify the cause of monocular diplopia without needing any neurological workup, since by definition the problem sits entirely within the affected eye’s own optical pathway.
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From Choroida — the team behind this siteCauses of Binocular Diplopia
Cranial nerve palsies affecting the third, fourth, or sixth nerve, each discussed in its own dedicated article on this site, are common causes, and the specific pattern of misalignment helps identify which nerve is involved.
Restrictive causes, including thyroid eye disease and Brown syndrome, both discussed in their own dedicated articles on this site, physically limit eye movement mechanically rather than through a nerve or muscle weakness.
Neuromuscular junction disease, most notably ocular myasthenia gravis, produces variable, fatigable misalignment that can mimic almost any pattern of cranial nerve palsy, which is part of why it deserves specific consideration whenever the pattern of diplopia seems to shift or vary.
Brainstem processes, including the crossed syndromes discussed in relation to Foville and Millard-Gubler syndrome in their own dedicated articles on this site, and orbital processes such as orbital apex syndrome, discussed in its own dedicated article on this site, round out the differential for less common but often more urgent causes.
Refining the Binocular Differential
The pattern of misalignment, whether horizontal, vertical, or a combination, and whether it is comitant, the same in all directions of gaze, or incomitant, varying with gaze direction, narrows the differential considerably.
A comitant deviation more often suggests a longstanding, decompensated strabismus or a neuromuscular junction process, while an incomitant deviation points more specifically toward a particular muscle or nerve being involved.
Associated symptoms and signs, including ptosis, pupillary involvement, pain, or other neurological findings, help localize further and often point toward one specific diagnosis discussed in more detail in its own dedicated article on this site.
Red Flags Warranting Urgent Evaluation
New diplopia with pain, particularly with pupillary involvement, raises concern for a compressive third nerve palsy and needs urgent neuroimaging, since this specific combination has a meaningfully different urgency than a pupil-sparing presentation.
Diplopia accompanied by other neurological symptoms, sudden onset in a patient with vascular risk factors, or a pattern suggesting brainstem involvement all warrant urgent rather than routine evaluation.
Diplopia in a child, particularly acute onset, deserves prompt evaluation as well, given the different and sometimes more concerning differential in a pediatric patient compared to an adult with the same symptom.
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From Choroida — the team behind this siteReferences
- American Academy of Ophthalmology. Basic and Clinical Science Course, Section 5: Neuro-Ophthalmology.
- Liu GT, Volpe NJ, Galetta SL. Neuro-Ophthalmology: Diagnosis and Management.
- Bowling B. Kanski’s Clinical Ophthalmology.
Test yourself
A few questions straight from this article.
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What is the single most useful first step in evaluating a patient with diplopia?
Covering one eye at a time and asking whether the double vision persists is the fastest, highest-yield question in the whole evaluation, and it splits the differential into two largely separate pathways. -
Which cause produces monocular diplopia?
Monocular diplopia comes from an optical or ocular surface cause in the affected eye itself, such as uncorrected refractive error, cataract, corneal irregularity, or a dislocated or decentered lens implant. -
Which workup usually identifies the cause of monocular diplopia?
A careful refraction, pinhole testing and slit-lamp examination of cornea and lens generally identify the cause without any neurological workup, since the problem sits entirely within that eye's optical pathway. -
Diplopia that resolves whenever either eye is covered indicates what?
Binocular diplopia means ocular misalignment, and the differential shifts toward a muscle, nerve or neuromuscular junction cause rather than anything optical. -
Which condition causes variable, fatigable misalignment that can mimic almost any nerve palsy?
Neuromuscular junction disease, most notably ocular myasthenia gravis, produces variable fatigable misalignment that can mimic almost any pattern of cranial nerve palsy, so it deserves consideration whenever the pattern shifts or varies. -
How do restrictive causes such as thyroid eye disease limit eye movement?
Restrictive causes, including thyroid eye disease and Brown syndrome, physically limit eye movement mechanically rather than through a nerve or muscle weakness. -
A comitant deviation, the same in all directions of gaze, more often suggests what?
A comitant deviation more often suggests a longstanding decompensated strabismus or a neuromuscular junction process, while an incomitant deviation points toward a particular muscle or nerve. -
New diplopia with pain and pupillary involvement raises concern for what?
New diplopia with pain, particularly with pupillary involvement, raises concern for a compressive third nerve palsy and needs urgent neuroimaging; this combination carries a meaningfully different urgency from a pupil-sparing presentation. -
Which feature makes new diplopia urgent rather than routine?
Diplopia with other neurological symptoms, sudden onset in a patient with vascular risk factors, or a pattern suggesting brainstem involvement all warrant urgent rather than routine evaluation. -
Why does acute diplopia in a child warrant prompt evaluation?
Diplopia in a child, particularly of acute onset, deserves prompt evaluation because the differential is different and sometimes more concerning than in an adult with the same symptom.
