01Purpose and principlesWhat the assessment is for and the core concepts behind it.
Diplopia means that a single object is perceived as two images. Patients also use the word 'double' for ghosting, poor focus or distortion, so begin by clarifying the experience. Ask whether the images are side by side, one above the other or tilted, and whether the separation changes with gaze or working distance. Then cover each eye in turn. Binocular diplopia disappears with either eye covered because misalignment requires input from both eyes. Monocular diplopia persists when the affected eye views alone and more often relates to the optical system, including tear film, cornea, refractive error or lens.
Ocular alignment depends on extraocular muscles, their cranial nerves, brainstem coordination, supranuclear control and neuromuscular transmission. Restrictive orbital disease can also prevent movement despite an intact nerve supply. This explains why a movement pattern should be interpreted alongside pain, pupils, lids, proptosis, visual function and the rest of the neurological examination. Ask about childhood squint, amblyopia, previous prisms, eye surgery and old photographs showing a compensatory head posture. A decompensated longstanding alignment problem is possible, but it should not be assumed before considering an acute neurological cause.
Observe the patient looking straight ahead before asking them to follow a target. Note a head turn or tilt, drooping or retracted lids, unequal pupils and obvious deviation. Hold a fixation target at a comfortable distance and move it smoothly through horizontal, vertical and diagonal positions, avoiding extreme positions that produce physiological discomfort or end-gaze nystagmus. Ask where two images appear or separate most, watch whether either eye fails to keep up and ask about pain. Keep the head still without physically restricting a patient with trauma or significant discomfort. Test individual eye movement if the binocular observation leaves uncertainty.
A cover-uncover test assesses manifest misalignment. With the patient fixing an appropriate target, cover one eye and watch the uncovered eye for a movement to take up fixation. Movement indicates that it was not directed at the target before the fellow eye was covered. An inward refixation movement means that eye was previously deviated outward; an outward movement corrects a prior inward deviation. Test both sides and consider distance and near because alignment may differ. Alternate cover testing interrupts fusion and can reveal the total dissociated deviation, including a latent component. These tests require clear fixation and experience; record what moved and in which direction rather than attaching an unsupported diagnostic label.
Classic patterns are useful but imperfect. Sixth-nerve weakness limits abduction and often causes horizontal separation worse at distance and on looking toward the affected side. Fourth-nerve dysfunction often produces vertical or torsional symptoms, particularly on looking down with the eye adducted. Third-nerve dysfunction can affect adduction, elevation and depression with ptosis and sometimes pupil involvement. A brainstem internuclear disorder can impair adduction with abducting nystagmus of the fellow eye. Partial palsies and restrictive disease may not fit a textbook pattern, so atypical findings increase the need for specialist localisation.
Intermittent or variable symptoms deserve a careful history rather than automatic reassurance. Fatigable diplopia and ptosis may suggest myasthenia, usually without an abnormal pupil response. Ask specifically about swallowing, speech, neck weakness and breathlessness; bulbar or respiratory involvement changes the urgency. Thyroid-associated orbitopathy can restrict movements and cause proptosis or lid retraction, and reduced acuity or colour function raises concern about optic-nerve compromise. After trauma, pain, nausea and restricted movement may indicate entrapment, including in children whose external bruising is deceptively mild.
Management starts with the cause and urgency. New binocular diplopia should receive prompt clinical assessment; severe headache, abnormal pupils, additional neurological signs or orbital warning symptoms requires emergency escalation. Short-term covering of one eye can reduce binocular confusion, but affects depth perception and field use and does not treat the disorder. Children require specialist advice before a sustained occlusion plan because visual development matters. Explain immediate safety precautions and arrange a defined referral or review rather than asking the patient simply to return if symptoms become intolerable.
Key points
- Ask the patient to describe two distinct images, blur or distortion, then test the effect of covering each eye in turn.
- Binocular diplopia resolves when either eye is covered; monocular diplopia remains when the affected eye views alone.
- Record onset, intermittency, image separation, gaze dependence, near versus distance symptoms, pain, trauma and neurological or bulbar symptoms.
- Inspect head posture, lids, pupils, alignment and proptosis before assessing movement through the principal gaze positions.
- Observe both eyes together and individual movements where needed; use cover testing to assess alignment rather than relying on gross movement range alone.
- A cranial-nerve pattern is a localisation clue, not proof of a benign cause; acute or unexplained binocular diplopia requires prompt assessment.
- Address falls, reading and driving safety; temporary occlusion may relieve symptoms while the underlying cause is assessed but does not establish fitness to drive.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
If doubling remains with the symptomatic eye open and the other covered, examine optical and anterior-segment causes and assess pinhole response. Persistent monocular symptoms still require explanation, especially with pain or reduced acuity.
Images separating more in a particular direction suggests an alignment or motility problem. Document the patient's description alongside observed limitation rather than inferring the responsible muscle from image separation alone.
Ask whether symptoms change during the day or with sustained use and look for variability during assessment. Associated dysphagia, dysarthria or breathing difficulty requires urgent medical evaluation for possible generalised neuromuscular weakness.
Proptosis, lid swelling, reduced vision, fever or a traumatic mechanism changes the differential toward orbital disease. Restriction after an orbital injury needs assessment even when the eye itself appears relatively quiet.
03Method and interpretationA systematic approach to the test and its findings.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Sequential monocular occlusionFirst step - Why
- Distinguish binocular misalignment from doubling that persists through one eye.
- Interpretation and limitations
- Cover the right and then left eye while the symptom is present. Document exactly which viewing condition preserves the double image; a vague statement that covering one eye helps can be misleading.
- 02
Alignment and movement through gaze positions - Why
- Identify limitations, pain and gaze-dependent changes in the symptom.
- Interpretation and limitations
- Compare both eyes together, then inspect individual movements if required. Avoid overinterpreting extreme gaze and record accompanying ptosis, nystagmus, pupil abnormalities and any restriction.
- 03
Cover-uncover and alternate cover testing - Why
- Assess manifest deviation and the effect of interrupting binocular fusion.
- Interpretation and limitations
- Observe refixation movements with a clear distance or near target and document the method. Poor acuity, poor fixation and incomplete understanding can limit the result and warrant orthoptic assessment.
- 04
Acuity, colour, pupils and visual fields - Why
- Detect associated retinal, optic-nerve or neurological dysfunction.
- Interpretation and limitations
- A new afferent deficit, reduced colour perception or field abnormality adds urgency and may point beyond an isolated ocular motor disorder. Preserve baseline pupil findings before any dilating medication.
- 05
Specialist neurological, orbital or neuromuscular investigation - Why
- Identify the cause of a clinically localised movement disorder.
- Interpretation and limitations
- Imaging, laboratory tests and orthoptic measurements depend on the syndrome and progression. An acute severe headache or pupil-involving palsy must not wait for routine outpatient motility testing.
04Clinical next stepsHow the result changes management or prompts escalation.
01Symptom definitionEstablish what the patient seesFirst stepA patient reports new or recurrent doubling, ghosting or visual confusion.+
- 1Clarify onset, image orientation and gaze dependence, then cover each eye in turn to distinguish monocular from binocular symptoms.
- 2Ask about pain, headache, trauma, previous squint, eye surgery, medicines and neurological or swallowing and respiratory symptoms.
- 3Record monocular acuity and inspect pupils, lids, head posture and orbital appearance before detailed movement testing.
02LocalisationCombine alignment with movement findingsBinocular symptoms suggest an ocular alignment or motility disorder.+
- 1Examine versions through comfortable gaze positions, ask where diplopia increases and assess individual ductions if the limitation is unclear.
- 2Use appropriate cover testing and extend the examination to fields, cranial nerves and relevant systemic signs, noting variable or restrictive patterns.
- 3Arrange ophthalmic or orthoptic and neurological assessment according to onset and findings, avoiding an unsupported benign label for an acute palsy.
03Urgent actionRespond to accompanying danger signsDiplopia occurs with severe headache, pupil change, neurological deficit or orbital warning symptoms.+
- 1Contact the emergency service best placed to assess the suspected neurological or orbital process and communicate the time course and complete examination.
- 2Provide safe assisted transfer, assess any bulbar or respiratory weakness promptly and avoid delaying urgent investigation for symptom-relieving optical treatment.
- 3Explain that temporary relief on covering an eye does not make the underlying problem safe, and document immediate restrictions on driving and hazardous tasks.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- Record alignment, eye movement limitations and associated pupil and lid findings in a reproducible way, including whether symptoms are constant or intermittent.
- At review ask about new pain, headache, visual loss, additional cranial-nerve symptoms or generalised weakness, which may change an earlier working diagnosis.
- Coordinate orthoptic follow-up for prisms, occlusion or other symptomatic treatment while ensuring investigation of the cause has an identified responsible team.
- With new diplopia advise stopping driving and checking notification requirements: DVLA applies in Great Britain and DVA in Northern Ireland; symptom relief alone does not grant clearance.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Full gross movements do not prove alignment
A small deviation can produce significant binocular diplopia while both eyes appear to move through the full range. Cover testing and orthoptic measurements can identify misalignment that a simple tracking examination misses.
The pupil-sparing shortcut is unsafe
Diabetes and hypertension can coexist with compressive or other neurological disease. A normal pupil does not independently exclude an aneurysm in an acute incomplete or evolving third-nerve palsy, so prompt specialist assessment remains necessary.
Two mechanisms can coexist
A patient may have optical ghosting from an irregular cornea and a separate binocular deviation. Repeat the occlusion sequence carefully if the description does not fit a single mechanism, and examine each eye individually.
Covering an eye changes daily function
Occlusion can reduce binocular double vision but also changes depth judgement and the use of peripheral vision. Discuss stairs, mobility and work, and obtain specialist advice before a prolonged patching plan, especially in a child.
07Common pitfallsFrequent interpretation and management errors.
- 01
Assuming that improvement when one eye is covered proves a benign cause rather than simply establishing binocular diplopia.
- 02
Diagnosing a microvascular palsy from age or diabetes without checking pupils, other neurological signs and the full movement pattern.
- 03
Forgetting proptosis, visual function and painful restriction when an apparent cranial-nerve pattern may instead arise from orbital disease.
- 04
Offering a patch as a complete management plan without investigating new symptoms or addressing driving and mobility safety.