Diplopia is a common complaint in neurology, neuro-ophthalmology, ophthalmology, and general internal medicine. It is broadly classified into monocular and binocular diplopia.
Monocular Diplopia
Definition: Diplopia that does not disappear when the unaffected eye is closed, but disappears when the affected eye is closed.
Main causes: Refractive errors (astigmatism, keratoconus), tear film abnormalities, cataracts, and other abnormalities of the eye itself. Characteristically improves with pinhole.
Rare causes: Bilateral monocular diplopia due to cortical lesions (extremely rare).
Binocular Diplopia
Definition: Diplopia that disappears when either eye is closed. Underlying ocular misalignment.
Main causes: Cranial nerve palsies such as abducens nerve palsy and trochlear nerve palsy, myasthenia gravis, thyroid eye disease.
Note: Trochlear nerve palsy is common; consider it first in vertical and torsional diplopia.
The key question to distinguish monocular from binocular diplopia is: “Does closing one eye make the double vision go away?”
If double vision persists when one eye is closed, it is monocular diplopia, often due to problems in the open eye itself (such as refractive error or cataract) rather than the closed eye. If double vision disappears when one eye is closed, it is binocular diplopia, suggesting misalignment of the eyes or cranial nerve issues.
The direction, distance, and onset pattern of diplopia provide diagnostic clues.
This is a stepwise diagnostic method to identify the causative extraocular muscle in vertical diplopia.
In the Bielschowsky head tilt test, tilting the head toward the affected side causes the affected eye to elevate (specific to trochlear nerve palsy).
In the Hess red-green test, the smaller pattern corresponds to the paretic eye, and the smallest direction corresponds to the action direction of the paretic muscle.
It is a stepwise diagnostic method to identify the causative muscle in vertical diplopia. It narrows down the causative muscle in three steps: identification of the hypertropic eye in primary position, confirmation of the gaze direction that worsens the deviation, and confirmation of the head tilt direction that worsens the deviation. A 5-step extended version is also used, adding cyclotorsion measurement with the double Maddox rod and confirmation of skew deviation in supine position.
The causes of diplopia are systematically classified according to the site of impairment in the ocular motor pathway.
Supranuclear
Progressive supranuclear palsy (PSP): Vertical supranuclear palsy (especially downgaze) + slowed saccades + postural instability. Poor response to levodopa.
Parkinson’s disease: Convergence insufficiency + exotropia worsening at near → horizontal diplopia.
Parinaud syndrome: Midbrain dorsal syndrome due to ischemia, tumor (pinealoma), ventricular enlargement, or demyelination.
Skew deviation: Vertical misalignment not localized to a specific muscle or nerve. Caused by brainstem or cerebellar vascular lesions, multiple sclerosis, or tumors. May improve in supine position.
Sagging eye syndrome: Age-related degeneration of the SR-LR connective tissue band → inferior displacement of the lateral rectus → abduction deficit → esotropia.
Nuclear and internuclear
Oculomotor nucleus lesion: Characterized by unilateral oculomotor palsy + bilateral superior rectus weakness + bilateral ptosis.
Trochlear nucleus lesion: Rare. Contralateral palsy due to decussation.
Abducens nucleus lesion: Ipsilateral horizontal gaze palsy. Cannot be overcome by head rotation.
Internuclear ophthalmoplegia (INO): Slowing of adduction saccades + contralateral abducting nystagmus. Caused by MLF lesion.
One-and-a-half syndrome: MLF + PPRF or CN VI lesion → only contralateral eye abduction remains in the horizontal plane.
Eight-and-a-half syndrome: One-and-a-half syndrome plus facial nerve nucleus disorder. Stroke is the most common cause.
Infranuclear (cranial nerve palsy)
Oculomotor nerve palsy: Ptosis, mydriasis, and extraocular muscle palsy (superior rectus, inferior rectus, medial rectus, inferior oblique). If accompanied by pupillary dilation, suspect posterior communicating artery aneurysm and require emergency management. Ischemic cases often resolve spontaneously within 1–3 months.
Trochlear nerve palsy: Common causes include trauma, microvascular ischemia, and congenital decompensation. Midline head trauma such as from motorcycle accidents can lead to bilateral involvement. Consider bilaterality if excyclotorsion is 10° or more.
Abducens nerve palsy: Horizontal diplopia that worsens with distance viewing. A Korean cohort study reported etiologies as vascular 56.6%, idiopathic 27.2%, neoplastic 5.6%, and traumatic 4.9% 1). For peripheral circulatory disorders, conservative observation for about 6 months is performed.
Orbit, muscle, and neuromuscular junction
Thyroid eye disease: The inferior rectus muscle is most commonly affected, causing limited upward gaze and diplopia on upward gaze. It presents as restrictive ophthalmoplegia.
Myasthenia gravis: Mimics any pattern of ocular muscle palsy without pain, normal pupils, and no proptosis. Commonly affects the medial rectus first, then the superior rectus. Shows diurnal variation.
Cavernous sinus lesions: Combination of CN III/IV/V1/V2/VI + Horner syndrome. Optic nerve involvement presents as orbital apex syndrome. Tolosa-Hunt syndrome is steroid-responsive (prednisone 50–60 mg/day).
Orbital metastatic tumor: Rare but requires attention to avoid oversight. Among 19 cases of orbital metastasis from pancreatic cancer, diplopia/ocular motility disorder was the most frequent symptom (81.8%), and in 66.7% of cases, orbital metastasis preceded the identification of the primary tumor6).
Infectious causes include brain abscess (a 63-year-old male with intermittent diplopia due to Prevotella oris 7)) and Gradenigo syndrome (petrous apicitis). The triad of Gradenigo syndrome is otorrhea, deep facial pain (trigeminal nerve first branch), and abducens nerve palsy, occurring as a complication of chronic otitis media. In a case caused by fluconazole-resistant Candida tropicalis, diplopia resolved within one month with appropriate antifungal therapy 8).
Oculomotor nerve palsy with pupillary dilation may be caused by compression from a posterior communicating artery aneurysm; urgent imaging (MRA) is needed to prevent subarachnoid hemorrhage from rupture. Diplopia with sudden severe headache also raises suspicion for subarachnoid hemorrhage and requires immediate evaluation.
A systematic patient history is directly linked to narrowing down the cause.
The purpose and characteristics of the main examination methods are described below.
| Examination Name | Purpose | Characteristics |
|---|---|---|
| Parks 3-Step Test | Identify the causative muscle in vertical diplopia | Systematically narrow down in 3 to 5 steps |
| Hess red-green test | Identification of paretic muscle/eye | The smaller pattern side is the paretic eye |
| Forced duction test | Differentiation between paralytic and restrictive strabismus | Resistance felt in restrictive cases |
Other major tests are as follows.
The doll’s eye phenomenon (vestibulo-ocular reflex) is useful for differentiation. VOR preserved → supranuclear lesion. Cannot be overcome → internuclear/infranuclear lesion.
Treatment of the underlying disease is the highest priority.
Performed concurrently with causal treatment.
Acquired paralytic strabismus tends to resolve spontaneously, so conservative treatment is generally performed for 6 months; if no improvement is seen, surgery is considered.
Surgical indications for abducens nerve palsy are as follows:
| Degree of palsy | Range of eye movement | Recommended procedure |
|---|---|---|
| Mild to moderate | Some abduction preserved | Horizontal muscle recession-resection |
| Severe | Does not cross midline | Vertical rectus muscle transposition |
| Severe | Does not cross midline | Full vertical rectus muscle transposition (minimally invasive) |
In thyroid eye disease, surgery is contraindicated during the inflammatory phase of the extraocular muscles. Surgery should be performed after waiting at least 6 months following stable eye position and at least 3 months after orbital decompression.
Ischemic cranial nerve palsy (caused by microvascular disorders related to diabetes or hypertension) often resolves spontaneously within 1 to 3 months. A Korean cohort study on abducens nerve palsy also recommends conservative observation for cases with peripheral circulatory disorders 1). However, if there is no improvement after 6 months, surgery should be considered.
The efferent ocular motor pathway is classified into four levels.
The rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF), the interstitial nucleus of Cajal (INC), and the posterior commissure (PC) are involved in the control of vertical gaze.
Autoantibodies against acetylcholine receptors at the postsynaptic neuromuscular junction are produced. They commonly affect extraocular muscles, which have high metabolism and are prone to fatigue, with the medial rectus followed by the superior rectus being most frequently involved. Diurnal variation (worsening in the evening or after fatigue) is characteristic.
Skew deviation is caused by an abnormality of the vestibulo-ocular reflex. In the ocular tilt reaction (OTR), the hypertropic eye intorts (in trochlear nerve palsy, it extorts). The deviation may decrease in the supine position.
Motor fusion is the ability to correct eye alignment, while sensory fusion is the ability to perceive two separate images from both eyes as a single image. The cyclofusional amplitude is wider than the vertical fusional amplitude (in adults) 5).
In a systematic review of 92 cases of trochlear nerve schwannoma, 4 of 10 patients without preoperative diplopia did not develop postoperative diplopia despite intraoperative transection of the trochlear nerve 5). It is speculated that motor and sensory fusion were acquired during the slow tumor growth process, allowing avoidance of diplopia even after surgery.
Isolated abducens nerve palsy associated with COVID-19 infection has been reported. A summary of 11 previously reported cases showed a median age of 44 years (range 32–71), median time from onset to diplopia of 4 days (range 3–27), bilateral involvement in 45.5%, spontaneous recovery without treatment in 63.6%, and all cases negative for anti-GQ1b antibodies 1). Differentiation from Miller Fisher syndrome (MFS) is important; in MFS, neurological symptoms appear approximately 8 days after onset, and anti-GQ1b antibodies are positive in about 90% of cases.
Ben-David et al. (2022) reported a case of a 44-year-old man 2). Left abduction limitation appeared 2 days after COVID-19 onset. Laboratory findings showed lymphopenia (1.28×10⁹/L), elevated CRP (92 mg/L), and elevated D-dimer (1.3 μg/mL). Diplopia resolved spontaneously 5 days after admission.
Kubota et al. (2022) reported a case of a 25-year-old Japanese man 1). Left abduction limitation appeared the day after COVID-19 onset. CSF protein was 55 mg/dL (albuminocytologic dissociation), anti-GQ1b antibody was negative, and head MRI was normal. He recovered without treatment.
Miller Fisher syndrome after COVID-19 vaccination has also been reported. A 53-year-old man developed diplopia, ataxia, and progressive ascending paralysis 8 days after Sinovac vaccination. CSF protein was 85 mg/dL (albuminocytologic dissociation), and nerve conduction studies showed conduction block. He fully recovered after 10 weeks with conservative treatment 3). All four cases in the literature presented with diplopia and ophthalmoplegia, occurring 14–18 days after vaccination.
Two cases of diplopia complicating anti-glutamic acid decarboxylase 65 (GAD65) antibody-associated epilepsy have been reported 4).
Chen et al. (2025) reported two cases: a 35-year-old woman (anti-GAD antibody 1:100) and a 25-year-old woman (anti-GAD antibody 1:10) 4). Both developed diplopia and nystagmus after epilepsy. In the first case, diplopia resolved with steroids plus mycophenolate mofetil. In the second case, gait instability persisted after IVIG. The authors suggest that diplopia and nystagmus may be prodromal symptoms of cerebellar ataxia. About 25% of patients with GAD65 antibody-associated ataxia experience dizziness before symptom onset.
Fujiwara et al. (2021) conducted a systematic review of 92 cases of trochlear schwannoma 5). Analysis of 10 cases without preoperative diplopia who underwent total resection revealed that 4 cases did not develop postoperative diplopia despite intraoperative transection of the trochlear nerve. This is thought to be due to the acquisition of motor and sensory fusion during the slow tumor growth process, with sensory fusion being particularly important for cyclotorsional deviation.
