Nonparetic diplopia is binocular diplopia not caused by dysfunction of any extraocular muscle. Eye movements are normal, and the essence is a disturbance of the normal fusion process.
It is also known as central-peripheral rivalry (CPR) type diplopia, macular diplopia, or foveal displacement syndrome.
Proper recognition of this condition has important clinical significance. Misdiagnosing nonparetic diplopia as paretic diplopia carries the risk of unnecessary and harmful strabismus surgery.
The main etiologies of non-paralytic diplopia are classified into the following five types.
Hemifield Slide Phenomenon
Concept: Latent strabismus becomes manifest due to temporal visual field restriction (e.g., chiasmal compression), causing half of the visual field to shift horizontally.
Cause: Typical example is chiasmal compression by pituitary adenoma.
Diplopia Due to Foveal Traction Syndrome (DFTS)
Concept: Caused by displacement of the fovea due to epiretinal membrane or subretinal neovascular membrane. Characterized by persistent diplopia in central vision. 1)
Epidemiology: 16–37% of patients with epiretinal membrane present with binocular diplopia. 2)
Supranuclear fusion anomaly
Convergence insufficiency: Diplopia at near. Involves supranuclear dysfunction of the accommodative pathway.
Divergence insufficiency: Horizontal diplopia at distance. Increases as distance increases.
Anisometropia / fusion fear
Aniseikonia: Differences in image size between the two eyes caused by compression or stretching of photoreceptors due to an epiretinal membrane.
Fusion phobia: Despite normal eye alignment, the patient experiences diplopia in all directions of gaze, accompanied by significant distress.
The importance of diplopia due to retinal diseases increases with age. The prevalence of epiretinal membrane is about 2% in individuals under 60 years old, but reaches up to 12% in those aged 70 and older. 2) Spontaneous improvement of diplopia is rare in patients with macular diseases such as epiretinal membrane, and diplopia often develops within days to weeks after the onset of macular disease.
Paralytic diplopia results from dysfunction of the extraocular muscles (e.g., nerve palsy, restrictive lesions) and is accompanied by limitation of eye movement. Non-paralytic diplopia occurs despite normal extraocular muscle function and is essentially a disorder of the fusion process. Differentiation based on cover test findings, ocular motility examination, and investigation of underlying diseases is important.
Hemifield slide phenomenon: Occurs due to a combination of temporal visual field restriction (chiasmal compression from pituitary adenoma, etc.) and manifestation of pre-existing latent strabismus. Loss of binocular correspondence in the vertical midline makes it difficult to maintain juxtaposition of the hemifields.
Central fixation traction diplopia syndrome (DFDS): Epiretinal membrane or subretinal neovascular membrane physically displaces the fovea, causing a mismatch between central and peripheral fusion (central-peripheral rivalry). 1) The prevalence of epiretinal membrane is about 2% in those under 60 years old and up to 12% in those over 70. 2)
Convergence insufficiency: Convergence ability decreases due to supranuclear disorders of the accommodative pathway. Causes may include midbrain dorsal lesions (Parinaud syndrome), thalamic hemorrhage, Parkinson’s disease, multiple sclerosis, etc. Many cases are idiopathic.
Divergence insufficiency: The neural mechanism is largely unknown. Lesions around the periaqueductal gray matter or near the abducens nucleus may be involved. More common in the elderly, often benign and may resolve spontaneously. Care must be taken to differentiate from bilateral abducens palsy, space-occupying lesions, and demyelinating diseases.
Aniseikonia: Compression (micropsia) or stretching (macropsia) of photoreceptors due to epiretinal membrane causes a difference in image size between the two eyes, making fusion difficult.
Fusion phobia: A condition in which fusion is not possible despite normal eye alignment. Long-term sensory deprivation, severe head trauma, post-viral syndrome, etc., may be triggers, but it is a diagnosis of exclusion.
Most patients with an epiretinal membrane do not experience double vision. Reports indicate that 16–37% of patients with epiretinal membrane have binocular diplopia, 2) but not all cases develop double vision. The degree of central and peripheral fusion disparity influences the onset of diplopia.
First, it is important to exclude paralytic and restrictive diplopia. The following diseases should be differentiated.
Diagnosis is made by combining the following main tests.
| Test | Purpose |
|---|---|
| Cover test | Assessment of eye misalignment and comitant/incomitant strabismus |
| Light-on/off test | Confirm DFDS |
| Optical coherence tomography (OCT) | Assessment of epiretinal membrane and foveal traction |
| MRI | Exclusion of cranial nerve palsy and space-occupying lesions |
Cover test: A basic test to differentiate comitant deviations (e.g., childhood strabismus, convergence insufficiency, divergence insufficiency) from incomitant deviations (e.g., cranial nerve palsy, restrictive diseases). If the eye position is normal but there is image disparity, it suggests macular disease or significant anisometropia.
Light-on/off test: This test is considered pathognomonic for confirming DFDS. 1) In a dark room, the patient fixates on a small white-on-black target, removing peripheral fusion cues. The test is positive if central fusion is possible. It is important to darken the entire room.
Optotype-frame test: An alternative to the light-on/off test. By separately evaluating an isolated optotype and a frame on a monitor, central and peripheral competition is assessed. 3)
OCT (Optical Coherence Tomography): An essential test for identifying epiretinal membrane and foveal traction.
MRI: Used to rule out cranial nerve palsy, space-occupying lesions (e.g., pituitary adenoma), and demyelinating diseases. MRI should be performed in divergence insufficiency to exclude conditions causing increased intracranial pressure or skull base lesions.
Amsler chart / M-Charts: Detection and quantitative evaluation of metamorphopsia.
Awaya test: Quantitative evaluation of aniseikonia.
Synoptophore (major amblyoscope): Used for more accurate quantification of fusion deficits.
Hess chart / Gaze field test / Binocular single vision field (BSV field): Used to record eye movements and the range of diplopia, aiding in differential diagnosis.
This test involves fixing the gaze on a small black-on-white target in a dark room to eliminate peripheral fusion cues. 1) In a normal bright room, peripheral fusion overrides central fusion, causing diplopia, but in a dark room, removing peripheral fusion cues allows central fusion to function and resolve diplopia. This is considered a pathognomonic finding for DFDS.
Since the underlying pathology of DFDS is difficult to address, treatment aims to alleviate symptoms.
Surgical removal of the underlying disease (e.g., pituitary tumor) is the definitive treatment.
Currently, there are no effective treatment options.
Currently, there is no effective treatment for fusion fear. Fusion fear is a condition in which fusion is not possible despite normal eye alignment, and it is a diagnosis of exclusion. Long-term sensory deprivation, severe head trauma, and post-viral infection syndromes have been reported as triggers.
When binocular correspondence of the vertical median strip is lost due to temporal visual field restriction (e.g., optic chiasm compression), pre-existing phoria becomes manifest. In esophoria, the hemifields separate horizontally; in exophoria, they overlap; and in hyperphoria, they separate vertically.
When an epiretinal membrane or subretinal neovascular membrane physically displaces the fovea, a mismatch (central-peripheral conflict) occurs between central and peripheral fusion. 1) Because the driving force of peripheral fusion exceeds that of central fusion, the periphery fuses while the fovea remains displaced, resulting in persistent central diplopia.
Two mechanisms are considered for DFDS.
Partial detachment of the vitreous cortex causes traction on the macula, leading to retinal thickening, distortion, cystoid changes, and macular displacement. White fibrous epiretinal membranes are more likely to cause symptoms than thin transparent types. OCT evaluation of the presence and degree of foveal traction is important for diagnosis.
Convergence insufficiency: Lesions in the cerebral cortex, rostral midbrain, superior colliculus, and cerebellum cause supranuclear damage to the accommodative pathway, leading to accommodative paralysis and secondary convergence insufficiency. It can also occur with dorsal midbrain lesions.
Divergence insufficiency: The neural mechanism of divergence is not fully understood. Associations with lesions in the periaqueductal gray matter, near the abducens nucleus, and the medulla oblongata have been reported, but many cases are of unknown cause.
Aniseikonia: Compression of photoreceptors by an epiretinal membrane causes micropsia, while stretching causes macropsia, making fusion difficult due to differences in image size between the two eyes.
Fusion fear: Due to acquired disruption of central fusion, the patient becomes unable to fuse or suppress images. Despite normal eye alignment, they perceive diplopia in all directions of gaze.
Hatt et al. (2019) reported the new onset and resolution of central-peripheral rivalry-type diplopia in patients who underwent epiretinal membrane peeling. 7) While some cases showed improvement in diplopia after surgery, new diplopia also developed postoperatively in other cases. When performing epiretinal membrane peeling, adequate informed consent regarding the outcome of diplopia is necessary.
Hatt et al. (2019) evaluated prism correction and fogging therapy for DFDS. 4) Prism correction alone is often transiently effective, but fogging was effective in some cases. Combination therapy using a Bangerter filter and Fresnel prism has also been reported to be potentially effective in cases where monotherapy is insufficient. 6)
