Inability to Accommodate with Binocular Vision

Understanding the Mechanism

Macular distortion creates dissimilar foveal images between the two eyes that are too different to fuse, even though peripheral retinal images can still align, according to the American Academy of Ophthalmology 1
Peripheral fusion dominates over central fusion when viewing with both eyes, forcing peripheral retinas into alignment while leaving the foveas misaligned, as reported by the American Academy of Ophthalmology 1, 2
This central-peripheral rivalry prevents proper accommodative function binocularly, though accommodation may work normally when each eye is tested individually, as noted by the American Academy of Ophthalmology 1

The lights on/off test is pathognomonic for this condition, also called dragged-fovea diplopia syndrome, where in a completely darkened room with no peripheral fusion cues, the patient can see a small test letter singly, but when room lights are turned on, peripheral fusion reasserts itself and the letter becomes double, according to the American Academy of Ophthalmology 2
An alternative is the optotype-frame test, where the patient fixates an isolated letter on a monitor—if the letter is single but maintaining single vision of the monitor frame causes the letter to become double, this confirms peripheral fusion is overriding central fusion, as described by the American Academy of Ophthalmology 2

Maculopathy is the primary underlying cause, with epiretinal membranes or subretinal neovascular membranes having a prevalence of 2% under age 60, up to 12% over age 70, and 16-37% of patients with these maculopathies develop binocular retinal diplopia, as reported by the American Academy of Ophthalmology 1

Prism alternate cover testing typically shows either no strabismus or only a small vertical deviation, and any diplopia relief from prism correction is transient at best, according to the American Academy of Ophthalmology 1, 2
Metamorphopsia can be identified on Amsler grid and quantified with M-Charts, and aniseikonia can be documented with the Awaya test, as noted by the American Academy of Ophthalmology 2

Fogging one eye is the most successful treatment for reducing symptoms, creating a central scotoma that eliminates the foveal conflict, with Scotch Satin tape or Bangerter foils on spectacle lenses being generally well-tolerated, according to the American Academy of Ophthalmology 2
Strabismus surgery is rarely helpful because it cannot resolve the mismatch of distorted macular images or the central-peripheral conflict, as reported by the American Academy of Ophthalmology 1, 2
Epiretinal membrane peeling may be effective in some patients, though this is a double-edged sword as some non-diplopic patients become diplopic following retinal surgery, as noted by the American Academy of Ophthalmology 2