What causes a patient to be unable to accommodate when looking with both eyes?

Inability to Accommodate with Binocular Vision

The most likely cause is binocular retinal diplopia from macular disease (such as epiretinal membranes or subretinal neovascular membranes), where peripheral fusion forces override central fusion, creating a central-peripheral rivalry that prevents accommodation when both eyes are open. 1

Understanding the Mechanism

This phenomenon occurs through a specific pathophysiologic process:

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

Diagnostic Confirmation

The lights on/off test is pathognomonic for this condition (also called dragged-fovea diplopia syndrome): 1

• In a completely darkened room with no peripheral fusion cues, the patient can see a small test letter singly (central fusion works)
• When room lights are turned on, peripheral fusion reasserts itself and the letter becomes double 1
• An alternative is the optotype-frame test: 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 1

Associated Conditions

Maculopathy is the primary underlying cause: 1

• Epiretinal membranes or subretinal neovascular membranes (prevalence 2% under age 60, up to 12% over age 70) 1
• 16-37% of patients with these maculopathies develop binocular retinal diplopia 1
• Retinal detachment involving the macula can also cause this 1

Clinical Examination Findings

Prism alternate cover testing typically shows: 1

• Either no strabismus or only a small vertical deviation
• Any diplopia relief from prism correction is transient at best 1
• Metamorphopsia can be identified on Amsler grid and quantified with M-Charts 1
• Aniseikonia (unequal images) can be documented with the Awaya test 1

Management Approach

Fogging one eye is the most successful treatment for reducing symptoms: 1

• Creates a central scotoma that eliminates the foveal conflict 1
• Scotch Satin tape or Bangerter foils on spectacle lenses are generally well-tolerated 1
• Occlusive contact lenses are an alternative 1
• Small amounts of prism combined with Bangerter foils may provide additional relief in difficult cases 1

Strabismus surgery is rarely helpful because it cannot resolve the mismatch of distorted macular images or the central-peripheral conflict 1

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 1

Important Caveat

Some patients with retinal misregistration have additional treatable causes of diplopia that should not be missed—a full orthoptic examination is warranted to identify coexisting strabismus that may be amenable to treatment 1