A summary video by Dr Andrew Lee can be accessed here: https://www.youtube.com/watch?v=Qr-EqZt70L0
Non-paretic binocular diplopia refers to diplopia that is not a result of dysfunction of any muscle. Etiologies include hemifield slide phenomenon, dragged-fovea diplopia syndrome, supranuclear fusional abnormalities, image disparity, and horror fusionis, which we will discuss in detail below. In all of these etiologies, there is impairment of the normal fusional process. It is important to recognize these phenomena to avoid unnecessary and potentially harmful strabismus surgery.
Hemifield slide phenomenon
The hemifield slide phenomenon occurs in patients with limited temporal visual fields, when pre-existing phorias develop into tropias . Normally, the nasal field of one eye overlaps with the temporal field of the contralateral eye to permit stable binocular fusion. It has been previously noted that a vertical median strip of the retina exists contains retinal ganglion cells that project to both optic tracts, allowing the visual fields to overlap and appear uniform with no interruption of midline. Loss of input from this central zone results in difficulty maintaining the two half fields juxtaposed. In the hemifield slide phenomenon, patients with pre-existing esophoria or intermittent esotropia may have hemifields that drift apart horizontally, with the right nasal hemifield drifting further to the left and the left nasal hemifield drifting further to the right. If patients develop an exotropia, the nasal hemifields will overlap slightly, resulting in superimposition of images. If the patient develops a hypertropia, the nasal hemifields will separate vertically.
Dragged-fovea diplopia syndrome (DFDS)
Macular pathologies such as epiretinal membrane, retinal wrinkling, and subretinal neovascularization can cause diplopia due to dragged-fovea diplopia syndrome . This syndrome may also be referred to as central-peripheral rivalry, foveal displacement syndrome, macular diplopia. It occurs when macular misalignment between the two eyes creates competition between central and peripheral fusion. Normally, retinal correspondence allows for point-to-point fusion of foveal and peripheral images. This allows for the points in the macula and periphery of the retina in each eye to be fused into one image. When the macula is displaced, fusion of the macular areas will cause misalignment of the peripheral points, and fusion of the peripheral points will cause misalignment of the macular points. The stronger peripheral fusion drive will overpower the central fusion drive, and the patient experiences persistent central binocular diplopia due to misalignment of the macula.
The “lights on-off test” can be used to evaluate for DFDS . The patient is asked to fixate on a 20/70 letter on an LCD screen. The examiner then turns the lights off, removing all peripheral stimuli. If the patient is then able to see a single letter, and therefore fuse centrally, this is a positive test. If the diplopia does not resolve, the test is negative for DFDS.
Supranuclear fusional abnormalities
Supranuclear lesions may occur in the cerebral cortex, rostral midbrain, superior colliculus, and cerebellum .
Convergence insufficiency is the inability to fuse at near distances and a common cause of diplopia at near . Supranuclear lesions can cause damage to the accommodation pathway, resulting in paresis of accommodation and often convergence insufficiency due to cross-coupling between the two systems . However, the cause of convergence insufficiency is often unknown. Pencil pushups may be effective in some cases. Other training exercises include base-out prisms for near viewing and convergence exercises.
Divergence insufficiency is a benign phenomenon causing horizontal diplopia at distance viewing . Esodeviation is comitant in all fields of gaze at distance. Patients often have spontaneous resolution; however, neuroimaging should be considered due to potential clinical similarity with bilateral sixth nerve palsies, mass lesions, and demyelinating disease.
Etiologies causing image disparity
Aniseikonia, or a difference in the perceived size of images between eyes, can also impair fusion . Objects may appear larger (macropsia) or smaller (micropsia). Aniseikonia may be a result of compression or stretching of photoreceptors from an epiretinal membrane. Additionally, torsion can impair the fusional process. These etiologies should be considered before a diagnosis of central disruption of fusion is made.
Horror fusionis refers to an inability to fuse even with eyes that are aligned . This is an acquired disruption of central fusion, associated with strabismus, and causes diplopia in all positions of gaze that cannot be eliminated by prisms. It is a diagnosis of exclusion and may occur in patients with prolonged sensory deprivation in one eye, severe head injury, and postviral syndromes. Patients with horror fusionis typically experience significant distress from their symptoms, as they can neither fuse nor suppress the images.
A diagnosis of non-paretic diplopia requires differentiating it from common causes of paretic binocular diplopia, such as third nerve, fourth, sixth palsies, as well as systemic diseases that impair ocular motility such as Graves disease and myasthenia gravis. Strabismus surgery is in general not useful for nonparetic diplopia, so it is important to make the correct diagnosis to avoid unnecessary and harmful procedures for the patient.
A thorough history should be taken for patients suspected of having non-paretic diplopia. History of trauma and vasculopathic risk factors should be assessed to determine likelihood of paretic versus nonparetic diplopia. Specific features of the diplopia should be considered, such as whether it is comitant or incomitant, worse on near or far vision, or worse at the end of the day. Ocular history can be used to guide risk assessment, such as the probability of DFDS in a patient with a known epiretinal membrane. Patients should be asked about symptoms associated with the disease in suspicion; for example, patients suspected of having hemifield slide phenomenon should be asked about associated symptoms of pituitary mass lesion such as visual field defects, galactorrhea, or headache.
Full ocular examination should be performed. Binocular diplopia should be evaluated via a cover test to determine if the diplopia is due to misalignment of the eyes . Incomitant strabismus may point to restrictive disease, cranial neuropathy, supranuclear disorders, ocular myopathies, and neuromuscular junction disorders. Comitant strabismus points to childhood strabismus or an acquired cause such as vergence paresis. If the eyes are aligned properly, dissimilar images may be indicative of macular disease or marked refractive asymmetry.
Some patients may present with specific features, depending on etiology. Patients with hemifield slide phenomenon may have features of increased intracranial pressure on examination if a large pituitary tumor is the cause. Patients with DFDS may have an epiretinal membrane that can be noted on slit lamp examination with dilated fundus examination and scleral depression. Convergence insufficiency and divergence insufficiency can be assessed during near and far vision.
MRI may be obtained to rule out causes of paretic diplopia such as cranial nerve palsy, mass lesion, or demyelinating disease. MRI may also be ordered if pituitary adenoma or mass causing hemifield slide phenomenon is suspected. OCT retina can identify an epiretinal membrane causing dragged-fovea diplopia syndrome or aniseikonia.
Management of nonparetic diplopia depends on etiology. Epiretinal membranes causing dragged-fovea diplopia syndrome generally do not benefit from prism therapy, vitreoretinal surgery, or strabismus surgery . Mass lesions such as tumors causing hemifield slide phenomenon may be removed. Convergence insufficiency may benefit from pencil pushups or base-out prisms for training exercises. Divergence insufficiency typically resolves on its own; however, base-out prisms may be used for relief of symptoms. Some causes of non-paretic diplopia, such as horror fusionis, lack treatment options.
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