Treatment of Strabismus

Functional and cosmetic results must be considered. Note a patient rarely complains about binocular status if the strab has been longstanding. Clinical objectives should be realistic in relation to the initial binocular status.

Surgery

• 15% of strabs have no refractive component so surgery is necessary. If only a small component of it is refractive then referral is probably necessary. It's not always the size of the strab rather the overall cosmesis that is important.
• GOAL OF SURGERY 1 - Functional binocularity (10%). Improvement of functional binocularity when prisms don't work, restoration of binocular alignment following injury or disease. Development of binocular vision by providing alignment or by improving comitance, prevention of sensory anomalies, corrections of head turns/tilts
• GOAL OF SURGERY 2 - Cosmetic improvement - Large angle strabismus is cosmetically unacceptable especially w/kids.

Timing of Surgery

• Infantile strabismus - Early as ocular alignment is important, later if there's already a limited prospect wrt binocular function
• Acquired childhood strabismus - a child up to the age of 6 who had binocular fusion - immediate restoration of BV essential
• Acquired adult strabismus - usually from disease. If incomitant immediately refer to determine aetiology. Comitant disorders should receive vergence training or prismatic correction. If these alternatives are unsatisfactory and a cosmetically unacceptable strab is present then consider surgery

Pharmacological Treatment

• Miotics can be used to reduce innervation to the vergence system as less accommodative drive is required. It produces an accommodation that is peripheral to the vergence system so less accommodative convergence is required at near. This reduced AC/A ratio, reducing an ESOT to and ESOP and maybe then try exercises.
  
• Miotics can also be used as a diagnostic trial to distinguish between an accommodative and a non-accommodative SOT in infants. If a miotic is instilled and the SOT is reduced then it's accommodative or related to the AC/A ratio.
• They can also be used as therapy but bifocals are preferred. Miotics caused reduced VA due to both the miosis and accommodation spasm.
• Atropine can be used long term to control accommodative esotropia although it's not advised due to the toxicity of the drug, the use of bifocals to read and photophobia. Cyclopentolate has some short term use

Optical Correction

• Correcting the rx provides a clear retinal image which prevents amblyopia and creates the correct balance between accommodation and convergence. Cycloplegic refraction should always be considered in young patients to determine the full extent of the underlying problem. If the refractive correction achieves binocular vision then consider prescribing the full amount.

Manipulation of AC/A Ratio Using Modified Presentation

• Over-correcting myopes can control XOP/T. It is not practical to undercorrect myopes as VA is reduced. Consider the maximum plus correction if BV achieved - supplemental exercises can be given to improve fusional reserves.

Bifocals

• Useful in non-refractive conditions w/abnormal AC/A ratio.
• Convergence excess - give extra plus at near
• Convergence insufficiency - give extra minus at near
• Divergence excess - increase minus at distance
• Divergence insufficiency - increase plus at distance (NB not v. practical)
• Use a large bifocal fitted to a lower pupil margin
• You can estimate the add lens power by measuring the AC/A ratio - for example if the distance = 18D exop/t and near = 8D exop and the pd is 60mm
• AC/A = 6 + (-8-(-18))/2.5

Prismatic Correction

• Used in small comitant vertical deviations and can also be useful in small horizontal phorias that aren't amenable to refractive correction or orthoptic exercises.
• Prism power is determined w/Sheard's criterion (often stated as "the opposing fusional reserve to the blur point should be at least twice the degree of the phoria" but can be up between two to four times)
• Sheard's Prism = (2xPhoria - fusional reserve)/Fixation Vergence

Fixation Disparity

• Remember to only give prism to px w/symptoms. Many patients have a fixation disparity but have no difficulty w/headaches etc. The amount of prism to neutralise the disparity is not always related to the angular subtense of the error. Stability plays a key role (?)

Orthoptic Therapy

• Preferred to surgery as it offers best chance of functional cure. Aim is to give stable, comfortable, functional BV.
• Can train sensory fusion, disparity vergence and accommodation. One principle is to establish sensory fusion prior to training motor fusion. Both can be trained simultaneously but priority should be given to anti suppression training.