Use of optotype-based tests for distance visual acuity are recommended as soon as children are able to respond appropriately. Vision screening protocols vary as to the visual acuity cut-off depending on the program and age of the children.  For example, some programs have recommended cutoffs of 6/12 (20/40) or 6/9.5 (20/32) for screening and referring children for further assessment. Using optotypes such as Tumbling E, or single line optotype (pre-school vision screening) are recommended as standardized tests in some parts of the world. ^[xvii] The evidence suggests that some charts, especially those without crowding bars, are unacceptable, as they can overestimate the level of visual acuity. ^{[xviii] [See Appendix 2. Tips for Appropriate Eye Chart Design] [xix]}

In preschool-aged children, recommended visual acuity testing is use of a single optotype such as HOTV letters or LEA Symbols surrounded by crowding bars at a 5-ft (1.5 m) test distance, with the child responding by either matching or naming. ^[xx] Other charts designed specifically for the preliterate child are available, but they are designed for a 10 ft (3 m.) test distance.

The use of smartphone-based systems such as the Peek school eye health system to identify and refer children with visual issues with a visual acuity cut-off of 6/12 (20/40) using a Tumbling E optotype also increased adherence to referral in underdeveloped countries with higher rates of visual impairment. ^[xxi]

Accurate assessment of refractive error is core to identifying whether a child is at risk for complications of myopia, hyperopia, astigmatism and their consequences.  Myopia as an impending epidemic is a primary risk factor for visual impairment and blindness due to complications of high myopia. ^[xxii]

The best screening technique to detect children with the highest risk for amblyopia is non-cycloplegic retinoscopy performed by a trained optometrist to perform this technique on children. ^[xxiii] However, when the distance visual acuity is equal to or worse than 6/12 (20/40), latent refractive error is suspected, thus cycloplegic refraction is advisable. ^[xxiv]

Performing retinoscopy needs considerable expertise in a vision screening set-up.  Most recent vision screening studies recommend using a handheld binocular autorefractor with pre-determined cut-off values for vision screening.  This becomes even more important in preschool age children when reliable estimates of visual acuity cannot be obtained. ^[xxv] There is potential to underestimate hyperopia and overestimate myopia when outcomes are compared with cycloplegic retinoscopy. ^[xxvi] Instruments that may be used, especially in the pre-school age group, include the following:

• Welch Allyn® Spot™ Vision Screener
• Plusoptix S12C Vision Screener
• Retinomax (Right Mfg. Co Ltd.- Tokyo, Japan) ^[xxvii]

The latest additions to the market are instruments that include wavefront technology, including the QuickSee and SVOne. ^[xxviii]

^{[See Appendix 3. Evidence-Based Screening Tool Examples]}

Whenever a child fails a screening, a referral should be made, and a mechanism should be put in place to assure that the child has received a comprehensive eye examination.  Cycloplegic refraction remains the gold standard to determine refractive error in children especially when they are prescribed with optical correction for the first time.^[xxix],[xxx]  In addition, current recommendations for the potential of myopia include not only the accurate assessment of refractive error but also the axial length of the eye as there are tables suggestive as to who might be at risk, though the availability of this procedure may be limited at this time.^[xxxi]

Binocular vision disorders such as types of accommodative/vergence dysfunction are prevalent, with up to 40% of some populations having at least one condition. ^[xxxii] It is not typical for these types of visual problems to be assessed during a screening program.  With complaints of vision problems associated with near work, even without objective findings, referral for a comprehensive eye examination is suggested. It is expected that a full examination would thoroughly evaluate these areas.

Strabismus (and amblyopia that often accompanies it) remains a concern among school children and represents a high priority for early detection in the vision screening programs.  There is no scientific evidence for a single tool to screen for strabismus or amblyopia.  The cover test is considered the gold standard technique to determine the presence of strabismus along with the magnitude and type. For vision screening that include eye care professionals, cover test in addition to automated refraction resulted in 15%–25% increased detection of strabismus.  In the absence of a cover test, the Brückner test followed by the Hirschberg test may suggest the presence of a manifest strabismus. ^[xxxiii] These gross tests of binocularity are reliable for large angle strabismus and constant eye turns and may miss small angle, intermittent and/or alternating strabismus.

To detect likelihood of strabismus and/or anisometropia, some studies recommend using the Randot stereo test, whereas others recommend using photorefraction based screening, The Preschool Assessment of Stereopsis with a Smile 2 (PASS 2) test with two forced choices has shown higher testability among preschool children to screen for vision disorders. ^[xxxiv] For screening tests administered by trained lay screeners, the addition of PASS 2 to automated refraction increased the detection of strabismus sensitivity by 21%. ^[xxxv] Blinq is an automated screening device which the manufacturer claims can detect misalignment of the eyes. ^[xxxvi]

For children who receive a comprehensive eye exam, it is expected that this would include an assessment of near point of convergence, phorias and compensating vergences (in phoropter or with a prism bar), accommodative status (including facility, posture, and stamina) and both distance and near cover test (unilateral and alternate).

The prevalence of color vision deficiency among school children ranges between 2-10% among boys. Detecting congenital color vision deficiency among boys would aid in appropriate counseling before they make career choices. The tests used to screen for color deficiency in school children in these studies included Ishihara pseudoisochromatic plates (38 plates edition), Color Vision Testing Made Easy color plates, Waggoner HRR Diagnostic Test color plates and the Richmond HRR 4th edition plates. ^{[xxxvii], [xxxviii]}

Vision screening should be able to identify the potential existence of eye disease conditions, especially if they are vision threatening. Retinal disorders, congenital cataract, glaucoma, corneal scarring and other media opacities, optic nerve pathologies, pathologic myopia and cerebral causes are known to cause visual impairment in children. Other conditions cause pronounced symptoms without vision impairment, including infectious and allergic conjunctivitis, blepharitis, and dry eyes. ^{[xxxix], [xl], [xli]}

Congenital cataracts remain a huge burden in many low- and middle-income countries.  Early detection, referral and appropriate management can ensure reducing the visual morbidity due to these visual threatening conditions with lifelong implications.  Referral compliance remains a challenge in ensuring that the children referred seek treatment. ^[xlii]

Retinoblastoma, the leading cause of childhood neoplasm, is usually detected before 5 years of age due to the typical presentation of white reflex (leukocoria) in one or both eyes, and often, strabismus. Due to the vision and life-threatening nature of the condition, urgent referral in all cases of leukocoria in children is recommended. ^[xliii]

Screening for issues such as Onchocerciasis, and Bitot’s spots due to Vitamin A deficiency needs to be considered when customizing screening programs for specific regions. ^{[xliv], [xlv]}

In settings in which an eyecare provider is participating in a screening, a hand-held slit lamp allows the best evaluation of detail of the anterior segment.  When an eyecare provider and/or slit lamp is not available, the external eye may be examined by illuminating with a penlight/torchlight.  A 20 D lens or other approximately 4-5x magnifier may enhance the view.

Viewing of the internal eye is not typically performed by lay screeners.  If an eyecare provider is participating, a direct ophthalmoscope allows a view of the fundus without pharmaceutical mydriasis.  Monocular ophthalmoscopy (such as the Welch Allyn® PanOptic™) allows a wider view of the fundus as compared to direct ophthalmoscopy and permits a greater working distance between the patient and the clinician.

A thorough internal and external eye health exam is typically performed as a part of the comprehensive eye examination. This would include functional assessments of extraocular muscles and pupils, as well as confrontation visual fields.

The vision screening process is not complete without enabling the provision of spectacles, referral for ocular diseases, and assessing compliance to spectacle wear and other recommendations. Due to the difference in eye health models across nations, there is no standard process to address referrals. Many countries have provisions for free spectacles in their school vision screening model.^[xlvi] A recent Cochrane review showed that vision screening with provision of free spectacles results in more children wearing spectacles after screening compared with giving the children a prescription on its own.^[xlvii] Health education to improve spectacle compliance does show varying degrees of benefits across studies.^[xlviii] If there is a referral to an eyecare provider for a comprehensive eye examination, a dilated fundus evaluation is expected.^[xlix]

Children’s visual demands such as involvement in sports, music or other avocations need to be considered to decide on appropriate refractive prescription, and prescription eyewear material. Also, children with visual issues such as amblyopia, unilateral vision loss, and strabismus need to be counseled for appropriate prescription eyewear material to provide additional protection.

Children with neuro-developmental disorders and multiple disabilities often need to be provided with a multidisciplinary care approach. Hence, referral to developmental pediatricians, occupational and physical therapy, and neurology need to be considered in addition to eye care consultation, if not already engaged. Parents need to be involved in the referral process, understand the need, and have their questions answered in order to ensure that the children are provided with appropriate care.

There remains a considerable variability in vision screening protocols across the world due to differences in optometry practice patterns, scope of practice, support systems available, training requirements, competence standards, funding, and support from school administrations and local government.

Many children will need referral for secondary and tertiary services (either directly or after a comprehensive eye examination).  The following guidelines can serve as a reference in such situations.