Clinical Practice Guideline

for

EXCESSIVE REFRACTIVE ERROR

Developed for the

Aerospace Medical Association

by their constituent organization

American Society of Aerospace Medicine Specialists

 

Overview: A refractive error is present when the optical power of the eye produces an object image that is not focused on the retina.  Myopia is present when the anterior-posterior diameter of the eye is too long relative to the refractive power of the cornea and lens.  The focal point of the object image occurs anterior to the retina.  Hyperopia is present when the anterior-posterior diameter of the eye is too short relative to the refractive power of the cornea and lens.  The focal point of the object image occurs posterior to the retina.  Myopia and hyperopia are spherical refractive errors and the optical components act with equal power in all meridians.  Astigmatism is present when there is variability in the optical powers of the eye in various meridians, or axes, thus creating more than one focal point.  Anisometropia is present when there is a difference in the refractive power between the two eyes.

 

Myopia has been divided into pathologic (also known as malignant, progressive, or degenerative) and physiologic (or simple).  Pathologic myopia is caused by excessive growth in the axial length of the eye while the rest of eye has normal growth.  These individuals show marked choroidal and retinal degenerative changes, high incidence of retinal detachment, glaucoma, and increased occurrence of staphyloma (ectasia) development.  Pathologic myopia occurs primarily in myopes with a refractive error > -6.00 diopters (D).  Physiologic myopia is associated with normal growth of each of the refractive components of the eye, the combination of which results in mild to moderate myopia.  Physiologic myopia will usually progress during the adolescent years and stabilize in the early 20s.  

 

A 2006 prevalence study of corrective lens use based on aircrew spectacle orders in the Spectacle Request Transmission System (SRTS) among US Air Force pilots showed 38.4% of active duty (AD) pilots require corrective lenses to meet vision standards for flight versus 39.4% in 1995.  The prevalence of corrective lens use was much lower among Air National Guard (ANG) and Air Force Reserve Component (AFRC) pilots at 24.0% and 20.7%, respectively.  The majority of AD pilots (86.4%) utilize single vision lens correction while multifocal lenses accounted for nearly one-quarter of spectacles worn by ANG and AFRC pilots.  The typical refractive correction among AD pilots using lenses was low myopia, with 80% of all spectacle orders falling between plano and -2.62 diopters.  One-third of all AD spectacle orders contained 0.75 diopters or more astigmatism correction.  High astigmatism correction (over 2.00 diopters) was rare occurring only 1.9% of the time.  Corrective lens use was relatively constant across aircraft platforms.  Pilots of high performance (high “G”) airframes were just as likely to be using lenses as were pilots flying tanker, transport, bomber (low “G”) aircraft.  The prevalence of corrective lens use was also relatively age independent for those pilots under 40; however, there was a small increase in lens use for pilots 41-45 and 76% of pilots over the age of 45 required lens corrections.

 

Severe (high) myopia (greater than 6 diopters) is more prevalent among FC III aviators and FC II flight surgeons since entry standards for pilot and navigator aircrew have been more stringent and have excluded those with higher risk due to higher levels of refractive error.  As the degree of myopia increases, the risk of retinal detachment also increases.  The risk of retinal detachment in normals is 0.06% over a 60 year time span compared to 2.5% in myopes with > -5.00 diopters refractive error.  Beyond -9.75 diopters, the risk increases to 24%.  However, the risk for retinal detachment dramatically increases in the presence of associated peripheral retinal lattice degeneration.  The lifetime risk for retinal detachment in myopes > -5.00 diopters is 35.9%.  Likewise, the prevalence of lattice degeneration rises as the level of myopia rises.

 

Aeromedical refractive error is based on the cycloplegic refraction for FC I/IA and for initial FC II and III if medical qualification is in question.  The authorized cycloplegic exam technique uses one percent cyclopentolate (Cyclogyl), 2 drops each eye, 5 to 15 minutes apart, with examination performed no sooner than one hour after the last drop and within two hours of the last drop of cyclopentolate.  The cycloplegic refractive error is the minimum refractive power needed to achieve 20/20 vision each eye separately.  The refractive error standard for aeromedical purposes is that produced “in any meridian” following transposition.  The rules of transposing are: (1) Algebraically add the cylinder power to the sphere power to determine the transposed power of the sphere (2) Change the sign of the cylinder (3) Change the axis by 90 degrees (do not use degrees greater than 180 or less than 0).   Note:  180 degrees is on the same axis as 0 degrees.

 

                                    Sphere              Cylinder            Axis

      Example 1:              +2.25                -1.50         X     179

      Transposed             +0.75                +1.50         X     089

 

      Example 2:  -           4.25                  -1.25         X     068

      Transposed             -5.50                 +1.25         X     158

 

Aeromedical standards and waiver requirements are based upon the magnitude of sphere power in the meridian that gives the largest value.  In example 1, +2.25 is the largest sphere power.  This value is achieved at axis 179 prior to transposition.  In example 2, -5.50 is the largest sphere power.  This value is achieved at axis 158 after transposition.  Myopia is represented by a negative diopter value in the sphere and hyperopia by a positive diopter value.  Astigmatism may be represented by either a positive or negative cylinder value depending on the axis referenced.

 

Aeromedical Concerns: Improper or unbalanced correction with spectacles or contact lens can degrade stereopsis and contrast sensitivity as well as induce generalized ocular pain and fatigue (asthenopia).  Myopia is more likely to progress, with respect to the degree of myopia, regardless of age, while hyperopia tends to remain static over time.  In addition, myopes may see halos or flares around bright lights at night and are also at risk for worsening under dim illumination and with pupil enlargement, a phenomena known as “night myopia.”  Myopes also have an increased risk of retinal detachment, open angle glaucoma and retinal degenerations, such as lattice.

 

Hyperopes, especially those with greater than +3.00D of correction, will experience greater problems with visual acuity after treatment with atropine or topical cycloplegic agents.  They have a greater predisposition for tropias, microstrabismus, and phorias that can decompensate under the rigors of flight.  They also have a higher prevalence for amblyopia due to the accommodative esotropia and anisometropia.   Moreover, hyperopes have more problems with visual aids, such as night vision goggles, as they develop presbyopia at earlier ages compared to myopes.  Lastly, hyperopes are more likely to develop angle closure glaucoma than myopes.

 

Higher levels of astigmatism or progressive astigmatism can indicate a potentially progressive corneal condition, such as keratoconus, that can degrade image quality and visual performance during productive years of flying career.  Anisometropias have greater association with diplopia, fusional discrepancies (e.g. defective stereopsis), and amblyopia, especially when greater than 2.00 diopters refractive error difference between the two eyes.

 

In general, corrective measures presently available to correct refractive errors include spectacles, contact lenses, and corneal refractive surgical techniques such as PRK and LASIK.  Spectacles interpose an additional other optical interface between the aircrew’s eyes and the outside world.  This increases the risk of internal reflections, fogging, reduces the light reaching the retina and can create visual distortion, especially in high myopes and in higher levels of astigmatism.  Finally, spectacle frames interfere with the visual fields, cause hot spots, and displace under G forces.  Depending on refractive errors, the lenses themselves can induce optical blind spots (scotomas), optical image size changes, and can create unacceptable effects on other visual performance parameters, such as stereopsis.  Contact lenses share some of these same problems, but reduce some of the drawbacks of spectacles, such as changes in image size, peripheral vision interference, hot spots from frames, fogging, and blind spots.  However, contact lenses introduce their own unique aeromedical problems particularly related to maintenance and wear.  In addition, further concern exists with the risk of acutely having to perform without the corrective lenses, such as after spontaneous lens loss, e.g. after ejection or during a deployment without adequate backups.

 

Medical Work-up: In military medicine for initial waiver for excessive myopia the aeromedical summary should include:

      A.  Cycloplegic refraction (for initial flying training applicants) to 20/20 each eye and manifest refraction to best corrected visual acuity each eye

      B.  Optometry/ophthalmology exam to include a dilated peripheral retina exam of each eye

 

In military medicine for initial waiver for excessive hyperopia the aeromedical summary should include:

      A.  Cycloplegic refraction (for initial flying training applicants) to 20/20 each eye and manifest refraction to best corrected visual acuity each eye

      B.  Results of stereopsis testing (OVT or VTA testing)

      C.  Optometry/ophthalmology exam to include :stereopsis, ocular motility and alignment testing

 

In military medicine for initial waiver for excessive astigmatism the aeromedical summary should include:

      A.  Cycloplegic refraction (for initial flying training applicants) to 20/20 each eye and manifest refraction to best corrected visual acuity each eye

      B.  Corneal topography imaging

      C.  Results of corrected vision with spectacle correction alone

      D.  Results of stereopsis testing (OVT or VTA testing)

      E.  Optometry/ophthalmology exam to include slit lamp and fundus exam

 

In military medicine for initial waiver for anisometropia the aeromedical summary should include:

      A.  Cycloplegic refraction (for initial flying training applicants) to 20/20 each eye and manifest refraction to best corrected visual acuity each eye

      B.  Results of stereopsis testing (OVT or VTA testing)

      C.  Optometry/ophthalmology exam to include: stereopsis, ocular motility and alignment testing.

      D.  History of asthenopic (eye pain/fatigue) symptoms, diplopia or fusional problems, to include negative responses

 

Aeromedical Disposition (military): The following tables cover the different flying classes, waiver potential and required review/evaluation for myopia, hyperopia, astigmatism and anisometropia.

 

Table 1:  Myopia

 

Flying Class

Refractive error

Waiver Potential

 

Expert review/evaluation

Pilot Training Applicant

> -1.50 but ≤ -3.00

 

 

> -3.00

Yes

 

No

 

No

Navigator Training Applicant

> -1.50 but ≤ -4.50

 

 

> -4.50

Yes

 

No

 

No

Pilot/Nav

> -4.00

Yes

 

No

Non-pilot

> -5.50

Yes*

 

No

 

Table 2:  Hyperopia

 

Flying Class

Refractive error

Waiver Potential

 

 

Expert

review/evaluation

Pilot Training Applicant

> +2.00 but ≤ +3.00 if waiverable degradation or no degradation in stereopsis

 

> +3.00 but ≤ +4.00 if no degradation in stereopsis

 

Yes

 

 

 

 

Yes

 

Yes

 

 

 

 

 

Yes

Navigator Training Applicant

> +3.00 but ≤ +4.00 if waiverable degradation in stereopsis

 

> +3.00 but ≤ +5.50 if no degradation in stereopsis

 

Yes

 

 

 

Yes

 

Yes

 

 

 

 

Yes

 

Pilot/Nav

> +3.50 if waiverable or no degradation in stereopsis

 

Yes

 

No

 

Non-pilot

> +5.50 if waiverable or no degradation in stereopsis

Yes*

 

No

 

Table 3:  Astigmatism

 

Flying Class

Refractive Error

Waiver Potential

 

Expert review/evaluation

Pilot Training Applicant

>1.50 but ≤3.00

Yes

 

Yes*

Pilot/Nav

>2.00

Yes

 

Yes, initial waiver

Non-pilot

>3.00

Yes

 

Yes, initial waiver

 

Table 4.  Anisometropia

Flying Class

Refractive error

Waiver Potential

 

 

Expert review/evaluation

Pilot Training Applicant

> 2.00 and if normal stereopsis or waiverable degradation in stereopsis* and no asthenopic symptoms or diplopia

 

Yes

 

Yes

Navigator Training Applicant

> 2.50 and if normal stereopsis or waiverable degradation in stereopsis* and no asthenopic symptoms or diplopia

Yes

 

Yes

Pilot/Nav

> 2.50 and if normal stereopsis or waiverable degradation in stereopsis* and no asthenopic symptoms or diplopia

Yes

 

No

Non-pilot

> 3.50 and if normal stereopsis or waiverable degradation in stereopsis* and no asthenopic symptoms or diplopia

Yes

 

No

·      Waiverable degradation of stereopsis means meets waiver criteria for defective depth

perception.

 

In U.S. Civil Aviation medicine there are no standards for refractive error.  There are also no standards for uncorrected visual acuity.  Many of the conditions associated with myopia or hyperopia can be considered for waiver. 

 

Aeromedical Disposition (civilian):

 

Waiver Experience (military): Review of US Air Force waiver databases showed 542 cases of myopia for all flying classes for 2006, of which 113 aeromedical summaries were reviewed; all disqualified (50) and 53 randomly selected.  Fifty-six (56/113) were waivers for photorefractive keratectomy (PRK) or LASIK; 15 disqualified and 41 granted waivers.  Of the 35 myopia cases disqualified, 20 were initial flying training, two were pilot/navigators, and 13 were non-pilots.  Of the 20 initial flying training, ten were outside waiver criteria, six had other eye conditions (defective color vision, not correctable to 20/20, defective depth perception) and four had other disqualifying conditions.  Of the two disqualified pilot/navigators, both were initial FC II and had other disqualifying conditions (one with additional eye conditions and another with desensitization to fire ants).  Of the 13 disqualified non-pilot aviators (all initial training candidates), four had myopia which was not correctable to 20/20, three with myopia only, three with other non-eye disqualifying conditions, two had myopia with history of retinal tears and one with defective color vision.

 

Review of a large military waiver database (ICD 9 code 367.0) showed 91 cases of hyperopia for all classes of flyers for 2005 and 2006, of which 51 aeromedical summaries were reviewed, all disqualified (35) and 16 randomly selected.  Six were disqualified for hyperopia only, 26 for hyperopia and additional eye conditions (defective stereopsis, astigmatism, anisometropia, defective color vision) and three were disqualified due to other non-eye conditions (asthma, stroke, medications usage).

 

Review of the same database (ICD 9 code 367.2 – 367.23 and “astigmatism”) showed 194 cases of astigmatism for all classes of flyers for 2006, of which 50 aeromedical summaries were reviewed, all disqualified (24) and 26 randomly selected.  Of the 50 aeromedical summaries reviewed, 30 (13 disqualified and 17 granted waivers) did not have astigmatism that was aeromedically disqualifying (required a waiver).  Therefore, of the 11 disqualified for astigmatism, six also had myopia, four had hyperopia and one was not correctable to 20/20.

 

Waiver Experience (civilian): There are no policy standards. 

 

References:

 

1. AFPAM 48-133, Physical Examination Techniques, 1 June 2000.

 

2. Baldwin, JB, Dennis, RJ, Ivan, DJ, Miller, RE, et. al.  The 1995 Aircrew Operational Vision Survey: Results, Analysis, and Recommendations.  SAM-AF-BR-TR-1999-0003.  May 1999.

 

3. Duane TD, Jaegar EA.  Clinical Ophthalmology.  Philadelphia: Harper & Row, 1993; 3:  27.9.

 

4. Wright, S, Clark, P, Miller, R, Tutt, R, Gooch, J, et al.  An Update on the Prevalence of Corrective Lens Use Within the USAF Pilot Population.  Abstract/poster presentation for Aerospace Medical Association 78th Annual Scientific Meeting, May 2007.

 

5. Miller RE II, Woessner WM, Dennis RJ, O’Neal MR, Green Jr RP.  Survey of spectacle wear and refractive error prevalence in USAF pilots and navigators.  Optometry and Vision Science.  1990; 67:  833‑9.

 

6. Waring III GO, Lynn MJ, McDonnell PH.  PERK Study Group.  Arch Ophthalmol  1994;112:1298-1308.

 

7. Miller, D and Scott, CA.  Chapter 10 - Epidemiology of Refractive Errors.  In Ophthalmology, 2d Edition.  Ed by Yanoff, M, et al.  Mosby, 2004.

 

8. Coats, D and Paysse, EA.  Refractive error and amblyopia.  UpToDate. Online version 14.3.  July 13, 2006.

 

 

7/22/07