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
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.
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
8. Coats, D
and Paysse, EA.
Refractive error and amblyopia. UpToDate. Online version 14.3.
July 13, 2006.
7/22/07