Clinical Practice Guideline
Developed for the
Aerospace Medical Association
by their constituent organization
Overview: Cones are the sensors in the retina of the eye that allow detection of colors. Humans have three different classes of cones that are each sensitive for a different range of visible wavelengths, but with distinctive peak sensitivities in the red, green and blue regions of the visible spectrum. The more acceptable nomenclature refers to red cones as long wavelength sensitive or “L” cones; green cones as middle wavelength cones or “M” cones; and blue cones as short wavelength or “S” cones. The combined input from all three-cone types responding normally to their specific wavelength range is needed to have normal color vision. There is some overlap of these normal sensitivity curves, but in some areas of the visible spectrum, a single cone type may be stimulated exclusively. The interaction generated by stimulation of the individual cone type, or not, provides input signals to the brain which are centrally integrated and processed to determine ultimately what colors are being perceived. This three cone system is known as normal trichromacy. Normal trichromacy can be further divided into red-green and blue-yellow systems, both of which must be intact to allow normal full-spectral color perception. Cone channels also provide brightness cues, particularly the red-green system.
In congenital color vision deficiencies, alterations (shifts) in the normal wavelength sensitivities of any of the cone types lead to color vision defects. These deficiencies can be absolute or relative, but all are aberrant. Acquired color vision deficiencies are different pathophysiologically from congenital defectives and can affect the visual pathways multifactorially and anywhere between the cones and the brain. Absolute loss of a particular wavelength sensitivity range of one of the three cone types leads to the condition called dichromatism, meaning that these individuals have only two functioning cone systems providing input. Actually, the loss of a particular cone type in congenital deficiencies occurs because its wavelength sensitivity curve shifts completely and overlaps with one of the other cones resulting in only two responsive cone types (dichromat). Such individuals have significantly altered color perception and are called “color blind.” They have lost the ability to make universal color determinations because only two cone types are responding. Dichromats make up approximately 2% of the male population.
The largest group of color deficient individuals (6%) is still trichromatic, but they have anomalies in their individual cone sensitivity curves. These sensitivity curves have incomplete shifts from their normal position in the visual spectrum towards one of the other cone types. These individuals are considered “color weak” rather than “color blind” because instead of an absolute deficiency or complete overlap, they have a partial overlap and a more relative deficiency. All three cones are still present, hence the term trichromat, but they are called anomalous trichromats. The partial overlap of the curves still produces aberrant input regarding the accuracy of color perception. They are further classified according to the specific cone group that is anomalous. Deuteranomalous defects in color vision result in difficulties with the green sensitive cones and protanomalous defects result in difficulties with the red sensitive cones. However, both of these groups have aberrant color perception and have trouble distinguishing between reds and greens, even confusing them with yellows and whites, if severe enough.
Congenital color vision defects are overwhelmingly red-green in type and occur almost exclusively in males, being linked to the X-chromosome in many varieties of allelic forms. The incidence of congenital red-green color disturbances in males is 8-10 percent, whereas it is only 0.5 percent in females. Congenital color vision defects are usually symmetric and almost always remain stable. They vary quantitatively and qualitatively depending on the extent of the genetic expression. Hence, many variations occur. In essence, no two color defectives are alike.
Most of the older color vision screening tests such as pseudoisochromatic plates (PIP) were designed to only detect the presence of congenital red-green deficiencies based on known inherited color confusion patterns. More recently, newer PIPs have been developed to screen for acquired and congenital deficiencies of all types. Failure of a PIP usually means that the likelihood of a color deficiency is high and mandates that further testing is needed. PIPs do not reliably determine the exact type of color vision defect. Current military standards for flying training dictate that applicants miss no more than 4 of 14 of the PIP I plates, and can only miss one of ten on the PIP II and PIP III plates.
It is paramount to understand what is meant by requiring normal color vision and the limitations of traditional color vision screening tests. The accurate detection of individuals who have any degree of color deficiency is integral to successful passing of color screening required by regulation to enter USAF undergraduate pilot training (UPT). It should be emphasized that the use of the older Farnsworth Lantern test does not adequately screen out all color deficient individuals and we should not be using it as such.
In the US Air Force, the gold standard for color vision testing is the anomaloscope. Anomaloscopes are expensive and sensitive pieces of equipment that take training to operate, maintain, and administer properly. Although more accurate in determining the actual color deficiency, they are not suitable for use as a mass screening test. They are not commonly available and are presently no longer mass produced, but can be special ordered. A comprehensive eye exam is also performed to rule out any acquired ophthalmic pathology.
Aeromedical Concerns: Color deficient individuals are at a distinct disadvantage in terms of receiving information in an efficient manner when using multicolor displays, terrain maps, and protective colored filters. The latter can compound these decrements and cause unpredictable effects in their residual color perception. Color defectives are more vulnerable to low-light and hypoxic effects on color vision than normals.
Another operational consideration with color defectives is the compounding effect induced by certain required protective or performance enhancing optical appliances that can potentially degrade existing levels of color perception even further. These currently include blue-blocker sunglasses, yellow high-contrast visors, and assorted laser eye protection devices. Whereas the impact of such devices on normal color perception can be predicted, the consequence of their use by individual color defectives is both unpredictable and highly variable. In many cases, these devices generate a chaotic color vision disturbance or can render color weak individuals totally color blind. These filters obviously also affect performance on multi-colored displays and they further disrupt brightness information.
The use of color as a means for more rapid and effective transfer of information is proliferating rapidly in modern operational environments. Color vision standards have been part of military physical standards for decades. In order to keep pace with new demands placed on the human in the more modern man-machine interface, color vision standards have undergone some changes. To keep pace with this evolution, the sensitivity and sophistication of the tests for the detection of color deficiencies have changed. The development of multi-color enhanced visual displays are based on the premise that end-users will have normal color vision, therefore, especially pilots, must possess normal color vision to be able to safely and effectively perform their missions.
Medical Work-up: A complete eye exam is required as well as a history of previous color vision testing results, a family history of color vision problems, all mediations used and occupational injuries/exposures that may impact color perception. The eye exam needs to include a good fundoscopic exam and as complete a color screening as possible.
Air Force: All color vision deficiencies are disqualifying for all flying classes in the US Air Force, particularly all classes of initial flying duties except flight surgeon applicants and initial flying class positions not requiring color vision normal. Trained aircrew can be considered for a waiver for defective color vision. For trained aircrew, waiver recommendations and management are primarily dependent on the etiology and severity of the color deficiency and can only be made on a case by case basis.
Army: Color vision is not disqualifying for entry into the US Army except for officers where the ability to distinguish vivid red and green is a requirement, however for Airborne, Special Forces and aviation training color vision standards do exist. The US Army standard is for COLOR SAFE, meaning those who may have a mild deficit will be accepted if they can still pass the prescribed screening test algorithm. The Army passing standard is PIP PASS (2 or fewer errors out of 14 presentations). If the PIP is failed, but FALANT (or OPTEC-900, no errors in 9 presentations) is passed, the standard is met. However in this case a one-time ophthalmology/optometry evaluation is required to define the color axis and specific type of deficiency, as well as assess for any underlying abnormalities. Failing PIP and FALANT fails the standard. The assessment of color vision occurs during the initial FDME as well as during any periodic or post-mishap comprehensive exam due to the recognition that though most color vision deficiencies are congenital, some occur as a result of ocular disease and/or medication side effects.
Navy: INFORMATION REQUIRED:
1. Based on a conference with the U.S. Air Force on vision standards and procedures, the Pseudo-Isochromatic Plates (PIP) are considered the preferred primary test.
a. For the Navy, 12 of 14 correctly identified plates constitute a passing score. The preferred lighting is the MacBeth lamp. If one is not available, a daylight fluorescent bulb may be used. Do not use incandescent lighting as this may allow persons with mild deuteranomalous (green weak) deficiencies to pass. Passing criteria is 12 or more plates correctly read, i.e., no more than 2 errors. Record the findings as the number of plates correctly read out of 14. For example: PIPs 13/14 correct "PASS" or PIPs 9/14 correct "FAIL.”
2. If member cannot pass the PIP, the FALANT may be administered as an alternative, if available.
a. Passing criteria for FALANT remains 9/9 or 16/18 correct responses.
3. If a designated crewmember fails both tests, evaluation is required to screen for acquired pathology, as well as a test of demonstrated ability, usually performed with the flight surgeon and safety officer as observers.
Civilian: There are several FAA acceptable tests for color deficiency in airmen. Currently, other than the standard pseudoisochromatic plates there are several other additional tests that the airman may obtain:
Should an airman fail any of these tests, they may seek out a facility that has an alternate and if they pass they must continue to take this test with each examination. If the airman fails one of these office based tests the AME is required to place a restriction on the medical certificate: NOT VALID FOR NIGHT FLYING OR BY COLOR SIGNAL CONTROL. Should the airman want to have this restriction removed, they may request permission to take a Signal Light Gun test. This test is administered by the airman’s local Flight Standards District Office by an FAA Aviation Safety Inspector. Currently there has been a change in policy. If the airman only requires a third-class medical certificate they must pass the signal light test and be able to properly identify objects on an aeronautical chart. This test is known as the OCVT or Operational Color Vision Test. If the airman fails the signal light test during the day they may only repeat it during the night and should they pass they are issued a restriction NOT VALID FOR DAYLIGHT FLIGHT USING COLOR SIGNAL CONTROL. An airman who desires to have a first- or second-class medical certificate must successfully pass the OCVT and take a Medical Flight Test, which includes a flight where they must point out pertinent airport lights, other aircraft lights, objects on avionic multifunctional displays, and all the various ground topography in order to make a successful emergency landing. Airmen who successfully pass these tests are issued a Letter of Evidence rather than a Statement of Demonstrated Ability such as in other functional defects.
Air Force: Query of AIMWTS revealed a total of 231 cases. All 36 initial pilot training cases resulted in a disqualification. Of the remaining 195 cases, 60 were disqualified.
Army: The Aeromedical Epidemiological Data Repository (AEDR) catalogs all Army flight physicals since 1960. There have been approximately 160,000 individual aircrew entered in this database. During this period of time, there have been 31 requests for waiver in the pilot population, 17 were applicants and 14 were already rated. Of the 17 applicants, 4 were granted exceptions to policy. Thirteen of the 14 rated aviators were granted waivers. There were also 76 aeromedical summaries submitted for non-rated crewmembers for which 68 were granted waivers.
Navy: Not available at this time.
Civilian: Based on the discussion in the aeromedical disposition above, the FAA currently codes the color deficiencies as those airmen who pass the signal light gun testing in the standard fashion and those airmen who pass the test only at night time. So in the first category there are 814 first-class airmen, 379 second-, and 1,070 third-class airmen currently issued and in the latter category there are 82 first-, 60 second-, and 192 third-class airmen.
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Prepared by Drs Doug Ivan, John Gooch, Karen Fox and Dan Van Syoc
Date: September 26, 2010