I was teaching my pharmacology class the medications prescribed for erectile dysfunction (ED). In particular, I was teaching a little-known side effect of the class of medications called phosphodiesterase inhibitors. Being the forever flight instructor, I related the side effect to a real-life story of pilots experiencing blue vision disturbances after taking a well-publicized blue tablet for ED. That same day, I saw an FAA mandatory color vision testing update set to start on January 1, 2025. When I combined the two worlds, I began to contemplate possibilities. I asked myself, is it possible or probable that medications may cause an applicant to fail the color blindness test?
The retina in the back of the eye contains two light-detecting cells called photoreceptors. The photoreceptors convert light into electrical signals sent to the brain via the optic nerve. These light-detecting cells are the rods and cones. Rods are photoreceptors that are highly sensitive to light and are responsible for night vision and peripheral vision. Rods are also receptive to motion. Cones in the human eye detect red, green, and blue light. Red-sensing cones make up about 60% of cones. Green-sensing cones make up about 30% of cones, and blue-sensing cones make up about 10% of cones. People with protanopia cannot perceive red light, those with deuteranopia cannot perceive green light, and those with tritanopia cannot perceive blue light. People with red and green deficiencies visualize murky greens where blues and yellows stand out. Browns, oranges, and shades of red and green are easily confused, and people with both types can also confuse some blues with some purples. Pale colors are also a problem.
Color blindness is inherited and primarily seen in males. Females are carriers of the color-blindness gene. Any abnormalities of the retina, optic nerve, and visual cortex in the brain can cause defects in color vision. Diseases like diabetes, glaucoma, and cataracts can alter color vision.
Color blindness in aviation is a significant issue due to our reliance to perceive red, green, and blue hues. Flying is not just black and white, pun intended. Pilots rely on identifying navigation lights, aircraft position lights, airport beacons, runway lights, taxiway markings, sectional maps, and cockpit instruments that all contain green, red, and blue colors. The inability to recognize these colors could lead to misinterpretations of visual objects, causing endangerment to the pilot and passengers.
The Ishihara test is the universal test for detecting color blindness. The test consists of pseudoisochromatic plates. Each plate depicts a solid circle of colored dots appearing randomized in color and size. Within the pattern are dots that form a number or shape visible to those with normal color vision and invisible, or difficult to see, to those with a red-green color vision defect. Other plates are intentionally designed to reveal numbers only to those with a red-green color vision deficiency and be invisible to those with normal red-green color vision. The entire test consists of 38 plates, but a severe deficiency is usually apparent after only a few plates. The Ishihara test is not infallible and not reliable one hundred percent of the time. I have seen many YouTube presentations and blogs on how colorblind applicants can beat the Ishihara test. Many sites provide all 38 plates that can be studied ahead of time. With some studying, one can memorize how the numbers appear as distinct patterns on the plate.
Starting January 1, 2025, digital testing equipment will be required for all initial medical certificates and current colorblind restrictions. If the colorblind applicant with a night flying restriction passes the digital test, the restriction will be removed and they will not have to retest at future FAA medical exams.
An example of one of the digital colorblind tests is the Waggoner Computerized Color Vision Test (CCVT). The new test will use digital equipment to screen for red/green and yellow/blue color deficiencies and can be taken with a laptop or iPad.
The risk of an acquired color vision deficiency is less than one percent and may be associated with medical conditions or medications. Sildenafil (Viagra) was approved for patients on March 27, 1998. In the fall of 1998, the Federal Air Surgeon’s Medical Bulletin had a statement from Dr. Donato Borolo: “Although only 3 percent of all patients report visual disturbances, this blue-green impairment could be dangerous while flying at night or in bad weather. The correct identification by the pilot of blue and green is necessary for safe flight. In addition, the use of color video terminal displays has increased in aviation and may present a problem in the color-deficient pilot.” I began sharing this information with my student pilots, biennial flight review pilots, and many friends flying for the airlines.
Viagra is a class of medications called phosphodiesterase inhibitors. Viagra was initially studied in patients with high blood pressure, and one of the side effects was an erection. This side effect led to the clinical trials of using Viagra in patients with erectile dysfunction. The mechanism of action of Viagra is an improvement of the blood flow to the penis, causing an erection. There are many different phosphodiesterase receptors. The phosphodiesterase inhibition by Viagra is on phosphodiesterase 5 (PDE5). There is a minor inhibitory action against PDE6, which is present exclusively in rod and cone photoreceptors. At higher doses, sildenafil causes mild and transient visual symptoms in a minority of patients. The vision changes are mainly blue vision tinge. The effect is seen in patients prescribed tadalafil (Cialis) as well. High-dose Viagra will take at least 24 hours to clear your system. Cialis may have an effect for 36 hours and take three to four days to leave the body. The FAA says to wait at least 8 hours after taking Viagra, while Cialis used prn (as needed) requires a 24-hour wait after use.
Digoxin is a medication prescribed for heart failure. Digoxin’s mechanism of action causes an increase in the heart’s contraction, which is weakened by heart failure. Digoxin has a fascinating history. Dr. William Withering used foxglove (Digitalis purpurea), also known as purple coneflower, to treat dropsy (congestive heart failure) in 1775. One of the side effects of digitalis is green/yellow halos around objects in the visual field. The story goes that Vincent Van Gogh had heart disease and was using foxglove for his heart symptoms. When Van Gogh had imbibed too much digitalis, he saw halos around everything. The mechanism of the halos may be due to stimulation of the chemoreceptor trigger zone in the brain. Visual disturbances, such as the appearance of halos and distortions in color perception, may also occur because of a direct effect of digoxin on the visual cortex. Objects often appear yellow or green.
Hydroxychloroquine is an immunosuppressant medication indicated for patients with rheumatoid arthritis and systemic lupus erythematosus. The original indication for hydroxychloroquine was for patients with malaria. When taking hydroxychloroquine, the color most commonly affected in color vision is red, with patients often reporting difficulty distinguishing shades of red or seeing them as washed out.
Tamoxifen is an antiestrogen medication used to treat estrogen receptor-positive metastatic breast cancer. Tamoxifen is also prescribed for postmenopausal women to help prevent bone loss, thus reducing the risk of osteoporosis. Tamoxifen can potentially cause changes in color vision. Studies have reported cases of mild to moderate color vision loss. Tamoxifen can affect the function of short-wavelength sensitive (SWS) cones in the retina. Patients may notice difficulty distinguishing blues and greens or a general washing-out of colors.
Alendronate (Fosamax) is a medication prescribed for osteoporosis. In a case report where the dose of alendronate was increased, red-colored visual disturbances were reported. In 2019, there were almost eight million prescriptions written for Fosamax.
If you plan on going for your initial medical certificate, have a color-blindness restriction, or are applying for a renewal, be aware that certain medications may affect your colorblind test. Notice if your AME has not obtained the new digital color vision testing equipment and you have been prescribed the above medications. Be well and fly safely.
