Functional visual loss

Before diagnosing Functional vision loss, the patient should be tested to rule out conditions which cause defective vision or even blindness in one or both eyes with normal anterior segment and a normal fundus. Some major conditions that should be considered include:

• Amblyopia- Amblyopia is a condition in which the brain fails to fully process input from one eye and over time favors the other eye and the vision in one become reduced.[7]
• Cortical blindness-Cortical blindness is the total or partial loss of vision in a normal-appearing eye caused by damage to the brain's occipital cortex.[8]
• Retrobulbar neuritis- Retrobulbar neuritis cause of visual loss with normal fundus but there will be relative afferent pupillary defect (RAPD) and abnormal visually evoked responses.[9]
• Cone dystrophy- Cone dystrophy can cause a blurring of vision, color vision defects and photophobia.[10]
• Chiasmal tumors- In chiasmal tumors, in addition to vision loss, sluggish pupillary reactions and characteristic visual field defects may be noted.[9]

The general behavior of the patient is important in this. If the patient easily enters and walks around the exam room without hesitation, it is a sign of malingering. If a person claims he has bilateral visual impairment, the following tests can be done to see if he is lying.

• Eye contact effect- If a patient claims to be completely blind in both eyes and makes eye contact with the examiner, it is definitely a case of malingering.[6]
• Obstructions- Ask the patient to walk through the exam room where there are obstacles such as waste baskets and chairs and observe if they identify and avoid them.[6]
• Looking at hand- Ask the patient to look at their hand. Those who are truly blind will hold out their hand, look at it, and say, "I know where my hand is, but cannot see it," but malingers, without looking at the hand, will assert, "I cannot see it."[6]
• Signature Test- If the patient is asked to sign with a pen and paper, people who are truly blind will sign easily and consistently multiple times, but those who are simulating blindness will generally scribble on the paper.[6]
• Menace test- If the patient closes their eyes when a hand is suddenly waved towards the patient's face, it means that that eye has vision.[6]
• Mirror test- Hold a large mirror in front of you and ask the patient to look into it. Then slowly turn the mirror sideways. If the patient changes their vision according to rotating the mirror, it means they have vision.[6]
• Reading from smallest letter- Start with the smallest line in the eye chart and gradually increase the size of the letters and ask the patient to read them. Also express surprise to the patient at not being able to see the letters even though they are doubled in size.[3] In this way the patient is more likely to read.
• Vision aids- Four lenses with zero power when placed together are given to the patient in a trial frame and say patient that the lenses are special magnifying lenses. If he says any change in vision, he is malingering.[3]
• Near Vision Testing- In general, near vision decreases in proportion to the decrease in distance vision. A large discrepancy between near visual acuity and distance acuity indicates that the problem is not organic.[3]
• Optokinetic response- In this test, the nystagmus movement of the eyes indicates at least 20/400 vision.[3]
• Psychogalvanic test- This test is useful in people who say they can't even see light. The patient is seated in front of a slit lamp and a bright light is suddenly flashed into his eyes. If they blink, it means they see light.[3]

If a person claims that he has defective vision or no vision at all in one eye, if there are no other signs, the following tests can be done to see if he is malingering.

• Relative afferent pupillary defect- If there is no refractive error or media opacity that cause disparity in visual acuity between the two eyes, if true pathology is present, the affected eye will have a Relative afferent pupillary defect.[3]
• Convex Lens Test- Place a weak convex or concave lens (0.25 D) in front of the non-sighted eye and a high-power convex lens (+10 D) in front of the good eye. If the patient can read all the letters far away, it should be understood that their claimed sightless eye has sight.[9]
• Prism base down test- Place a prism upside down in front of the good eye and ask it to look at a distant light source. If the patient admits seeing two lights, it means that both eyes have vision.[9]
• Prism Base Out Test. Ask the patient to look at a light source. A prism of 10 D is then placed in front of the non-sighted eye with base out. Malingering is confirmed if the eye moves inward (brain activity to eliminate diplopia).[9]
• Red-Green Test- Patient is asked to read the red-green duochrome chart after placing a red glass in front of the good eye. If the patient reads all the letters, it means he has good vision in the said defective eye also.[9]
• Cycloplegic test- Cycloplegic test can be used in children those are not myopic.[3] Put a cycloplegic drug in the good eye and saline in the so called blind eye. After accommodation is paralyzed in the good eye, ask the patient to read small near prints with both eyes open.[3] Cycloplegia in the good eye will blur near vision in that eye, so if the patient reads without any discomfort, it means both eyes have good vision.[3]

If a person claims that he has defective visual field, if there are no other signs, the following tests can be done to see if he is lying.

• Visual field test- Visual field tests including Confrontation testing can be used to rule out defects in the field of vision.[3]