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Myopia: Causes, Prevention, and Treatment of an Increasingly Common Disease (2024)

Chapter: Appendix C: Glossary

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Suggested Citation: "Appendix C: Glossary." National Academies of Sciences, Engineering, and Medicine. 2024. Myopia: Causes, Prevention, and Treatment of an Increasingly Common Disease. Washington, DC: The National Academies Press. doi: 10.17226/27734.

Appendix C
Glossary

Accommodation of the eye	Refers to the eye’s ability to adjust its focus to see objects clearly at different distances. This is achieved by changing the shape and curvature of the eye’s natural lens to bring images into sharp focus on the retina.
Agonist	A molecule that binds to a receptor and initiates a response. Receptors often behave like switches; agonists turn the switch “on.” Agonists can be natural substances such as hormones or neurotransmitters, or they can be synthetic. Many drugs are agonists. See also “antagonist.”
Allele	One of two or more versions of a DNA sequence (a single base or a segment of bases) at a given genomic location. An individual inherits two alleles, one from each parent, for any given genomic location where such variation exists. If the two alleles are the same, the individual is homozygous for that allele. If not, the individual is heterozygous.
Anisometropia	The presence of asymmetric refraction between the two eyes, typically defined by a difference of 1 or more diopter.
Antagonist	A molecule that binds to a receptor but does not initiate a response. Receptors often behave like switches; antagonists turn the switch “off” by blocking the receptor binding site. Many drugs are antagonists. See also “agonist.”
Axial length	In layman’s language, this is the distance in millimeters from the front of the eye to the back of the eye. Technically, axial length is measured along the optical axis from the corneal apex to the front surface of the retina. The axial length of the eye in an emmetropic human adult is approximately 23 mm.
Blur	A loss of sharpness in an image, especially the image on the retina. Blur can be caused by defocus, by optical aberrations other than those that lead to defocus, or by a motion of the image.
Cerebral cortex or cortical brain	Outermost layer of tissue in the brain. The cerebral cortex, also referred to as gray matter, covers the cerebrum, which is the largest portion of the brain. The cerebral cortex is responsible for integrating sensory impulses, directing motor activity, and controlling higher intellectual functions.
Choroid	A layer of tissue lying beneath the retina that has a very rich blood supply. A key role of the choroid is to provide nutrients and oxygen for the retina.
Ciliary body	A part of the middle layer of the wall of the eye. The ciliary body is found behind the iris and includes the ring-shaped muscle that changes the shape of the lens when the eye focuses. It also produced the clear fluid that fills the space between the cornea and the iris.

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Cones (or cone cells)	See Photoreceptors.
Cost-effective measure	Informally, a preventive measure or treatment that provides good value for money spent; sometimes simply “cost-saving” or “less expensive.” More formally, a preventive measure or treatment that leads to improved outcomes at lower costs or one that costs less than a threshold monetary level per improved outcome. The U.S. government does not use such thresholds; some other countries do.
Cost-effectiveness analysis	Informally, a study looking at both resources used and outcomes for a preventive measure or intervention. More formally, an analysis in which the costs and effects of two or more interventions are compared. If one is more effective and less expensive it is said to “dominate;” in turn, the one that is more expensive and less effective is “dominated.” In general, one is interested in the amount of resources that need to be spent for improved outcomes; in the United States this is expressed in dollars spent for each unit of change in outcome. The ratio is called an “incremental cost-effectiveness ratio.”
Cross-sectional study	A type of research study in which a group of people is observed, or certain information is collected, at a single point in time or over a short period of time.
Cycloplegia	The paralysis of the ciliary muscle of the eye resulting in dilatation of the pupil and paralysis of accommodation. This can be achieved by instilling cycloplegic agents such as atropine, cyclopentolate, or tropicamide eye drops.
Defocus	In optics, defocus is the aberration in which an image is out of focus. This aberration causes loss of sharpness and contrast. It will be familiar to anyone who has used a camera, video-camera, microscope, telescope, or binoculars. Optically, defocus refers to a translation of the focus along the optical axis away from the detection surface.
Degenerative myopia	Also known as malignant, pathological, or progressive myopia, is characterized by marked fundus changes, such as posterior staphyloma, and associated with a high refractive error and subnormal visual acuity after correction. This form of myopia gets progressively worse over time. Degenerative myopia has been reported as one of the main causes of visual impairment.
Direct cost	A cost that actually involves the exchange of money, which may occur electronically. This is in contrast to indirect or productivity or time costs, which in analyzing vision care costs are used with the recognition that treatments and intervention require time for the patient (and often caregiver) that could be used to produce economic output (even within the home) and that changes in educational attainment that are related to being able to see better can also lead to changes in economic output (even within the home).
Dopamine	Also known as DA, an abbreviation for 3,4-dihydroxyphenethylamine, dopamine is a neuromodulatory molecule that plays several important roles in cells. An organic chemical of the catecholamine and phenethylamine families, it constitutes about 80% of the catecholamine

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	content in the brain. In the brain, dopamine functions as a neurotransmitter—a chemical released by neurons (nerve cells) to send signals to other nerve cells. Neurotransmitters are synthesized in specific regions of the brain but affect many regions systemically. The brain includes several distinct dopamine pathways, one of which plays a major role in the motivational component of reward-motivated behavior.
Ecologic (or ecological) study	An epidemiology study design in which the unit of observation is the population or community. Ecologic studies of geographical associations between disease prevalence and risk factor levels may provide a broader range of exposure to the risk factor than within-population studies.
Emmetropization	The developmental process that matches the eye’s optical power to its axial length so that the unaccommodated eye is focused at distance. This is a process whereby the refractive components and the axial length of the eye come into balance during postnatal development in order to induce emmetropia (vision with no refractive error). A person with emmetropia may be referred to as an emmetrope.
Epidemiology	The study of the determinants, occurrence, and distribution of health and disease in a defined population.
Epigenetics	The study of how changes in behavior and the environment cause changes in gene expression. Unlike genetic changes, epigenetic changes are reversible and do not change a person’s DNA sequence, but they do change how the body reads a DNA sequence.
Federally qualified health centers (FHQCs) and rural health clinics (RHCs)	Safety-net providers that deliver prevention and primary services in an outpatient clinic setting.
Form deprivation myopia	Myopia that occurs when the eye is deprived of clear vision by light scattering, not because of a focusing error. In a young human or animal whose eyes are still in their growth phase, form deprivation acts as a strong signal for the eye to grow and become myopic.
Fovea	Short for fovea centralis, it is a small, central pit located in the macula lutea, at the back of the retina, and is composed of closely packed cone cells.
Genome wide association study (GWAS)	A test of hundreds of thousands of genetic variants across many participants to find those statistically associated with a specific trait or disease. This methodology has generated a myriad of robust associations for a range of traits and diseases, and the number of associated variants is expected to grow steadily as GWAS sample sizes increase. GWAS results have a range of applications, such as gaining insight into a phenotype’s underlying biology, estimating its heritability, calculating genetic correlations, making clinical risk predictions, informing drug development programs and inferring potential causal relationships between risk factors and health outcomes.
Glutamate	An amino acid and also acts as a neurotransmitter in the central nervous system.

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Glutamatergic	Refers to neurons or synapses that release glutamate as a neurotransmitter. These neurons play crucial roles in many cognitive functions and are involved in various neurological disorders.
Glutamine	A conditionally essential amino acid that performs many functions in the body. Glutamine is essential for the homeostasis of glutamatergic neurotransmission. Glutamine is also the most abundant extracellular amino acid in the body. Glutamine is a precursor to glutamate, which acts as a neurotransmitter in glutamatergic signaling pathways in the brain and nervous system.
Glutaminergic	Refers to neurons or synapses that use glutamine as a precursor to synthesize glutamate. Glutaminergic neurons convert glutamine into glutamate through the enzyme glutaminase.
Glutaminergic amacrine cells (GACs)	Glutaminergic amacrine cells are specialized neurons found in the retina, part of the eye’s neural network that processes visual information. These cells are called “glutaminergic” because they use glutamine, which can be converted into glutamate, to mediate their functions.
Glycosaminoglycan (GAG)	Also known as mucopolysaccharides, are negatively-charged polysaccharide compounds. They are long, unbranched polysaccharide molecules composed of repeating disaccharide units that are present in every mammalian tissue.
High myopia	This is generally defined as nearsightedness requiring a glasses correction of 6.00 diopters or more and is sometimes referred to as degenerative myopia or pathological myopia.
Hyperopia	Farsightedness, that is, the condition of the eyes in which light from distant objects is brought to a focus behind the retina when accommodation is relaxed. In older individuals this leads to blurry vision, but in children and young adults it often leaves them able to see clearly by accommodative focusing of the crystalline lens. Hyperopia occurs because the eye is too short relative to the focusing power of its optical elements.
Intraocular pressure	Similar to blood pressure, the eye maintains a stable pressure. The intraocular pressure helps ensure that the eye retains its correct shape. Intraocular pressure is measured in units of mm Hg. Commonly abbreviated to “IOP”.
Intrinsically photosensitive retinal ganglion cells (ipRGCs)	Also called photosensitive retinal ganglion cells (pRGC), or melanopsin-containing retinal ganglion cells (mRGCs), ipRGCs are a type of neuron in the retina of the mammalian eye. These are photoreceptor cells that are particularly sensitive to the absorption of short-wavelength (blue) visible light. They communicate information directly to the area of the brain called the suprachiasmatic nucleus, also known as the central “body clock,” in mammals.
Lateral geniculate nucleus (LGN)	Also called the lateral geniculate body or lateral geniculate complex, it is a structure in the thalamus and a key component of the mammalian visual pathway. It is a small, ovoid, ventral projection of the thalamus where the thalamus connects with the optic nerve.

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Mendelian randomization (MR)	A term that applies to the use of genetic variation to address causal questions about how modifiable exposures influence different outcomes. The principles of MR are based on Mendel’s laws of inheritance and instrumental variable estimation methods, which enable the inference of causal effects in the presence of unobserved confounding.
Melanopsin	A type of photopigment belonging to a larger family of light-sensitive retinal proteins called opsins and encoded by the gene Opn4. In the mammalian retina, there are two additional categories of opsins, both involved in the formation of visual images: rhodopsin and photopsin (types I, II, and III) in the rod and cone photoreceptor cells, respectively. In humans, melanopsin is found in intrinsically photosensitive retinal ganglion cells (ipRGCs). It is also found in the iris of mice and primates. Melanopsin plays an important non-image-forming role in the setting of circadian rhythms as well as other functions.
Muscarinic acetylcholine receptors, or mAChRs	Acetylcholine receptors that form G protein-coupled receptor complexes in the cell membranes of certain neurons and other cells. They play several roles, including acting as the main end-receptor stimulated by acetylcholine released from postganglionic fibers in the parasympathetic nervous system.
Myopia	A type of refractive error of the eye characterized by blurry distance vision, yet sharp near vision (nearsightedness).
Oblate and prolate spheroid shapes	If an ellipse is rotated about its major axis, the result is a prolate spheroid, elongated like a rugby ball (less pointy than an American football). If an ellipse is rotated about its minor axis, the result is an oblate spheroid, flattened like a lentil or a plain M&M candy piece.
Opsin	See Melanopsin
Orthokeratology	A modality of contact lens wear that serves as an alternative to spectacles, to provide clear vision for patients with myopia. By contrast to conventional contact lenses that are worn during the daytime, orthokeratology contact lenses are worn overnight and not in the daytime. During overnight wear, the shape of the orthokeratology contact lens and the pressure of the eyelid re-mold the front surface of the cornea, such that the refractive error of the eye becomes less myopic. The contact lenses need to be worn every night. Also known as “ortho-K.”
Photopic vision	Vision in lighting conditions as bright as ordinary room illumination or outdoors during daylight.
Photoreceptors	Specialized cells for detecting light. Two types of photoreceptors reside in the retina: cones and rods. The cones are responsible for daytime vision, while the rods respond under dark conditions. The cones come in three varieties that each responds to a different portion of the visible spectrum, allowing for color vision: L, M, and S types (for long, middle, and short wavelength). Rods have a spectral sensitivity that differs from that of cones.
Plus or minus lenses	Types of lenses include (A) converging (convex or plus) lenses, and (B) diverging (concave or minus) lenses. The focal point of a plus lens occurs where parallel light rays that have passed through the lens converge to

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	form an image. The focal point of a minus lens occurs where parallel light rays entering the lens appear to diverge.
Prolate shape	See Oblate and prolate spheroid shapes
Randomized controlled trial (RCT)	A study in which the participants are divided by chance into separate groups to compare different treatments or other interventions. A randomized controlled trial is a form of scientific experiment used to control factors not under direct experimental control. Examples of RCTs are clinical trials that compare the effects of drugs, surgical techniques, medical devices, diagnostic procedures, or other medical treatments.
Refractive error	A type of vision problem that makes it hard to see clearly. Such an error happens when the shape of the eye keeps light from focusing correctly on the retina (see below). Refractive errors are the most common type of vision problem. The most common refractive errors are myopia (nearsightedness), hyperopia (far-sightedness), astigmatism, and presbyopia (difficulty reading up close due to lens stiffness, which most commonly affects individuals above the age of 40 years).
Refractive surger Retina	Any surgical procedure that corrects the refractive error of the eye. A layer of photoreceptor cells and glial cells within the eye that captures incoming light and transmits signals representing (primarily) color and luminance along neuronal pathways as both electrical and chemical signals for the brain to perceive a visual picture.
Retinitis pigmentosa	An inherited retinal disease, also known as an inherited retinal dystrophy, characterized by progressive loss of peripheral (side) vision. The condition often occurs in families. Inheritance can follow different pattern in different families, depending on which gene is affected.
Rods	See Photoreceptors
Sclera	an opaque, tough layer of tissue that forms the outer shell of the middle and back of the eye. The tissue of the sclera is continuous with the cornea. The sclera largely determines the size and shape of the eye.
Scleral collagen	Collagen is a protein that forms long, strong fibers. The fibers are woven together to form tough yet pliable tissues known as “connective tissue.” Examples of connective tissue include tendon, skin, and the sclera.
Scotopic vision	Vision in very dim lighting conditions.
Vision screening	Vision screening uses a limited set of procedures designed to detect problems with the eye or vision that can then be diagnosed and treated through a comprehensive eye examination.
Visual field	Similar to field of view, the visual field is the angular extent that can be seen by the eye. Because the retinal image is an inverted picture of the world, the visual field is seen by the opposite retina (e.g., objects in the temporal visual field of the right eye are seen by that eye’s nasal retina).