8.4: Causes of Blindness and Low Vision
- Page ID
- 178846
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Blindness and low vision have many different causes affecting different parts and functions of the eye, including optical defects, ocular motility disturbance, and external and internal problems of the eye (Smiley et al., 2022). Vision relies on an interaction between the eye and the brain. First, light passes through the cornea, which is the clear front layer of the eye. The cornea is shaped like a dome and bends light to help the eye focus. Some of this light enters the eye through an opening called the pupil. The iris, the colored part of the eye, controls how much light the pupil lets in. Next, light passes through the lens, a clear inner part of the eye. The lens works with the cornea to focus light correctly on the retina. When light hits the retina, a light-sensitive layer of tissue at the back of the eye, special cells called photoreceptors turn the light into electrical signals. These electrical signals travel from the retina through the optic nerve to the brain. The brain then turns the signals into images (National Eye Institute, 2022a).
Optical Defects
There are three major optical defects: myopia, hyperopia, and astigmatism. These are refraction errors. Refractive errors are a type of vision problem that makes it hard to see clearly. Refraction errors happen when the shape of the eye keeps light from focusing correctly on the retina.
- Myopia (i.e., nearsightedness) makes faraway objects look blurry.
- Hyperopia (i.e., farsightedness) makes nearby objects look blurry.
- Astigmatism can make faraway and nearby objects look blurry or distorted.
Symptoms of refraction errors include double vision, hazy vision, or a glare or halo around bright lights. Students may also experience headaches or eye strain or have trouble focusing when reading or looking at electronic devices (National Eye Institute, 2022b). Astigmatism is usually present at birth. However, myopia and hyperopia can develop throughout the lifespan (Smiley et al., 2022).
Ocular Motility Disturbance
Ocular motility disturbance refers to any abnormal eye alignment or difficulty controlling eye movements. These conditions involve the brain and the muscles that control eye movement and include strabismus, nystagmus, and amblyopia. Strabismus is a condition where the eyes are not facing in the same direction. It is a relatively common condition in children and may be present at birth. Symptoms of strabismus may include eyes that do not point in the same direction, double vision, and poor depth perception.
Amblyopia is a condition in which the brain fails to process the information from one eye. This may occur because one eye is misaligned (strabismus) and incorporating that information would cause double vision. It may also occur if one eye has much better vision than the other, and the brain selects the clearer image. Nystagmus is characterized by fast, uncontrollable eye movements. This may be present at birth (Houston Methodist, 2022).
Types of External Eye Problems
External eye problems may affect the orbit, eyelids, and cornea. Orbital problems may include protruding, recessed, or abnormally small eyeballs. Abnormalities of the eyelids include drooping of the upper eyelids, outward or inward rolling of the eyelids, eyelids that do not close completely, and inflammation of the lids or glands around the eye (Smiley et al., 2022). Students may also experience corneal problems such as growths, thinning of the cornea, and inflammation of the cornea, which can lead to problems with vision, pain, and tearing of the cornea (National Eye Institute, 2022c).
Types of Internal Eye Problems
There are also several conditions that may affect the internal components of the eye, such as retinopathy of prematurity (ROP). ROP is an eye disease that may occur in babies who are premature or who weigh less than 3 pounds at birth. ROP happens when abnormal blood vessels grow in the retina. Some babies with ROP have mild cases and get better without treatment. But some babies need treatment to protect their vision and prevent blindness (National Eye Institute, 2022d).
In addition, retinitis pigmentosa (RP), a group of rare eye diseases that affect the retina, causes cells in the retina to break down slowly over time, causing vision loss. RP is genetic, with symptoms beginning in childhood, resulting in significant vision loss (National Eye Institute, 2022e). Cortical visual impairment (CVI) is a disorder caused by damage to the parts of the brain that process vision. A child with CVI has vision problems caused by their brain that cannot be explained by a problem with their eyes. Normally, the eyes send electrical signals to the brain, and the brain turns those signals into images. Children with CVI have trouble processing and understanding these signals. CVI is caused by an injury to the brain. These injuries usually happen before, during, or shortly after birth (National Eye Institute, 2022f). Finally, traumatic brain injury may also cause vision problems in children (Smiley et al., 2022).
Smiley, L. R., Richards, S.B., & Taylor, R. (2022). Exceptional students: Preparing teachers for the 21st century (4th ed.). McGraw Hill.
National Eye Institute. (2022a). How the eyes work. https://www.nei.nih.gov/learn-about-eye-health/healthy-vision/how-eyes-work
National Eye Institute. (2022b). Refraction errors. https://www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/refractive-errors
Houston Methodist. (2022). Ocular motility disturbances. https://www.houstonmethodist.org/neurology/neuro-ophthalmology-diseases/ocular-motility-disturbances/
National Eye Institute. (2022c). Corneal conditions. https://www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/corneal-conditions
National Eye Institute. (2022d). Retinopathy of prematurity. https://www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/retinopathy-prematurity
National Eye Institute. (2022e). Retinitis pigmentosa. https://www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/retinitis-pigmentosa
National Eye Institute. (2022f). Cerebral visual impairment. https://www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/cerebral-visual-impairment-cvi