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Disorders of the Eye

Chapter 34 | Harrison's 22e

KEY CLINICAL POINTS

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[PAGE 220] 220 PART Cardinal Manifestations and Presentation Diseases FURTHER READING termed the fovea, packed exclusively with cones, provides the best Aquino G et al: Towards the neurobiology of insomnia: A systematic visual acuity. review of neuroimaging studies. Sleep Med Rev 73:101878, 2024. Photoreceptors hyperpolarize in response to light, activating bipolar, Cash RE et al: Association between sleep duration and ideal cardio- amacrine, and horizontal cells in the inner nuclear layer. After process- vascular health among US adults, National Health and Nutrition ing of photoreceptor responses by this complex retinal circuit, the flow Examination Survey. Prev Chronic Dis 17:E43, 2020. of sensory information ultimately converges on a final common path- Chinoy ED et al: Unrestricted evening use of light-emitting tablet way: the ganglion cells. These cells translate the visual image impinging computers delays self-selected bedtime and disrupts circadian timing on the retina into a continuously varying barrage of action potentials and alertness. Physiol Rep 6:e13692, 2018. that propagates along the primary optic pathway to visual centers Cribb L et al: Sleep regularity and mortality: A prospective analysis in within the brain. There are a million ganglion cells in each retina and the UK Biobank. Elife 12:RP88359, 2023. hence a million fibers in each optic nerve. Holth JK et al: The sleep-wake cycle regulates brain interstitial fluid Ganglion cell axons sweep along the inner surface of the retina in tau in mice and CSF tau in humans. Science 363:880, 2019. the nerve fiber layer, exit the eye at the optic disc, and travel through Landrigan CP et al: Effect on patient safety of a resident physician the optic nerve, optic chiasm, and optic tract to reach targets in the schedule without 24-hour shifts. N Engl J Med 382:2514, 2020. brain. The majority of fibers synapse on cells in the lateral geniculate Lee ML et al: High risk of near-crash driving events following night- nucleus, a thalamic relay station. Cells in the lateral geniculate nucleus shift work. Proc Natl Acad Sci USA 113:176, 2016. project in turn to the primary visual cortex. This afferent retinoge- Liblau RS et al: The immunopathogenesis of narcolepsy type 1. Nat niculocortical sensory pathway provides the neural substrate for visual Rev Immunol 24:33, 2024. perception. Separate classes of ganglion cells project to subcortical Scammell TE: Narcolepsy. N Engl J Med 373:2654, 2015. visual nuclei involved in other functions. Pupillary constriction and Scammell TE et al: Neural circuitry of wakefulness and sleep. Neuron circadian rhythms are governed by ganglion cells that are intrinsically 93:747, 2017. light sensitive, owing to a pigment named melanopsin. Pupil reflexes Sletten TL et al: The importance of sleep regularity: A consensus are mediated by a projection to the pretectal olivary nuclei. Their statement of the National Sleep Foundation sleep timing and vari- output is supplied to the Edinger-Westphal nuclei, which provide para- ability panel. Sleep Health 9:801, 2023. sympathetic innervation to the iris sphincter via an interneuron in the ciliary ganglion. Circadian rhythms are timed by melanopsin ganglion cells that project to the suprachiasmatic nucleus. Visual orientation and VIDEO 33-1 A typical episode of severe cataplexy. The patient is joking and eye movements are served by retinal input to the superior colliculus. then falls to the ground with an abrupt loss of muscle tone. The electromyogram Gaze stabilization and optokinetic reflexes are governed by a cluster recordings (four lower traces on the right) show reductions in muscle activity of small retinal targets known collectively as the brainstem accessory during the period of paralysis. The electroencephalogram (top two traces) shows optic system. wakefulness throughout the episode. (Video courtesy of Giuseppe Plazzi, University The eyes must be rotated constantly within their orbits to place of Bologna.) and maintain targets of visual interest on the fovea. This activity, called foveation, or looking, is governed by an elaborate efferent VIDEO 33-2 Typical aggressive movements in rapid eye movement (REM) sleep motor system. Each eye is moved by six extraocular muscles that behavior disorder. (Video courtesy of Dr. Carlos Schenck, University of Minnesota are supplied by cranial nerves from the oculomotor (III), trochlear Medical School.) (IV), and abducens (VI) nuclei. Activity in these ocular motor nuclei is coordinated by pontine and midbrain mechanisms for smooth pursuit, saccades, and gaze stabilization during head and body move- ments. Large regions of the frontal and parietooccipital cortex con- trol these brainstem eye movement centers by providing descending supranuclear input. Section 4 Disorders of Eyes, Ears, Nose, CLINICAL ASSESSMENT OF VISUAL FUNCTION and Throat REFRACTIVE STATE In approaching a patient with reduced vision, the first step is to decide 34 Disorders of the Eye whether refractive error is responsible. In emmetropia, parallel rays from infinity are focused perfectly on the retina. Sadly, this condition is enjoyed by only a minority of the population. In myopia, the globe Jonathan C. Horton is too long, and light rays come to a focal point in front of the retina. Near objects can be seen clearly, but distant objects require a diverging lens in front of the eye. In hyperopia, the globe is too short, and hence, THE HUMAN VISUAL SYSTEM a converging lens is used to supplement the refractive power of the The visual system provides a supremely efficient means for the rapid eye. In astigmatism, the corneal surface is not spherical, necessitating assimilation of information from the environment to aid in the guid- a cylindrical corrective lens. Most patients elect to wear eyeglasses or ance of behavior. The act of seeing begins with the capture of images contact lenses to neutralize refractive error. An alternative is to perma- focused by the cornea and lens on a light-sensitive membrane in nently alter the refractive properties of the cornea by performing laser the back of the eye called the retina. The retina is actually part of in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK). the brain, banished to the periphery to serve as a transducer for the With the onset of middle age, presbyopia develops as the lens within conversion of patterns of light energy into neuronal signals. Light is the eye becomes unable to increase its refractive power to accommo- absorbed by pigment in two types of photoreceptors: rods and cones. date on near objects. To compensate for presbyopia, an emmetropic In the human retina, there are 100 million rods and 5 million cones. patient must use reading glasses. A patient already wearing glasses for The rods operate in dim (scotopic) illumination. The cones function distance correction usually switches to bifocals. The only exception is a under daylight (photopic) conditions. The cone system is special- myopic patient, who may achieve clear vision at near simply by remov- ized for color perception and high spatial resolution. The majority ing glasses containing the distance prescription. of cones are within the macula, the portion of the retina that serves Refractive errors usually develop slowly and remain stable after ado- the central 10° of vision. In the middle of the macula, a small pit lescence, except in unusual circumstances. For example, the acute onset [PAGE 221] 221 CHAPTER Disorders the Eye of diabetes mellitus can produce sudden myopia because of lens edema in conjunction with weak or absent tendon reflexes in the lower induced by hyperglycemia. Testing vision through a pinhole aperture is extremities. This benign disorder, which occurs predominantly in a useful way to screen quickly for refractive error. If acuity is improved healthy young women, is assumed to represent a mild dysautonomia. by viewing through a pinhole, the patient nee