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  • 12
  • Retina

Gabriele K. Lang and Gerhard K. Lang

  • 12.1
  • Basic Knowledge

The retina is the innermost of three successive layers of the globe. It comprises two parts:

❖

A photoreceptive part (pars optica retinae), comprising the first nine of

the 10 layers listed below.

A nonreceptive part (pars caeca retinae) forming the epithelium of the cil-

❖

iary body and iris.
The pars optica retinae merges with the pars ceca retinae at the ora serrata.

Embryology: The retina develops from a diverticulum of the forebrain (proen-

cephalon). Optic vesicles develop which then invaginate to form a doublewalled bowl, the optic cup. The outer wall becomes the pigment epithelium, and the inner wall later differentiates into the nine layers of the retina. The retina remains linked to the forebrain throughout life through a structure known as the retinohypothalamic tract.

Thickness of the retina (Fig. 12.1)

Layers of the retina: Moving inward along the path of incident light, the individual layers of the retina are as follows (Fig. 12.2):
1. Inner limiting membrane (glial cell fibers separating the retina from the vitreous body).
2. Layer of optic nerve fibers (axons of the third neuron). 3. Layer of ganglion cells (cell nuclei of the multipolar ganglion cells of the third neuron; “data acquisition system”).
4. Inner plexiform layer (synapses between the axons of the second neuron and dendrites of the third neuron).
5. Inner nuclear layer (cell nuclei of the bipolar nerve cells of the second neuron, horizontal cells, and amacrine cells).
6. Outer plexiform layer (synapses between the axons of the first neuron and dendrites of the second neuron).
7. Outer nuclear layer (cell nuclei of the rods and cones = first neuron). 8. Outer limiting membrane (sieve-like plate of processes of glial cells through which rods and cones project).

300 12 Retina

Thickness of the retina.

At the equator: 0.18 mm

Close to the ora serrata: 0.12 mm

Around the fovea: 0.23 mm

Fovea centralis: 0.10 mm

At the optical disk: 0.56 mm

Fig. 12.1 Retinal tears most often occur close to the ora serrata.

9. Layer of rods and cones (the actual photoreceptors).
10. Retinal pigment epithelium (a single cubic layer of heavily pigmented epithelial cells).
11. Bruch’s membrane (basal membrane of the choroid separating the retina from the choroid).

Macula lutea: The macula lutea is a flattened oval area in the center of the retina approximately 3–4 mm (15 degrees)temporal to and slightly below the optic disk. Its diameter is roughly equal to that of the optic disk (1.7–2 mm). The macula appears yellow when examined under green light, hence the name macula lutea (yellow spot). Located in its center is the avascular fovea

Fig. 12.2 a Layers of the retina and examination methods used to diagnose abnormal ̈ processes in the respective layers (EOG = electro-oculogram; ERG = electroretinogram; VEP = visual evoked potential). b Histologic image of the 10 layers of the retina.

12.1 Basic Knowledge

301

Histology and function of the layers of the retina.

L i g h t

1. Inner limiting membrane

2. Layer of optic nerve fibers

Optic nerve

VEP

3. Layer of ganglion cells

Pattern ERG

4. Inner plexiform layer

Amacrine cells

5. Inner nuclear layer

Bipolar cells

6. Outer plexiform layer

Horizontal cells

ERG

7. Outer nuclear layer

Photoreceptors

8. Outer limiting membrane

9. Layer of rods and cones

Supporting cells of Müller

10. Retinal pigment epithelium

EOG

11. Bruch's membrane

123

456

789
10

Fig. 12.2

302 12 Retina

centralis, the point at which visual perception is sharpest. The fovea centralis contains only cones (no rods)each with its own neural supply, which explains why this region has such distinct vision. Light stimuli in this region can directly act on the sensory cells (first neuron)because the bipolar cells (second neuron)and ganglion cells (third neuron)are displaced peripherally.

Vascular supply to the retina: The inner layers of the retina (the inner limiting membrane through the inner nuclear layer)are supplied by the central artery of the retina. This originates at the ophthalmic artery, enters the eye with the optic nerve, and branches on the inner surface of the retina. The central artery is a genuine artery with a diameter of 0.1 mm. It is a terminal artery without anastomoses and divides into four main branches (see Fig. 12.8).

Because the central artery is a terminal artery, occlusion will lead to retinal infarction.

The outer layers (outer plexiform layer through the pigment epithelium)contain no capillaries. They are nourished by diffusion primarily from the richly supplied capillary layer of the choroid. The retinal arteries are normally bright red, have bright red reflex strips (see Fig. 12.8)that become paler with advancing age, and do not show a pulse. The retinal veins are dark red with a narrow reflex strip, and may show spontaneous pulsation on the optic disk.

Pulsation in the retinal veins is normal; pulsation in the retinal arteries is abnormal.

The walls of the vessels are transparent so that only the blood will be visible on ophthalmoscopy. In terms of their structure and size, the retinal vessels are arterioles and venules, although they are referred to as arteries and veins.

Venous diameter is normally 1.5 times greater than arterial diameter. Capil-

laries are not visible. Nerve supply to the retina: The neurosensory retina has no sensory supply.

Disorders of the retina are painless because of the absence of sensory supply.

Light path through the retinal layers: When electromagnetic radiation in

the visible light spectrum (wavelengths of 380–760 nm)strikes the retina, it is absorbed by the photopigments of the outer layer. Electric signals are created in a multi-step photochemical reaction. They reach the photoreceptor synapses as action potentials where they are relayed to the second neuron. The signals are relayed to the third and fourth neurons and finally reach the visual cortex.

Light must pass through three layers of cell nuclei before it reaches the photosensitive rods and cones. This inverted position of the photoreceptors is due to the manner in which the retina develops from a diverticulum of the forebrain.

12.1 Basic Knowledge

303

Sensitivity of the retina to light intensity: The retina has two types of pho-

toreceptors, the rods and the cones. The 110–125 million rods permit mesopic and scotopic vision (twilight and night vision). They are about 500 times more photosensitive than the cones and contain the photopigment rhodopsin.

Twilight vision decreases after the age of 50, particularly in patients with additional age-related miosis, cataract, and decreased visual acuity. Therefore, glaucoma patients undergoing treatment with miotic agents should be advised of the danger of operating motor vehicles in twilight or at night.

The six to seven million cones in the macula are responsible for photopic vision (daytime vision), resolution, and color perception. There are three

types of cones:

❖

blue cones, green cones, red cones.

❖❖

Their photopigments contain the same retinal but different opsins. Beyond a certain visual field luminance, a transition from dark adaptation to light adaptation occurs. Luminance refers to the luminous flux per unit solid angle per unit projected area, measured in candelas per square meter (cd/m2). The cones are responsible for vision up to a luminance of 10 cd/m2, the rods up to 0.01 cd/m2 (twilight vision is 0.01–10 cd/m2; night vision is less than 0.01 cd/m2).
Adaptation is the adjustment of the sensitivity of the retina to varying degrees of light intensity. This is done by dilation or contraction of the pupil and shifting between cone and rod vision. In this manner, the human eye is able to see in daylight and at night. In light adaptation, the rhodopsin is bleached out so that rod vision is impaired in favor of cone vision. Light adaptation occurs far more quickly than dark adaptation. In dark adaptation, the rhodopsin quickly regenerates within five minutes (immediate adaptation), and within 30 minutes to an hour there is a further improvement in night vision (long-term adaptation). An adaptometer may be used to determine the light intensity threshold. First the patient is adapted to bright light for 10 minutes. Then the examining room is darkened and the light intensity threshold is measured with light test markers. These measurements can be used to obtain an adaptation curve (Fig. 12.3).

Sensitivity to glare: Glare refers to disturbing brightness within the visual field sufficiently greater than the luminance to which the eyes are adapted such as

the headlights of oncoming traffic or intense reflected sunlight. Because the retina is adapted to a lesser luminance, vision is impaired in these cases. Often the glare will cause blinking or elicit an eye closing reflex. Sensitivity to glare can be measured with a special device. Patients are shown a series of visual symbols in rapid succession that they must recognize despite intense glare.

304 12 Retina

Normal and abnormal dark adaptation curves.

cd/m2

–7

.
3,2 10

–5

.
3,2 10

–3

.
3,2 10

3,2

  • 0
  • 10
  • 20
  • 30
  • 40
  • 50 min

Fig. 12.3 X axis: adaptation time in minutes. Y axis: luminance of the respective test marker in candelas per square meter. The blue curve shows normal progression with Kohlrausch’s typical discontinuity indicating the transition from cone to rod vision. The red curve in retinitis pigmentosa is considerably less steep.

The sensitivity to glare or the speed of adaptation and readaptation of the eye is important in determining whether the patient is fit to operate a motor vehicle.

  • 12.2
  • Examination Methods

Visual Acuity see Chapter 1.

  • 12.2.1
  • Examination of the Fundus

Direct ophthalmoscopy (Fig. 12.4a; see also Fig. 1.13): A direct ophthalmo-

scope is positioned close to the patient’s eye. The examiner sees a 16-power magnified image of the fundus.

Advantages. The high magnification permits evaluation of small retinal find-

ings such as diagnosing retinal microaneurysms. The dial of the ophthalmoscope contains various different plus and minus lenses and can be adjusted as necessary. These lenses compensate for refractive errors in both the patient

Fig. 12.4

̈

a Direct ophthalmoscopy: the examiner sees an erect fundus image of the patient. b Indirect ophthalmoscopy: the examiner sees a virtual inverted fundus image. c Position of examiner and patient for indirect ophthalmoscopy.

12.2 Examination Methods

305

Ophthalmoscopy.

  • Examiner
  • Patient

a

Light source

  • Examiner
  • Loupe
  • Patient

b

Light source

  • c
  • Fig. 12.4

306 12 Retina

and the examiner. They may also be used to measure the prominence of retinal changes, such as the prominence of the optic disk in papilledema or the prominence of a tumor. The base of the lesion is brought into focus first and then the peak of the lesion. A difference of 3 diopters from base to peak corresponds to a prominence of 1 mm. Direct ophthalmoscopy produces an erect image of the fundus, which is significantly easier to work with than an inverted image, and is therefore a suitable technique even for less experienced examiners.

Disadvantages. The image of the fundus is highly magnified but shows only a small portion of the fundus. Rotating the ophthalmoscope can only partially compensate for this disadvantage. Direct ophthalmoscopy also produces only a two-dimensional image.

Indirect ophthalmoscopy (Figs. 12.4b and c): A condensing lens (+ 14 to + 30

diopters)is held approximately 13 cm from the patient’s eye. The fundus appears in two to six-power magnification; the examiner sees a virtual inverted image of the fundus at the focal point of the loupe. Light sources are available for monocular or binocular examination.

Advantages. This technique provides a good stereoscopic, optimally illuminated overview of the entire fundus in binocular systems.

Disadvantages. Magnification is significantly less than in direct ophthalmoscopy. Indirect ophthalmoscopy requires practice and experience.

Contact lens examination: The fundus may also be examined with a slit lamp when an additional magnifying lens such as a three-mirror lens (see Fig. 12.5)or a 78 to 90 diopter lens is used.

Examination of the fundus with a Goldmann three-mirror lens.

Three-mirror lens

1

Retinal tear

3
2

1

4

  • 4
  • Slit-lamp light

2

3

  • Examiner
  • Patient

  • a
  • b

Figs. 12.5a and b Principle of the examination: The lens is placed directly on the eye after application of a topical anesthetic. The various mirrors of Goldmann threemirror lens visualize different areas of the retina: 1) posterior pole, 2) central part of the peripheral retina, 3) outer peripheral retina (important in diagnosing retinal tears), 4) gonioscopy mirror for examination of the chamber angle.

12.2 Examination Methods

307

Advantages. This technique produces a highly magnified three-dimensional image yet still provides the examiner with a good overview of the entire fundus. The three-mirror lens also visualizes “blind areas” of the eye such as the angle of the anterior chamber. Contact lens examination combines the advantages of direct ophthalmoscopy and indirect ophthalmoscopy and is therefore the gold standard for diagnosing retinal disorders.

Where significant opacification of the optic media (as in a mature cataract) prevents direct visualization of the retina with the techniques mentioned above, the examiner can evaluate the pattern of the retinal vasculature. The sclera is directly illuminated in all four quadrants by moving a light source back and forth directly over the sclera. Patients with intact retinas will be able to perceive the shadow of their own vasculature on the retina (entoptic phenomenon). They will see what looks like “veins of a leaf in autumn”. Patients who are able to perceive this phenomenon have potential retinal vision of at least 20/200.

Ultrasonography: Ultrasound studies are indicated where opacification of the optic media such as cataract or vitreous hemorrhage prevent direct inspection of the fundus or where retinal and choroidal findings are inconclusive. Intraocular tissues vary in how they reflect ultrasonic waves. The retina is highly reflective, whereas the vitreous body is normally nearly anechoic. Ultrasound studies can therefore demonstrate retinal detachment and distinguish it from a change in the vitreous body. Optic disk drusen are also highly reflective. Ultrasound is also helpful in diagnosing intraocular tumors with a prominence of at least 1.5 mm. The specific echogenicity of the tissue also helps to evaluate whether a tumor is malignant, for example in distinguishing a choroidal nevus from a malignant melanoma (Fig. 12.6).

Ultrasound studies can demonstrate retinal detachment where the optic media of the eye are opacified (due to causes such as cataract or vitreous hemorrhage). This is because the retina is highly reflective in contrast to the vitreous body. Ultrasound can also be used to confirm the presence of malignant choroidal processes.

Fundus photography: Abnormal changes can be recorded with a single-lens reflex camera. This permits precise documentation of follow-up findings. Photographs obtained with a fundus camera in green light provide high-contrast images of abnormal changes to the innermost layers of the retina such as changes in the layer of optic nerve fibers, bleeding, or microaneurysms.

Fluorescence angiography (with fluorescein or indocyanine green): In

fluorescein angiography, 10 ml of 5% fluorescein sodium are injected into one of the patient’s cubital veins. Blue and yellow-green filters are then placed along the optical axis of a single-lens reflex camera. The blue filter ensures that only blue light from the light source reaches the retina. The yellow-green

308 12 Retina

Ultrasound examination of the fundus.
Fig. 12.6 Ultra-

sound findings in malignant melanoma (arrow).

filter blocks the blue components of the reflected light so that the camera records only the image of the fluorescent dye (Fig. 12.7).

Fluorescein angiography is used to diagnose vascular retinal disorders such as proliferative diabetic retinopathy, venous occlusion, agerelated macular degeneration, and inflammatory retinal processes. Where the blood-retina barrier formed by the zonulae occludentes is disturbed, fluorescein will leak from the retinal vessels. Disorders of the choroid such as choroiditis or tumors can also be diagnosed by this method; in these cases indocyanine is better than fluorescein.

12.2.2 Normal and Abnormal Fundus Findings in General

Normal fundus: The retina is normally completely transparent without any

intrinsic color. It receives its uniform bright red coloration from the vascula-

ture of the choroid (Fig. 12.8). The vessels of the choroid themselves are

obscured by the retinal pigment epithelium. Loss of transparency of the retina is a sign of an abnormal process (for example in retinal edemas, the retina appears whitish yellow). The optic disk is normally a sharply defined, yel- lowish orange structure (in teenagers it is pale pink, and in young children significantly paler)that may exhibit a central depression known as the optic or physiologic cup. Light reflection on the inner limiting membrane will normally produce multiple light reflexes on the fundus. Teenagers will also exhibit

a normal foveal reflex and wall reflex surrounding the macula, which is

caused by the transition from the depression of the macular to the higher level of the retina (Fig. 12.9).

12.2 Examination Methods 309

Fluorescein angiography of the fundus.

White light
Blue light

Light source

a

Blue filter
Yellow-green light reaches the camera
Yellow-green light is emitted; blue light is reflected

Camera

Yellow-green filter

b

Fig. 12.7 Blue and yellow-green filters are placed along the optical axis of a singlelens reflex camera. a First the blue filter ensures that only blue light from the light source reaches the retina. This excites the previously injected fluorescein dye in the vessels of the fundus. b The excited fluorescein emits yellow-green light, and the blue light is reflected. The yellow-green filter blocks the blue components of the reflected light so that the camera records only the image of the fluorescent dye.

Normal fundus.
Fig. 12.8 The

macula lutea lies about 3 – 4 mm temporal to and slightly below the optic disk. The fundus receives its uniform bright red coloration from the vessels of the choroid. Venous diameter is normally 1.5 times greater than arterial diameter.

310 12 Retina

Wall reflex surrounding the macula.
Fig. 12.9 Typical

highly reflective fundus in a teenager (see wall reflex).

Age-related changes: The optic disk turns pale yellow with age, and often the optic cup will become shallow and will be surrounded by a region of choroidal atrophy. The fundus will become dull and nonreflective. Drusen will be visible in the retinal pigment epithelium and middle peripheral reticu-

lar proliferations of pigment epithelium will be present. The arterioles will be elongated due to loss of elasticity with irregular filling  due to thickening of the vascular walls. Meandering of the venules will be present with crossing signs,

i.e., the sclerotic artery will be seen to compress the vein at the arteriovenous crossing, reducing the diameter of the column of venous blood. In extreme cases venous blood flow will be cut off completely.

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    CHAPTER 3 Vitreous and Developmental Vitreoretinopathies Kevin R. Tozer, Kenneth M. P. Yee, and J. Sebag Invisible (Fig. 3.1) by design, vitreous was long unseen the central vitreous and adjacent to the anterior cortical as important in the physiology and pathology of the eye. gel. HA molecules have a different distribution from col- Recent studies have determined that vitreous plays a sig- lagen, being most abundant in the posterior cortical gel nificant role in ocular health (1) and disease (1,2), includ- with a gradient of decreasing concentration centrally ing a number of important vitreoretinal disorders that and anteriorly (6,7). arise from abnormal embryogenesis and development. Both collagen and HA are synthesized during child- Vitreous embryology is presented in detail in Chapter 1. hood. Total collagen content in the vitreous gel remains Notable is that primary vitreous is filled with blood ves- at about 0.05 mg until the third decade (8). As collagen sels during the first trimester (Fig. 3.2). During the second does not appreciably increase during this time but the trimester, these vessels begin to disappear as the second- size of the vitreous does increase with growth, the den- ary vitreous is formed, ultimately resulting in an exqui- sity of collagen fibrils effectively decreases. This could sitely clear gel (Fig. 3.1). The following will review vitreous potentially weaken the collagen network and destabilize development and the congenital disorders that arise from the gel. However, since there is active synthesis of HA abnormalities in hyaloid vessel formation and regression during this time, the dramatic increase in HA concentra- during the primary vitreous stage and biochemical abnor- tion may stabilize the thinning collagen network (9).
  • MOC PORT/DOCK Practice Core Modules Content Outline

    MOC PORT/DOCK Practice Core Modules Content Outline

    MOC PORT/DOCK Practice Core Modules Content Outline 1 Cataract and Anterior Segment (10%) 1.1 Basic Anatomical and Scientific Aspects 1.1.1 The Normal Lens 1.1.1.1 Anatomy 1.2 Assessment Techniques 1.2.1 History-Taking and Preoperative Examination of Ocular Structures 1.2.1.1 Take patient history 1.2.1.2 Examine ocular structures 1.2.1.3 Signs and symptoms 1.2.1.4 Indications for surgery 1.2.1.6 External examination 1.2.1.7 Slit-lamp examination 1.2.1.8 Fundus evaluation 1.2.1.9 Impact on visual functioning and quality of life 1.2.2 Measurement of Visual and Macular Function 1.2.2.1 Visual acuity testing 1.2.2.2 Test visual acuity 1.2.2.5 Visual field testing 1.2.2.6 Test visual field 1.2.2.7 Cataract's effect on visual acuity 1.2.2.8 Estimate cataract's effect on visual acuity 1.2.2.10 Measure visual (and macular) function 1.2.4.3 Evaluate glare disability: subjective and objective 1.3 Clinical Disease Conditions and Manifestations 1.3.1 Types of Cataract 1.3.1.1 Identify types of cataracts 1.3.2 Anterior Segment Disorders 1.3.2.1 Diagnose anterior segment disorders 1.3.2.2 Cataract associated with uveitis 1.3.2.3 Diabetes mellitus and cataract formation 1.3.2.4 Lens-induced glaucoma 1.3.2.4.1 Lens particle 1.3.2.4.2 Phacolytic 1.3.2.4.3 Phacomorphic Copyright and Use Policy for ABO Content Outlines © 2017 – American Board of Ophthalmology.
  • Genes in Eyecare Geneseyedoc 3 W.M

    Genes in Eyecare Geneseyedoc 3 W.M

    Genes in Eyecare geneseyedoc 3 W.M. Lyle and T.D. Williams 15 Mar 04 This information has been gathered from several sources; however, the principal source is V. A. McKusick’s Mendelian Inheritance in Man on CD-ROM. Baltimore, Johns Hopkins University Press, 1998. Other sources include McKusick’s, Mendelian Inheritance in Man. Catalogs of Human Genes and Genetic Disorders. Baltimore. Johns Hopkins University Press 1998 (12th edition). http://www.ncbi.nlm.nih.gov/Omim See also S.P.Daiger, L.S. Sullivan, and B.J.F. Rossiter Ret Net http://www.sph.uth.tmc.edu/Retnet disease.htm/. Also E.I. Traboulsi’s, Genetic Diseases of the Eye, New York, Oxford University Press, 1998. And Genetics in Primary Eyecare and Clinical Medicine by M.R. Seashore and R.S.Wappner, Appleton and Lange 1996. M. Ridley’s book Genome published in 2000 by Perennial provides additional information. Ridley estimates that we have 60,000 to 80,000 genes. See also R.M. Henig’s book The Monk in the Garden: The Lost and Found Genius of Gregor Mendel, published by Houghton Mifflin in 2001 which tells about the Father of Genetics. The 3rd edition of F. H. Roy’s book Ocular Syndromes and Systemic Diseases published by Lippincott Williams & Wilkins in 2002 facilitates differential diagnosis. Additional information is provided in D. Pavan-Langston’s Manual of Ocular Diagnosis and Therapy (5th edition) published by Lippincott Williams & Wilkins in 2002. M.A. Foote wrote Basic Human Genetics for Medical Writers in the AMWA Journal 2002;17:7-17. A compilation such as this might suggest that one gene = one disease.
  • Passport to Success

    Passport to Success

    The following terms and other boldface terms in the chapter are defined in the Glossary accommodation choroid After careful study of this chapter, you should be able to: cochlea conjunctiva 1. Describe the function of the sensory system convergence 2. Differentiate between the special and general senses and give examples of each cornea 3. Describe the structure of the eye gustation 4. List and describe the structures that protect the eye lacrimal apparatus 5. Define refraction and list the refractive parts of the eye lens (crystalline lens) 6. Differentiate between the rods and the cones of the eye olfaction 7. Compare the functions of the extrinsic and intrinsic muscles of organ of Corti the eye ossicle 8. Describe the nerve supply to the eye proprioceptor 9. Describe the three divisions of the ear refraction 10. Describe the receptor for hearing and explain how it functions retina 11. Compare static and dynamic equilibrium and describe the sclera location and function of these receptors semicircular canal 12. Explain the function of proprioceptors sensory adaptation 13. List several methods for treatment of pain sensory receptor 14. Describe sensory adaptation and explain its value tympanic membrane 15. Show how word parts are used to build words related to the vestibule sensory system (see Word Anatomy at the end of the chapter) vitreous body PASSport to Success Visit thePoint or see the Student Resource CD in the back of this book for definitions and pronun- ciations of key terms as well as a pretest for this chapter. ® Paul’s Second Case: Seeing More of the Sun’s Effects aul glanced once again at the postcard condition, and it does have a hereditary fac- sitting on his entranceway table as he ar- tor.” The doctor dilated Paul’s eyes with drops Prived home in the evening.