11 Posterior segment
Anatomy
Vitreous
Composed of 99% water and a few type II (mainly) and type IX collagen fibers; viscous, gel-like quality from mucopolysaccharide and hyaluronic acid that is folded into coiled chains and holds water like a sponge; volume ∼4 cm3; syneresis (liquefaction) occurs with aging (Figures 11-1, 11-2).
Figure 11-1 Vitreous anatomy according to classic anatomic and histologic studies.
(From Schepens CL, Neetens A: The Vitreous and Vitreoretinal Interface. New York, Springer-Verlag, 1987.)
Retina (Figure 11-3)
Neurosensory Retina (9 Layers)
Inner refers to proximal or vitreous side of retina
Peripheral Retina
Extends from macula to ora serrata, nonpigmented epithelium is contiguous with pars plana; defined as any area of the retina with a single layer of ganglion cells (Figure 11-5)
Macula
Area of retina in which ganglion cell layer is more than 1 cell thick (5–6 mm in diameter)
Centered 4 mm temporal and 0.8 mm inferior to optic nerve
Differentiation of the macula does not occur until age 4–6 months old
Fovea (Figures 11-6, 11.7)
Central depression of inner retinal surface (1.5 mm in diameter) within macula
Figure 11-6 Foveal margin, foveal declivity, foveola, and umbo.
(From Schubert HD: Structure and function of the neural retina. In: Yanoff M, Duker JS (eds) Ophthalmology. London, Mosby, 1999.)
Figure 11-7 Normal fundus with macula encompassed by major vascular arcades.
(From Schubert HD: Structure and function of the neural retina. In: Yanoff M, Duker JS (eds) Ophthalmology. London, Mosby, 1999.)
Foveola
Central area of fovea (350 µm in diameter)
Absence of ganglion cells and other nucleated cells; avascular
Clinical Correlation
Collateral vessels: occur at site of obstruction, across horizontal raphe, and at disc
RPE
Monolayer of hexagonal cells with apical microvilli and basement membrane at base
Merges anteriorly with pigmented epithelium of ciliary body
Functions
RPE cells may undergo hypertrophy, hyperplasia, migration, atrophy, and metaplasia
Choroid
Layers
Figure 11-8 Human choriocapillaris, retinal view.
(From Fryczkowski AW: Anatomical and functional choroidal lobuli. Int Ophthalmol 18:131–141, 1994.)
Endothelium is permeable to large molecules
Drains via vortex veins to superior and inferior ophthalmic veins
Physiology
Visual pigments
4 types, each composed of 11-cis-retinal (vitamin A aldehyde) + a protein (opsin); 3 cone pigments and 1 rod pigment (Table 11-1)
Photoreceptor | Pigment | Peak sensitivity |
---|---|---|
Rod | Rhodopsin | 505 nm |
Red cones | Erythrolabe | 575 nm |
Green cones | Chlorolabe | 545 nm |
Blue cones | Cyanolabe | 445 nm |
Rod photoreceptor membranes
lipid bilayer in which rhodopsin is an integral component
Luminosity curves
Electrophysiology
Electroretinogram (ERG)
Measures mass retinal response; useful for processes affecting large areas of retina
Photoreceptors, bipolar and Müller’s cells contribute to flash ERG; ganglion cells do not
Components (Figure 11-10)
Photopic (light adapted)
Scotopic (dark adapted)
measures rod function; dark adapted for 30 min (Figure 11-12)
Disease states (Figure 11-14, Table 11-2)
Extinguished ERG abnormal photopic, normal ERG | Normal a-wave, reduced b-wave | Abnormal photopic, normal scotopic ERG |
---|---|---|
RP | CSNB; Oguchi’s disease | Achromotopsia |
Ophthalmic artery occlusion | X-linked juvenile retinoschisis | Cone dystrophy |
DUSN | CVO | |
Metallosis | CRAO | |
RD | Myotonic dystrophy | |
Drug toxicity (phenothiazine; chloroquine) | Quinine toxicity | |
Cancer-associated retinopathy |
Pattern ERG (PERG)
waveform similar to flash ERG, but different test to measure ganglion cell activity; stimulus is an alternating checkerboard pattern that gives a constant illumination to the retina (Figure 11-15)
Electro-oculogram (EOG)
Indirect measure of standing potential of eye (voltage difference between inner and outer retina) (Figure 11-16)
Retinal Imaging
Optical Coherence Tomography (OCT)
Creates cross-sectional image of tissue using light
Useful for optic nerve (glaucoma) and macular pathology (edema, hole, pucker); can compare thickness in cases of macular edema from one visit to next; can diagnose and differentiate vitreomacular pathology e.g. stage 1 macular hole vs full-thickness hole (≥stage 2) vs pseudohole or lamellar holes (Figures 11-17, 11-18)
Figure 11-17 Optical coherence tomography principle.
(Adapted from Shuman JS, Hee MR, Puliafito CA et al: Quantification of nerve fiber layer thickness in normal and glaucomatous eyes using optical coherence tomography. Arch Ophthalmol 113:586-596, 1995. From: Yanoff M, Duker JS (eds) Ophthalmology. London, Mosby, 1999.)
Ultrasound
Acoustic imaging of globe and orbit
A-scan
1-dimensional display (amplitude of echoes plotted as vertical height against distance) (Figures 11-19 to 11-21)
Figure 11-19 A-scan ultrasound demonstrating high internal reflectivity.
(From Friedman NJ, Kaiser PK, Pineda R II: The Massachusetts Eye and Ear Infirmary Illustrated Manual of Ophthalmology. 3rd ed. Philadelphia, Elsevier, 2009.)
B-scan
2-dimensional display (amplitude of echoes represented by brightness on a grey scale image) (Figure 11-22)
Specific lesions
(Tables 11-3 and 11-4) (Figures 11-22 to 11-27)
Figure 11-24 B-scan ultrasound demonstrating scleral thickening and the characteristic peripapillary T sign.
(From Kaiser PK, Friedman NJ, Pineda R II: Massachusetts Eye and Ear Infirmary Illustrated Manual of Ophthalmology, 2nd edn. Philadelphia, Saunders, 2004.)
Figure 11-25 B-scan ultrasound demonstrating elevated mass with underlying thickened choroid.
(From Kaiser PK, Friedman NJ, Pineda R II: Massachusetts Eye and Ear Infirmary Illustrated Manual of Ophthalmology, 2nd edn. Philadelphia, Saunders, 2004.)
Figure 11-26 B-scan ultrasound demonstrating dome-shaped choroidal mass.
(From Kaiser PK, Friedman NJ, Pineda R II: Massachusetts Eye and Ear Infirmary Illustrated Manual of Ophthalmology, 2nd edn. Philadelphia, Saunders, 2004.)
Fluorescein Angiogram (FA)
Phases
choroidal filling, arterial, venous, recirculation
Characteristics
Indocyanine Green (ICG)
98% bound to serum proteins (80% to globulins); therefore, leaks very slowly from choroidal circulation allowing enhanced imaging of the choroidal circulation; CNV often appears as hot spot (bright area; occurs within 3–5 min, lasts 20 min) (Figure 11-33)
Disorders
Vitreous Abnormalities
Asteroid Hyalosis
Refractile particles (calcium soaps) suspended in vitreous
More common in older patients and those with diabetes; 25% bilateral
Synchisis Scintillans (Cholesterol Bulbi)
Cholesterol crystals derived from old vitreous hemorrhage; with PVD crystals settle inferiorly
Occurs after blunt or penetrating trauma in blind eyes
Crystals sink to bottom of globe because no fixed vitreous framework