Posterior segment

11 Posterior segment




Anatomy





Retina (Figure 11-3)



Neurosensory Retina (9 Layers)


Inner refers to proximal or vitreous side of retina





















RPE


Monolayer of hexagonal cells with apical microvilli and basement membrane at base


RPE and outer segments of photoreceptors have apex-to-apex arrangement, resulting in a potential subretinal space


Merges anteriorly with pigmented epithelium of ciliary body



Functions













RPE cells may undergo hypertrophy, hyperplasia, migration, atrophy, and metaplasia








Choroid


Posterior part of uveal tract that extends from ora serrata (outer layers end before inner) to optic nerve. Attached to sclera by strands of connective tissue at optic nerve, scleral spur, vortex veins, and long and short posterior ciliary vessels; derived from mesoderm and neuroectoderm; 0.22 mm thick posteriorly and 0.1–0.15 mm thick anteriorly




Physiology








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


Light is delivered uniformly to entire retina, and electrical discharges are measured with a corneal contact lens electrode



Components (Figure 11-10)


















Disease states (Figure 11-14, Table 11-2)























Table 11-2 ERG patterns for various ocular diseases



































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    



Electro-oculogram (EOG)


Indirect measure of standing potential of eye (voltage difference between inner and outer retina) (Figure 11-16)



Depolarization of basal portion of RPE produces light peak; normal result requires that both RPE and sensory retina be normal





Retinal Imaging




Optical Coherence Tomography (OCT)


Creates cross-sectional image of tissue using light


Provides retinal thickness measurements and cross-sectional retinal imaging to ∼5–10 µm depending on light source; anterior segment spectral domain OCT is useful to image anterior segment, in particular the cornea and angle


Superluminescent diode fires beam of infrared light through fiberoptic Michelson interferometer at both the eye and a reference mirror; the reflected light from the retina is compared with the light from the reference mirror and analyzed so that the tissue reflectivity (similar to ultrasound) and density can be determined. With time-domain OCT (TDOCT), the reference mirror moves; with spectral-domain OCT (SDOCT) the mirror does not move and a Fourier transform is used to obtain imaging information (this makes SDOCT much faster than TDOCT)


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)








Ultrasound


Acoustic imaging of globe and orbit










Specific lesions


(Tables 11-3 and 11-4) (Figures 11-22 to 11-27)













Fluorescein Angiogram (FA)



Phases


choroidal filling, arterial, venous, recirculation















Disorders



Vitreous Abnormalities








Jun 4, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on Posterior segment
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