EXAM AND IMAGING

VENOUS ENGORGEMENT Seen with cryoglobulinemia, Waldenstro¨m’s macroglobulinemia, and Takayasu’s arteritis.

VESSEL ATTENUATION Caused by Behc¸et’s syndrome, RP, ARN, and CRAO.

VITREOUS HEMORRHAGE PDR (causes 39–54% of VH), retinal break (12–17%), PVD (7–12%), RD (7–10%), and NV (3–10%); also may be seen with X-linked retinoschisis, pars planitis, tumor (RB, melanoma, leukemia), CNVM, trauma, Coats’ disease, vein occlusion, sickle cell retinopathy, FEVR, VKH, OHS, Eales’ disease, and hypertension. Can lead to hemosiderosis and ghost cell glaucoma.

VITRITIS Associated with Irvine-Gass syndrome, sarcoidosis, amyloidosis, Eales’ disease, VKH, pars planitis, any active retinitis (toxoplasmosis), active retinal vasculitis, multiple sclerosis, MEWDS, MCP, birdshot choroiditis, AMPPE, serpiginous choroidopathy, psoriatic, Fuchs’ heterochromic iridocyclitis (Fuchs’ HIC), Whipple’s disease, masquerade syndromes (intraocular lymphoma), spillover from anterior uveitis, recent trauma, retinal detachment, retinal tear, and giardiasis.

Exam and Imaging

ELECTRO-OCULOGRAPHY (EOG) Measures the standing electrical potential of the RPE and the light response of the RPE from depolarization of the basal membrane of the RPE (is thus an indicator of retinal disease). Compares the negative charge of the RPE to the positive charge of the cornea.

Expressed as a ratio of the maximal light-adapted peak (Lp) to the minimal dark-adapted trough (Dt). The RPE has increased voltage in

the light and requires about 8 minutes. Normal Lp:Dt ratio is >2.0; if <1.75, it is abnormal, as in CRAO, ocular ischemic syndrome, ophthalmic artery occlusion, gyrate atropy, and Best’s disease.

EOG usually parallels the ERG, except in Best’s vitelliform dystrophy and its carriers, who have normal ERG but abnormal EOG.

ELECTRORETINOGRAM (ERG) Mass response of retina to light (full-field Ganzfeld system). The cornea is in electrical continuity with the front of the retina and the eyelid with the back of the retina.

ERG waveforms and measurements: no waveform for the ganglion cell.

A wave: represents photoreceptor hyperpolarization; a negative deflection from photoreceptor cell inner segment sodium flux.

B wave: measures the inner retina (Mu¨ller and bipolar cells); positive deflection from increased extracellular Kþ concentration.

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232RETINA AND VITREOUS

C wave: RPE positive deflection.

Early receptor potential: small negative deflection at the very beginning of the A wave from rhodopsin cascade initiation in the photoreceptor outer segments.

Implicit time: measurement of the trough of the A wave to the peak of the B wave. B-wave implicit times are slowed as the rods dark adapt because rods are a reservoir of vitamin A for cones; thus, widespread diseases with rod loss, such as early RP, may have delayed B-wave implicit time even with normal amplitudes.

Testing parameters:

Scotopic (rod-isolated): single flashes of dim blue light tests rods (100–200 mV, below cone threshold), as the patient is dark adapted.

White light: mixed cone and rod response (350–700 mV).

Photopic (cone-isolated): white flicker stimulus (30 flashes/sec) tests cones (50–100 mV), as the patient is light adapted to bleach the rods. Rods are unable to cycle quickly enough and fuse at 20 cycles/second. White flicker is mainly extramacular test because 90% of the cones are outside the macula (and are thus not affected by macular scar).

Pattern-evoked ERG: small focal ERG using checkerboard pattern, allowing ganglion cell influence (macula tested, not full-field), and thus is abnormal in POAG.

Selective abnormalities:

Selective B-wave reduction: seen in CSNB (defect in rod synapse to bipolar cells), Oguchi’s disease, X-linked juvenile retinoschisis (NFL pathology affects Mu¨ller end plates and ILM and probably bipolar synapse), quinine toxicity, CRVO and CRAO (flat B wave, but cones are preserved via choroidal circulation).

For CRVO that is nonischemic, the B:A ratio is >1.0; for ischemic CRVO, the B:A ratio is <1.0.

Flat ERG: seen in ocular ischemic syndrome and ophthalmic artery occlusion.

ERG is decreased in myopia and African-American patients (decreased reflection of light). ERG is increased in albino patients.

ERG is not affected by glaucoma, ON transection, or cortical disease.

FLUORESCEIN ANGIOGRAPHY (FA) Sodium fluorescein is an orange-red, crystalline, low molecular weight hydrocarbon that is 80% bound to plasma protein. It diffuses through the choriocapillaris but does not leak from intact retinal vessels. It is excited by blue light at 490 nm and emits green light at 530 nm. The camera filters block the blue light and film the light emitted from the fluorescein. Fluorescein, which is excreted by the kidney

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and liver within 24 hours, may cause yellow skin and urine. Adverse reaction with nausea and vomiting seen in 5% of patients; rarely vasovagal episode or allergic reaction; death reported in 1/250,000. Affects digoxin serum test for 12 hours and cortisol, thyroxine, and quinidine tests for up to

3hours.

Normal FA: the choroidal phase begins within 10 to 12 seconds and is very rapid. Arterial phase (from central retinal artery) occurs 1 to 3 seconds later. The arteriovenous phase proceeds 1 to 2 seconds later, and the choriocapillaris is filled. The laminar venous phase occurs 5 to 10 seconds later. The perifoveal capillary network is best seen at 20 to 25 seconds. Recirculation occurs >30 seconds. Late fluorescence is normally seen of choroid, sclera, and edge of disk.

Hypofluorescence:

Blockage: density between camera and choriocapillaris (e.g., blood, exudate), deposited in the retina or choroid.

Vascular filling defect: arterial, venous, or capillary defect in retina, disk, or choroid. For example, loss of choriocapillaris seen with staphyloma and choroideremia.

Fovea is naturally dark because tall macular RPE cells have increased melanin and lipofuscin, the foveal avascular zone (FAZ) has no retinal capillaries, and fovea has xanthophyll in the OPL.

Hyperfluorescence:

Autofluorescence: ON drusen, astrocytic hamartoma

Window defect: focal hyperfluorescence without late leakage from RPE loss, which normally blocks the easily seen lake of choroidal fluorescein (e.g., drusen, laser scar, geographic atrophy).

Abnormal vessels: either from the retina (NV, aneurysms, tortuous vessels) or choroid (CNVM, tumor vessels)

Leakage: hyperfluorescence that increases with time. May be subretinal from RPE tight junction breakdown (e.g., ICSR, OHS, CNVM), intraretinal from capillary tight junction breakdown (e.g., CME), or vitreal from neovascularization.

VISUAL EVOKED POTENTIAL (VEP) Records the electroencephalogram (EEG) over the visual cortex. Used to gauge visual function (e.g. cortical blindness, malingering) and is abnormal in optic neuritis (prolonged latency but normal amplitude unless there is active disease).

ULTRASONOGRAPHY High-frequency (8–10 MHz) radio signals with short wavelengths used to create images and measure distances in the globe from echos that are generated by acoustic interfaces. Indications include opaque ocular media; can be used to evaluate posterior segment pathology (see also Chapter 2).

Posterior segment tumors: obtain measurements, determine if solid versus cystic.

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234 RETINA AND VITREOUS

Choroidal detachment: dome-shaped elevation of the retina and choroid; little mobility with eye movement. Detachment may extend to the CB, and is segmental in appearance because the choroid attaches at the vortex veins but not to the ON, as seen with a retinal detachment. If serous, is acoustically clear, in contrast to hemorrhagic choroidal detachment, which has subchoroidal echoes. A-scan shows rapidly rising, usually 100% high, thick or double-spiked echo.

RD: retina remains attached to the ON and extends to the ora; typically bullous appearance with shifting fluid. Spike amplitude stays same all along the retinal interface and is 100% high and narrow.

PVD: thin echo spike lessens as it moves toward the ora.

Also used to evaluate ON cupping, elevation or drusen, IOFB, trauma,

endophthalmitis (rule out RD before antibiotic injection or to follow vitritis course). Silicone oil has lower sound velocity, and echos appear much larger.

Fig. 7–3 shows the patterns of some common electrodiagnostic tests.

Figure 7–3 Normal patterns of selected electrodiagnostic tests. The electroretinogram (ERG) records the electrical response from the retina and the electrooculogram (EOG) from the retinal pigment epithelium. Dark adaptation, as tested with the Goldmann-Weekers dark adaptometer, records the time latency for the eye to adjust to scotopic conditions. The visual evoked potential (VEP) is the recorded brainwave activity over the occipital cortex following visual stimulation.

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