Ординатура / Офтальмология / Английские материалы / Electrophysiology of Vision_Lam_2005

Cisplatin . . . . 434

Indomethacin . . . . 435

16. Non-Organic Visual Loss and Other Ocular and

Systemic Disorders . . . . . . . . . . . . . . . . . . . . . . 449

Non-Organic Visual Loss . . . . 450

Ocular Disorders . . . . 452

Systemic Disorders . . . . 458

17. Optic Neuropathies and Central Nervous

System Disorders . . . . . . . . . . . . . . . . . . . . . . . . 473

Optic Neuropathies . . . . 474

Central Nervous System Disorders . . . . 487

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 507

Table 1.1 Summary of International Standard for Clinical FullField Electroretinography (ERG)

Table 1.1 Summary of International Standard for Clinical FullField Electroretinography (ERG) (Continued )

Table 1.1 (Continued )

test that assesses rod-generated activity. The full-field ERG is essential in the diagnosis of numerous disorders including cone dystrophy, retinoschisis, congenital stationary night blindness, Leber congenital amaurosis, rod monochromatism, and paraneoplastic retinopathies. The ERG should be used in conjunction with a thorough ocular examination and when necessary other tests such as visual field and fluorescein

angiography. The full-field ERG provides no topographical information about localized defects, and an isolated macular lesion is unlikely to decrease the full-field ERG response substantially. In such cases, testing with focal or multifocal ERG is helpful.

RETINAL ELECTRICAL RESPONSES

The ERG response is produced by light-induced movements of ions in the retina. This activity is measured indirectly at the cornea with a recording electrode. The extracellular movement of predominantly positive potassium (Kþ) and sodium (Naþ) ions occurs as a result of opening (depolarization) and closure (hyperpolarization) of channels of the cellular membranes. The small size and short axons of the retinal cells are such that a change in ionic activity of one part of the cell is adequate to affect its synaptic activity. Both neuronal and non-neuronal retinal cells contribute to this light-evoked electrical current. The full-field ERG receives virtually no contributions from retinal ganglion cells that form the optic nerve. The ganglion cells utilize conventional all-or-none spike action potentials and are more response to the stimulus of the pattern ERG.

CLINICAL RECORDING OF FULL-FIELD ERG

Scotopic and Photopic Recordings

Dark-adapted or scotopic ERG responses are recorded after a period of at least 20 min of dark adaptation but a period of 30–40 min is preferable to allow more complete dark adaptation of the rods. Scotopic recordings are rod-driven although cones also contribute if bright stimuli are used. Light-adapted or photopic ERG responses are recorded with a lit white background after at least 10 min of light adaptation (2). Light adaptation is achieved with exposure to ambient room light or alternatively can be better standardized by exposing the patient to 10 min of the photopic background of the full-field dome. Photopic responses are cone-driven.

Full-Field ERG Flash Stimulus

The white flash ERG stimulus is delivered in a white full-field dome. The maximal flash is called the standard flash with a recommended intensity of 1.5–4.5 cd s m 2. However, a brighter flash in the range of 10–12 cd s m 2 elicits a larger better-defined a-wave for assessing photoreceptor activity. The flash is brief lasting usually well under 1 msec. For scotopic rod responses, the flash is dimmed with built-in filters. During photopic cone response recordings, the background of the full-field dome is lit with an even and steady white luminance of 17–34 cd m 2 to maintain light adaptation and to suppress rod activity. A centrally located built-in small dim red light is typically used as a fixation target. Proper calibration and periodic re-calibration with a photometer ensure that appropriate light intensities are utilized and no substantial changes occur over time. International guidelines for calibration of stimulus are available on the ISCEV Internet site (3). Periodic calibration of the amplifier by passing a known square wave or sine wave signal through each recording channel ensures that the amplifiers are working accurately and the output of each channel is identical.

Although the flash ERG stimulus is defined by the luminance emitted from the reflecting surface of the full-field dome and the stimulus duration (candela-second per square meter, cd s m 2), a more precise measure of the effective light stimulus received by the retina is retinal illuminance expressed in troland. For clinical ERG, stimulus luminance is usually a more readily available measure than retinal illuminance, but some recording systems can automatically calculate retinal illuminance. The troland (td) is the stimulus luminance in cd m 2 multiplied by the pupillary area in mm2, and the retinal illuminance from a flash is expressed as troland multiplied by the stimulus duration (td s). Photopic troland differs from scotopic troland because the sensitivity of the eye to light wavelengths is different under light and dark adaptation, and the spectral distribution of the stimulus needs to be taken into account. The standard spectral sensitivity function of the eye under photopic condition (Vl) is a