Ординатура / Офтальмология / Английские материалы / Small Animal Ophthalmology Secrets_Riis_2002

FIGURE 21. Cocker spaniel fundus with linear scars (*). Diagnosis: retinal folds or retinal dysplasia.

FIGURE 22. Portuguese water dog tapetal fundus showing vascular attenuation and hyperreflectivity. Diagnosis: progressive retinal atrophy.

FIGURE 23. Labrador retriever fundus with tapetal hyperreflectivity (*), vascular attenuation (**), and nontapetal pigmentation (***). Diagnosis: progressive retinal atrophy.

FIGURE 24. Springer spaniel fundus with scarring, unpigmented (*1 and pigmented (**1. Diagnosis: retinal dysplasia.

FIGURE 25. Siberian husky with an albinotic tigroid fundus. Diagnosis: Bergmeister's papilla (*) (pearl-gray spherical body in center of optic disc). FIGURE 26. Springer spaniel fundus. Diagnosis: retinal dysplasia (*) and optic disc hypoplasia.

FIGURE 27. German shepherd fundus. Diagnosis: flat retinal detachment (*). FIGURE 28. Dog fundus. Diagnosis: bullous retinal detachment (*).

FIGURE 29. Owl fundus. Diagnosis: retinal hemorrhage (*) adjacent to and involving pecten secondary to trauma. FIGURE 30. Cat fundus. Diagnosis: multifocal retinal detachments secondary to antifreeze toxicity.

FIGURE 31. Sable collie fundus. Diagnosis: large optic disc coloboma (*) and choroidal hypoplasia (**). FIGURE 32. Dog fundus. Diagnosis: choroidal hemorrhage secondary to hypertension.

FIGURE 33. Dog fundus. Diagnosis: focal retinal detachment (*). FIGURE 34. Chinchilla fundus. Diagnosis: normal.

FIGURE 35. Dog fundus. Diagnosis: multiple retinal scars secondary to distemper.

FIGURE 36. Rabbit fundus. Diagnosis: central optic disc depression and myelinated nerve fibers.

I.General

1.ELECTRICAL ACTIVITY OF THE VISUAL SYSTEM: THE ERG AND THE VER

Ellis R. Loew, Ph.D.

1.What is the electroretinogram (ERG)?

The first step in the process of vision is the absorption of light by the primary receptor cells

and the transduction of this event into a membrane potential change. This change in potential is called the receptor potential. The primary receptor cells (i.e., photoreceptor cells) of the visual system are the rods and cones. They are derived from ciliated cells and respond to a light increase with a membrane hyperpolarization due to the closing of Na + channels. There is also a redistribution of ions in the interand intracellular spaces that results in extracellular currents. The membrane potential change leads to a change in transmitter release that affects the electrical activity of neuronal elements proximal to the photoreceptors. These subsequent changes also produce extracel ular currents in the retina. In the end, stimulation of the ganglion cells leads to the production of action potentials that move over the inner retinal fiber layer and exit the eye via the optic nerve. Only the ganglion cells produce action potentials. All other electrical activity in the retina is graded. By measuring the changes in retinal activity in response to known stimuli, one can deduce many of the characteristics of retinal processing underlying the visual process.

2.What is the electrical basis of the ERG?

There are always ions flowing within the retina because of the action of pumps, membrane leak-

age and other potential differences. Moving charges are called a current, and a current flowing through the resistance provided by the retinal structure produces a voltage according to Ohm's law (E = IR where E is voltage, I is current, and R is resistance). To measure voltage, place two electrodes attached to some kind of voltmeter at two points in the current path separated by some distance. If the line joining the electrodes is parallel to the current path, the full ohmic voltage is measured. If the line joining the electrodes is normal to the path, and the current flow is uniform along the path, then no voltage is measured. The magnitude of the measured voltage is a sine function of the electrode axis relative to the current path. This is of great importance in measuring neuronal potentials because it points out the importance of electrode geometry in making accurate measurements.

The electrical events recordable from the retina are of two types. Intracellular recordings make lise of microelectrodes and can measure the activity of single cells. The main voltage source being measured is the transmembrane membrane "battery." Extracellular recording measures the potential differences that arise between different parts of a cell or between cells. In these cases, the electrodes do not pierce the cell membrane, and, in fact, the electrodes can be quite large and remote from the cells of interest. Extracellular measurements of retinal activity take advantage of the geometry of the retina, the location of "voltage sources," and the presence of a resistive layer (remember, current flowing through a resistor produces a voltage-that is Ohm's law). Remember that there is a continuous extracellular dark-current flowing from the inner segment to the outer segment of rods and cones. This current is moving in a radial direction. Electrodes placed at the ends of the photoreceptor cell would record a voltage that was proportional to the current and the extracellular resistance (from Ohm's law!). If the electrodes were placed on either side ofthe outer segment, no po-

tential difference would be recorded. If we imagine a layer of parallel outer segments with the electrodes "bridged" to the ends of all the cells using a conductive solution, the voltage we measure would be the sum of the contributions from all the cells. This is called a mass-cell potential. The concept to be gained from the above example is that those cells having currents flowing radially in the eye can be recorded with the electrodes placed across the retina, whereas the cells whose extracellular currents flow in the plane of the retina will not be measured. Also, the potential seen by distant electrodes represents the sum of all the radial currents flowing through the retinal resistance.

3. How is the ERG performed?

The vertebrate ERG takes advantage of the above geometrical considerations so that the response of the retina or whole eye can be recorded using external electrodes. In practice, a reference electrode is placed on some part of the animal unresponsive to light but close to the eyes (e.g., the forehead or the bridge of the nose), while a second, active electrode is placed in contact with the cornea. Because the eye is filled with conductive solution and also sits in a conductive "pocket," these electrodes are effectively recording across the whole retinal layer. The active and reference electrodes are connected to a differential amplifier whose output is the difference between these two inputs. In this way, any signals such as motion artifacts or power line noise are minimized or eliminated since signals common to the two inputs will cancel each other out. A third, common or ground electrode placed on the ear or nape of the neck establishes the zero level of the system.

4.What are the ERG components?

The retina responds to a flash of light with a series of electrical changes representing the pas-

sage of the stimulus through the retinal circuitry. As cells in the information pathway change their electrical state, they contribute to the overall current flow within and across the retina. The recording electrodes sum these changes in space and time so that the potential measured at any instant in time represents the activity of all the cell types electrically active at that time. This leads to the appearance of a complex waveform of varying amplitude and polarity recorded under the corneal electrode (Fig. I). In order to understand what is happening in the retina in response to a stimulus, it is necessary to ascertain how each contributing current generator in the retina responds separately. Theoretically, if one knew the response characteristics of each cell type in isolation, the complex ERG waveform could be reproduced by simple summation. There are three main components, or processes (P), that sum to produce the ERG: PI, PII, and PIII following the designation of Granit (for a more detailed discussion of these and a number of minor components the reader is directed to reference 3). PI is a slow, cornea positive wave that appears to be generated by the pigment epithelium. It is not normally used in veterinary ERG diagnostics and will not be further considered. PII is a fast cornea positive wave localized to the bipolar and Miiller cells. PIII is a cornea negative wave issuing from the photoreceptors.

5. How do the components sum to produce the multiphasic ERG?

Figure I demonstrates how these components sum. The generation of the ERG proceeds from the absorption of a photon train (a flash of light) by the photoreceptor cell outer segments. This leads to activation of PlII, which pulls the corneal potential negative resulting in the initial negative deflection of the ERG. The electrical changes in the photoreceptors, reflected in PIII, lead to a change in transmitter release that, after a time delay, result in changes in bipolar and horizontal cell activity as well as changes in extracellular ion concentrations activating PII. The horizontal cell currents flow mainly in the plane of the retina and thus contribute little to the ERG. The risetime of PII is faster and its amplitude greater than PIII because of amplification between the photoreceptors and the more proximal cells. This leads to a rapid increase in corneal positivity and a resulting negative peak in the ERG, producing the a-wave. PII peaks rapidly and with its decay produces another peak and identifiable wave, the b-wave. Because the decay of PII is faster than that of PIlL there is often a late negativity in the ERG. Note that the only part of the ERG that can be unequivocally correlated with the activity of a single cell type is the descending limb of the a- wave, which is a pure photoreceptor cell.

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MULLER CELLS

Figure I. Component analysis of the vertebrate ERG. As seen, the negative monophasic wave generated by the photoreceptor cells sums with the positive monophasic wave generated by the Miillerlbipolar cells to produce the multiphasic ERG at the top.

The waveforms and amplitudes of the components depend on the intensity, duration, and wavelength of the stimulus as well as adaptation state. As for all sensory responses, rise times and amplitudes are proportional to the log of the stimulus intensity (i.e., as the stimulus intensity increases response amplitude increases and the time-to-peak decreases). For the ERG, this means that the amplitudes of the a- and b-waves will increase as the stimulus intensity increases and their time-to-peak or implicit times will decrease. These changes can be seen in Figure 2. Over a defined range, stimulus duration multiplied by stimulus amplitude is a constant. However, with increasing duration, there is the risk of initiating adaptational processes that can affect ERG waveform. Wavelength affects response characteristics because of the spectral sensitivity of the photoreceptors as determined by their contained visual pigments.

6. What kinds of stimuli are best for eliciting the ERG?

Although the retina can follow slowly changing stimuli, a rapid rise time stimulus such as that produced by a photographic strobe, fast shutter, or light emitting diode (LED) ensures a rapid activation of the transductional cascade and good temporal segmentation of the ERG waves. The geometry of the stimulus is also important. The ERG is a mass cell response. Therefore, the best responses will occur when the entire retina is stimulated. This is best achieved by placing the eye in an integrating sphere ensuring that the visual field is filled with a uniform stimulus (a Ganzfeld stimulator). A uniformly irradiated, frosted contact lens covering the pupil is a good approximation of a true Ganzfeld. Sources that do not stimulate the entire retina can also be used successfully as long as they stimulate a large retinal area. Intraocular scatter will ensure that areas not directly stimulated will ultimately contribute to the ERG (this is particularly true for tapetal animals).

Figure 2. The effect of increasing stimulus intensity on the canine ERG (brightest at top). Note the a- and b- wave amplitude increases and decreases in implicit times. The stimulus was a 50-msec flash from a 470-nm blue LED.

7. What are the minor ERG components?

A number of minor components and ERG features may be of diagnostic significance in certain situations:

The c-wave. This is a slow, positive wave arising from PI and is believed to be due to changes in RPE activity. It is best produced using long duration stimuli on the order of seconds. It is also sensitive to adaptation state. Because it is a slow wave, it is best recorded using a direct current (DC) amplifier or one with a long time constantboth of which require stable preparations.

The d-wave. This is an off effect again seen with long stimuli. Interestingly, it usually has the same polarity as the normal "on" ERG event and is therefore not a typical rebound effect. This may come from the fact that on and off responses are carried by different retinal pathways and show different response properties. The d-wave shows the same changes with stimulus amplitude as the on event.

The e-wave. This is a late event coming many seconds after an appropriate stimulus. Unlike other waves, the implicit time of the e-wave increases with increasing stimulus intensity. It has been associated with afterimage activity as the result of bright flashes given under defined adaptation states.

Oscillatory potentials (P waves). Under conditions of fast rise times and bright stimuli, oscillations may be seen on the ascending limb of the b-wave extending over the peak. These arise from local feedback circuits operating between the retinal layers.

8. What parameters are measured in the clinical ERG and how do they relate to retinal abnormalities?

In order to use the ERG diagnostically, it is necessary to establish normal values for the ERG parameters most likely to be affected by retinal pathologies. This means generating ERGs with

11. How is the VER measured?

The exact protocols may vary, but the general idea is to place an active electrode over the occipital region and reference this to an electrode placed at the vertex or nasion. Stimuli from a strobe lamp, an ultrabright LED, or pattern stimulator are presented to the eyes and the responses averaged over a second. The VER is seen as a series of waves occurring after the main ERG components, which are usually present as an artifact.

12. How is the VER used diagnostically?

In human medicine, VER is used to confirm the patency of the visual pathways, measure visual acuity. check for albinism, and measure progression of multiple sclerosis by using delays in the appearance of VER peaks. However, in companion animals, the VER normally is used only to test for visual pathway patency.

13. Why is the VER not a standard diagnostic tool for companion animals?

Although it is easy to obtain a VER from a companion animal, standardization and the development of normal patterns to be used diagnostically are problematic. The measurement of the VER is not as straightforward as the ERG because of large variations in the geometry of the skull and the three-dimensional nature of the VER "generator" - the visual cortex. Variations among breeds have not been cataloged nor have there been adequate age studies. The lack of quantitative data limits the usefulness of the VER. There is also the problem of electrode placement standardization. In humans, the standard 10-20 EEG positions are used with the electrodes over the visual cortex (0, and O2) being successively connected to one side of an amplifier whereas reference electrodes are attached to the other. Standardization is possible because head dimensions vary very little when expressed as percentage distances between landmarks such as the inion and nasion and preauricular depressions. However, this is not the case for dogs and cats where different breeds can have radical differences in head dimensions and placement of the head relati ve to the rest of the body. This makes it almost impossible to develop a standard for dogs or cats. In addition, interference from the ERG is a problem. In some breeds the relationship between skull shape and eye placement makes it difficult to distinguish between late ERG components and the VER, especially when bright flashes are used as stimuli. Although late VER waves can usually be detected, any value of the early components is lost because of this uncertainty.

BIBLIOGRAPHY

I. Armington JC: The Electroretinogram. New York, Academic Press, 1974.

2.Fishman GA: The Electroretinogram and Electrooculogram in Retinal and Choroidal Disease. Rochester, NY, American Academy of Ophthalmology, 1975.

3.Heckenlivelly JR, Arden GB (eds): Principles and Practice of Clinical Electrophysiology of Vision. St. Louis. Mosby, 1991.