|Year : 1962 | Volume
| Issue : 2 | Page : 27-35
Modern photopic perimetry
A Dubois Poulsen
|Date of Web Publication||29-Mar-2008|
A Dubois Poulsen
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Poulsen A D. Modern photopic perimetry. Indian J Ophthalmol 1962;10:27-35
It is unnecessary to state how useful is perimetry in ophthalmology and neurological clinical work.
It represents one of the few, if not the only method, able to detect the state of intracranial optical pathways, and it allows precious observations in cases of affections more specifically ocular. Apart from the new technics,-perimetry in darkness, perimetry at mesopical levels, perimetry after dazzling, flicker perimetry, one of the most fruitful ideas seems to have been to replace the former perimetry with single variable stimulus by two variable ones, which are intensity and test surface.
Let us state what older perimetry consisted of. Methods were based on the notion of test visibility in the visual field and sensibility gradients from center to retinal periphery.
Visibility of an object, whatever it is, depends on numerous factors of which the most important are:
(i) its surface area (ii) its luminous intensity and (iii) its contrast with the background upon which it is presented, that is the luminous level of this background.
Moreover it depends on its excentricity in the visual field, in other words on the retinal region upon which its image is produced.
To give these different factors their accurate value, one has endeavoured, as in every physical experiment, to vary only one parameter at a time, the other remaining constant.
The experiment has been then repeated as many times as necessary, giving different but fixed values to secondary parameters. Families of curves have thus been deduced which are most valuable in visual physiology.
To vary only one parameter at a time evidently is the simplest and easiest idea. The most directly accessible variable is the test surface. Therefore, in clinical work, RONNE'S wellknown technic of the black or grey background with constant lighting and white tests with variable and known dimensions, has been adopted. TRAQUAIR, whose book includes every conclusion possible, brought that technic to its utmost perfection.
The larger the test, the more peripheral the site in which it can be observed in the visual field. Sensibility can be defined as represented by the inverse of the test size. So, carrying excentricities in abscissa the fixation point being quoted O, a characteristic curve of retinal sensibility from center to periphery can be obtained. On 3 and not 2 dimensions TRAQUAIR'S famous "island, washed in a sea of blindness," is produced, characterised by the central peak of foveal vision, close to which is found the crater of Mariotte's blind spot. The shore slopes are more or less inclined according to the meridian of the visual field examined.
This, introduced in perimetry the isopter notion that is to say level curves joining heights of equal sensibility on the island, thus tracing closed curves in the visual field.
In pathology the pattern of curves and their gradients must be studied. TRAQUAIR'S conclusions are well known about the early damage of central or peripheral isopters according to the lesion observed as well as his considerations on soft slopes of evolutive or unstable lesions in opposition to the steep slopes in the case of fixed or stable one:3.
This is a very elaborate technic. It has allowed accurate diagnosis. Recently HARMS substituted the surface variation of the test by the luminous intensity one. On every point of the visual field the luminous intensity is increased, all other variables being kept constant until visibility is reached. In fact it is a research for local thresholds, but still proceeding from the one vanable parameter idea.
Perimetry is in fact meant to search for thresholds through quicker and coarser clinical procedures than physiological methods. It really tends to find in the visual field the points corresponding to a given threshold defined by the qualities of the chosen test. Therefore, there is no objection to basing perimetric technic upon physiological principles of research for thresholds. Now, these are depending on numerous parameters the variations of which are not independent but interlinked by laws.
To make use of these interaction laws is the object of new perimetry. To thrive on new signs clinical work calls on theory.
| Spatial Summation|| |
Threshold variation factors are exactly similar to visibility factors above mentioned, i.e. test angular dimensions, luminous intensity, position or eccentricity, colour, time of exposure, contrast with background, all of which cannot evidently be considered. Laws linking test surface or its luminous intensity to TIME have not yet been considered in clinical perimetry where the test has always been presented with limited fixed time. So far COLOUR is not a factor in the examination because of our lack of physiological knowledge on its peripheral perception. It is usual practice to consider therefore white tests only and laws linking the luminous intensity of the test to its surface. Those are named laws of spatial summation . Let us define them briefly, state the physiological and quantic interpretation given and establish the way in which they can be used in clinical work and then in reverse see how clinical work can, in turn, help to understand them. INTENSITY AND SURFACE being two of the main threshold variables, it is legitimate to qualify the new perimetry of two variable technics, to contrast it to the former one of one variation factor, all the others remaining constant during the experiment.
Relation between luminous intensity and test surface area must be established both for luminous absolute threshold and differential threshold.
ABSOLUTE THRESHOLD. By absolute threshold is meant the ability to recognise a test object varying in size or intensity on a uniformly lighted background.
Authors who applied themselves to threshold mensuration, do not always give the relation found between surface area and luminous intensity, but it is implicited in their results and can in any case be deduced from their given curves expressing intensity in function with surface.
Others have expressed through a mathematical law the relation between the two quantities. According to the former results there should be a constant quantity of light necessary to reach the sensibility threshold and expressed by the wellknown formula:
I S = const.
Where I is intensity and S is surface area.
It is RICCO'S law, dating from 1887, taken over again by CHARPENTIER and rediscovered later by LOHLEIN in 1905 and HENNIUS in 1909. But is this law valuable for the whole retinal surface ? According to GEHLOER and SCHERRING (1919) RICCO'S law is accurate for peripheral retina but not for fovea. At the latter level constancy is assured by the product of intensity by surface square root.
In 1903, however, PIPER stated that under the I√S = const. formula the law could be applied to the whole retina hence the name of PIPER'S law given to this new formula.
PIERON, in 1929, reconsidered the question in a stricter manner with the head immobilised and a fixation point for the eyes very carefully studied.
The curves expressing relation between surface and luminance are exponential functions whatever the wave length of light and expressed by equations of the form:
I S k = const.
where k is called summation exponent.
PIERON proposes I S 0.33 = const.
or I 3√S = const. ELSBERG and SPOTNITZ, American authors have taken it up again in 1937, much later than PIERON to find again the I S 0.33 = const. law, which ever since has been called after them. GRAHAM Brown and MOTE, and BAUMGARDT found again similar figures.
Nothing seems more perplexing than this research of relations between surface and intensity. There are three possible laws. All have the I S k = const. formula but the exponant k varies according to authors. Thus it is unit for Ricco, square root for Piper, cubic root for Pieron and it is even possible for the exponent to vary still more and reach intermediary values between 1 and 0.7, e.g. 0.9, 0.8 etc.
Variables interfere in various ways. Variations in test size may, for instance, condition the form of equation, With a small surface, the exponent may equal one, with a larger surface it may take smaller values than I, for instance 0.5 or even to abolish itself. Similarly, luminance may introduce modifications and finally test excentricity is itself a prevailing factor. Diameter of the pupil, test colour and optical imperfection factors also play their part.
It is sufficient, to make use of these laws in clinical work, to settle the limit of their validity and pick out the simplest. After considering the problem at length, we prefer to rely upon a simple and constant relation between test luminance and surface size, for whichever region of the retina that is to be explored. One must adopt test-objects smaller than 1 ° in surface-size and not higher than 10-9 stilbs (between 10-9 and 10-7) in luminance. This relation holds better in the retina more peripheral to 30' from the fixation. These figures should be the limits for clinical exploration for absolute thresholds.
DIFFERENTIAL THRESHOLDS : By the differential sense is meant the ability to recognise two similar test objects as distinctly separate. The classical wav to search for differential thresholds would be to recognise two contiguous luminous stimuli but because of the varying sensitivity in the different parts of the retina this method would introduce difficulties. It can be estimated in another way that is by searching for a differential fraction, that is to search for a threshold for recognising a small luminous object over a larger less luminous background. This may appear to be just the same as when testing with a luminous object on a uniformly lighted total visual field as while using a Goldmann's perimeter, but we shall see the difference presently.
Author's curves concerning differential fraction expressing a certain contrast between a test object and its lighted background may correspond to several test dimensions and several different lighting levels. Isopter method allows them to conceive spatial summation laws in an easier and practical way in clinical perimetry. In fact, one isopter corresponding to a differential threshold may be registered on the same place of the field with other intensity and test-surface-area combinations. A large contrast and a small dimension test-object may be equal to a weak contrast and to a larger dimension test-object. There is thus one more law for relations between surface and contrast. It is another spatial summation law.
There is thus a definite relation between size of object and contrast of luminance to produce the same visual response in the retina. It is possible to determine corresponding isopters for different combinations of size-dimensions and luminance contrast. The determination of these isopters constitutes the essence of PERIMETRY WITH TWO VARIABLES. When isopters determined by two different combinations of size and luminance which should normally correspond, do not correspond then there is a disturbance of the differential sense. We shall call it SUMMATION TROUBLE.
FERREE and RAND seem to have first tried to express with curves, relations between the visual field. dimensions and the two variables, but their system is heavy and difficult to handle.
GOLDMANN'S system is coherent and practical in application. On the perimeter, named after him, a lamp insures the test luminous intensity and its variations are obtained by interposition of filters. He reaches a formula of form 1 S k =const. where K = 0.84, which, according to GOLDMANN is the best average. It is clearly shown that to obtain equal isopters with different sizes of tests, surfaces must increase in geometrical progression, while filter permeability must decrease in another progression.
These laws are only applicable within certain limits. The more peripheral the zone in the field and the larger the test, the better are summation phenomena.
It is necessary not to cover a surface over 1.1 mm 2 at 33 cm. that is to say once again 1◦, for the law to remain accurate for the whole field area.
We have, ourselves, gone through these experiments again with particular attention to the formula exponant. It is precisely the exponant which characterises the summation -1.0 according to RICCO'S law, 0.5 in PIPER'S law, 0.3 with PIERON'S. So we thought it necessary to determine this exponant directly in a normal subject as well as in a pathological case. We have modified GOLDMANN'S apparatus, which, with its filters allows only a variation of luminous intensity step by step. With GOLDBERG's wedge we have been able to obtain an uninterrupted variation. This apparatus allows the drawing of an isopter with a first test of given surface and intensity. Then, picking out another surface area one feels one's way about to find out what intensity to give to the test so that the isopter places itself in the same site of the visual field. I am not going to enter into details, however, very simple of logarithmic calculations.
With 5 individuals trained to physiological experimentation and observed on S isopters, one has been able to find an average exponant of 0,952-almost 1 which corresponds to RICCO's law.
Sixteen untrained individuals have however allowed to find an exponant equal to 0.818.
The average between the two figures is 0.85, very close to 0.84, GOLDMANN's exponant. This 0.85 is quite a worth while clinical value in practice, but it is better to keep nearer stricter experiences and adopt 0.90.
This exponant varies in an anarchical way through the visual field according to the excentricity of the isopter observed. But this variation is in keeping with the margin of errors and one may consider the exponant as constant, in the whole field area for the sizes of the tests examined. The central area upto 5 ° however has not been explored.
It is out of the question to use intricate calculations in clinical work.
The spatial summation relation is rendered quite easy with GOLDMANN's apparatus. There is one number for each test surface and each intensity. It is enough to pick out the summated intensity--surface stimulus in such a way as to have the sum of their numbers constant in order to obtain the whole series of tests belonging to the same isopter. An extremely practical Pythagoras' table indicates all surface and intensity combinations. They are found on the diagonals of the table.
When curves thus defined by different but corresponding tests do not coincide, there should be some summation trouble . One must however keep in mind that GOLDMANN's chosen exponant is an average, constant only for minor tests and that summation is only really applicable at field periphery.
Normal isopters do not always coincide completely and it is better to consider only important variations over 5° .
It must be immediately emphasised that differential sensibility cannot characterise alone the perimetric properties of a retinal zone. It is necessary to determine the summation capacity characteristic of the observed area. HARM's embossed model or TRAQUAIR's schemes expressing sensibility variations in the name of only one variable are thus not sufficient enough. It is here that the application of two variable perimairy in clinical work gives a striking proof of this. Many perimetrical lesions escape in fact, to a certain test combination which can be detected by other combinations which rigorously equivalent in a normal subject, become non-corresponding in a pathological case.
But we must not anticipate. We must briefly say what are, in nervous physiology, laws of correspondence between intensity and test surface.
| Physiological Spatial Summation in Retina|| |
Phenomena studied through two variable perimetry are in reality the ones grouped under the name of spatial summation. There are different types of physiological summations.
A lesser stimulus than the threshold does not elicit a response. However, if repeated at a suitable interval, it elicits an answer from the examined system. It is temporal summation in which influxes tend to add themselves in time. If several axon reflexes tend to converge on one single cell, in sufficient abundance it can be activated. This is another variety of spatial summation in which influxes come from several points of space. In retinal physiology, when the surface of an infraliminary, luminous intensity test is enlarged numerous sensorial cells are excited, the influxes of which tend to converge on one or several ganglion cells. This creates an addition of influxes on the ganglion cells and a spatial summation is realised. Heterogeneity of the retina in its different parts strongly conditions this problem. At foveal level., each cone is joined to a bipolar cell which converges on a ganglion cell, summation is thus perfect there. This type of summation is better seen when the stimulus is peripheral. Actually, a more complete interpretation of the facts can be given, based on statistical laws and corpuscular theories of light. It is known that radiations have undulatory characteristics, their absorption by matter takes place under quanta form. One quantum constitutes the smallest parcel of radiant energy capable of isolated existence. It is carried by photon. A weak radiation carries few photons. It is ruled by hazard laws and its statistical character is sometimes richer, sometimes poorer. The well-known threshold fluctuation is then explained by fluctuation of weak intensity flux and it is not necessary to bring nervous system in this theory.
Having gathered the idea of unit of foveal conduction as opposed to group conduction in the retinal periphery one can explain the varying behaviour of those two retinal zones under weak lighting. Foveal conduction units which are very small can pick very few photons strong enough to strike a sensorial cell and produce stimulus in a ganglion cell, while peripheric groups easily pick up larger number of the weaker photons which are summated to produce a stimulus strong enough to cause stimulation of the common ganglion cell in which they terminate. Hence we find central scotoma in the dark and peripheric sensibility maintained under cones zoons of weak illumination (PIRENNE).
To explain summation phenomena one must refer to retinal histology notions where convergent dispositions of cells of one with another play their part. Therefore, the inference that summation in the problems of vision has a purely retinal substratum is but a short step, too easily taken by physiologists.
| Geniculate Bodies and Occipital Cortex|| |
Two additional organs may be present with retina centers for nervous reductions and convergences. They are the corpus geniculatum and occipital cortex.
It is now necessary to make research in clinical work to reach a more perfect understanding of summation phenomena in the visual domain. We have given ample time to back high levels of theory to reach the lower regions of practice. Let us however take notice that from our point of view clinical work is not an intruder but an absolute scientific necessity.
To get a rapid idea of the value of the spatial summation trouble research, we have gone over visual fields measured in the Quinze-Vingts hospital with Goldmann's perimeter (about 3000 fields).
We have made two separate lists of cases : the former concerning those with summative trouble-the second where none were present. In a third list, we classified affection about which we could not say anything, precise perimetric documents being lacking.
Checking on these lists allows a first classification. Summation trouble only exists in lesions of the pregeniculate part of optical pathways, they do not exist in the regions of the retrogeniculate part except when a stasis or vascular oedema is present.
RETINAL LESIONS AND CHORIORETINAL OEDEMA seem to be particularly , concerned with summation difficulties.
Certain diseases require a more detailed commentary. It is not sufficient to use only ophthalmoscope to follow choroidal evolution. At the acute phase, perilesional and lesional oedema is marked by soft isopter slopes and spatial summation trouble.
Only disappearance of these signs can allow us to say that the lesion has improved or healed and in particular that there is an absence of oedema. In retinal vascular process summation troubles are constant. They are evidently found in venous thrombosis and vascular obliterations as well as in slight vascular troubles such as capillarosis.
In ARTERIAL HYPERTENSION, they indicate forthcoming oedema before its ophthalmoscopic appearance and are a danger-signal. At the neuro-retinopathia oedematous stage, summations are evidently very disturbed, but are not of the same interest.
In DETACHMENT OF THE RETINA , summations are extremely modified. At first stage, with diagnosis still doubtful, the revealing notch in isopters may be discovered with a certain surface-luminosity combination only and not with another. This summation trouble is of great importance. It takes a greater signification after intervention. It still persists during the weeks following around coagulated retinal spots. Its disappearance means a sound and firm reapplication.
Summation study takes a great importance in CHRONIC GLAUCOMA. They are disturbed in irritative glaucoma, that is to say in the variety of glaucoma comporting oedema. The first sign of field deterioration in that case is the well-known "bearing of the blind spot" of English authors.
In many cases, it is only possible to detect this sign through several surface and intensity combinations of the same isopter. One combination inscribes an isopter outside of the blind spot, whereas another equivalent one may inscribe inside it. This sign may disappear with the pilocarpine test. As it is, at this precise stage, interventions have least chance to be dangerous, it is so extremely important to detect early signs of retinal damage. Moreover summative trouble is a sign of oedema as precious if not more than the angular scotomas.
IN ACUTE GLAUCOMA, summations are evidently deeply disturbed, but one is rarely able to study it. In all cases oedema spreads all over the retina.
PAPILLARY OEDEMA evidently causes deep modifications in spatial summations. If summation difficulties are evident on medium and central isopters which is expressive of oedema of the posterior pole, the region around the blind spot must particularly be explored. In fact, curves defining the outline of the spot arise gradually one over another according to the intensity and surface of the applied test. It is thus possible to detect summations. They are deeply disturbed. This trouble evidently has little diagnostical value when oedema is seen with an ophthalmoscope, On the other hand, like every functional sign, it may precede by far the pathological anatomical aspect. Stasis, for instance can often be foreseen before it actually really appears.
Later, we will see that any homonymous hemianopsia with summation trouble around the spot is caused by tumour or a vascular process, photometric disharmony of which gives a peripheric signature.
Summative sign clarifies numerous diagnosis. Its presence strikes out false papillary oedemas (myelin fibres, papillary drusen, hypermetropic pseudo-neuritis).
Unfortunately, it does not allow a diagnosis between papillitis and stasis. It must be obtained perimetrically on other signs.
Spatial summation study at the retinal level is thus dominated by an oedema notion. It is certainly the factor in modifications of threshold sensibilities. Affections not creating summative troubles are the ones which do not comport any oedema (choroidal sarcoma, non-evolutive choroidosis, pigmentary retinitis etc.).
Leaving retinal domain to go up along optical pathways, summative trouble is less definite and less constant. It is detected in optic nerve and chiasmatic lesions. It is found especially in inflammatory lesions and not often in compressions.
It is then difficult to be positive but it seems again to be in relation with oedematous lesions.
Higher up, above corpus geniculatum , no summative trouble can be found whatever be the lesion. Photometric disharmony is always an indication of peripheric lesion and when found at retro-geniculate level, it implies stasis at the first stage or a vascular process traduced on retinal plane. With homonymous hemianopsia, it is an indication of tumour, with stasis threatening, or with vascular process at retinal level.
To end this survey, it may then be said that theoretical laboratory work has given perimetric clinical work a most precious sign, but, on the other hand, clinical work completes laboratory research. In fact if there is no summation beyond corpus geniculatum, it is because it is of retinal origin before anything, though there may be a weak indication of its presence in corpus geniculatum but there is none in pathways belonging to the occipital cortex in which there must not be any convergence or influx reduction. This is no proof, however that it does not exist in other pathways of occipital associations.
In pathology, summation trouble seems to be able to be easily interpreted. It is mostly found with retinal oedema. Distended by imbibition water, retinal sensory cells are stretched. They are less numerous by surface unity. Then, summation does not occur as easily. But it remains to be explained why summations can be disturbed in lesions of the conduction ways up to corpus geniculatum, and it remains to find out which is the modification of the summation exponant in various pathological processes. We have undertaken this research the results of which will be brought out later and which, already keeps in store riddles of most intricate interpretation. It must not be forgotten that it is not absolute threshold but a variety of differential thresholds which is studied in clinical work, which may well deprive of all legitimately mathematical explanations arrived at upto this time.
We mentioned them because actual conception is based on them and they have allowed the elaboration of the two variable system, but they very likely will be widely discussed in a near future.
Whatever happens, the practical use of a two variable perimetry instead of one has proved extremely rewarding. The first great acquisition can be summed up in a lapidary formula. The trouble of laws ruling relations between luminous intensity and surface tests belongs to the infrageniculated peripheric level of optical pathways and is not found at retrogeniculate levels.
It is especially found with oedema. These considerations find their application in the prognosis and diagnosis of many troubles and yet they derive from theoretical, mathematical, physical and physiological considerations, intricated enough. They are a definite demonstration that theoretical research is not in opposition with clinical work but completes it and helps it to progress.
| Cases Where the Summation Law is Disturbed|| |
Evolutive myopic choroidosis
Detachment of retina
Atypical forms of pigmentary retinitis
Vascular diseases of the retina
Von Hippel disease
Tabetic optic atrophy
Optic nerve compression
Chiasmatic diseases with oedema
| Cases Where the Summation Law is not Disturbed|| |
Some chronic non-evolutive glaucomas
Colobomatous hole in papilla
Toxic amblyopias :- alcohol-nicotin
- C. S. 2.
Optic nerve trauma
Optic nerve compression without stasis
Chiasmatic diseases without oedema Chiasmatic gliomas
Homonymous hemianopsias without stasis and without vascular retinal disturbances
Temporal lobe tumours without stasis
Parietal lobe tumours without stasis
Occipital lobe tumours without stasis
Retrogeniculate lesions without stasis or retinal oedema.
| Case Where the State of Summation Law is Unknown|| |
Visceral hemorrhages with visual field disturbances
Without tension glaucoma : Von Graefe's disease
Toxic amblyopias :- lead
Optical tracts lesions
Corpus geniculatum lesions
Visual field disturbances in squint