Análisis de la entrevista y observación de los estudiantes

1.7.2.1 Gonioscopy

The technique of gonioscopy was developed by Troncoso Visualisation of the angle by gonioscopy is of paramount importance in managing PAC. The indirect gonioscopy lenses, which give an inverted image reflected in a mirror, are

established as the most convenient for regular clinical usage. In particular the “Goldmann" (one and two mirror) and “Zeiss” (four mirror) models are probably the most useful in regular practice.

The Goldmann lens has the advantage of giving a clear view even for a relatively inexperienced observer, and being stable on the eye once inserted. The lens curvature is usually steeper than that of the cornea, and hence requires a coupling medium. The need for a viscous medium such as 2% hypromellose can be

overcome with practice, and 0.5% hypromellose or normal saline substituted. This reduces the subsequent blurring of vision, which results from disturbance of the ocular surface. The Zeiss lens, being “flatter than K” does not require a coupling medium, and having four mirrors, gives a rapid means of inspecting all four quadrants of the angle-recess.

“Pressure” or dynamic gonioscopy was first described in 1957 as a method of evaluating the extent of peripheral anterior synechiae in the operating theatre Indentation of the central cornea forces aqueous into the periphery of the anterior chamber. On the premise that appositional angle-closure is reversible and that due to synechiae is not, this technique has found wide usage in the evaluation of cases of angle-closure, and in determining their prognosis and management. Although originally described using the Zeiss lens it is feasible using the Goldmann lens The Zeiss lens is more effective for indentation gonioscopy when there is a high pressure gradient between anterior and posterior chambers or in the presence of a very thick iris. It may not be possible to accurately

determine the extent of synechial closure in an eye with a substantially raised

lOP^l

The single most important structure to identify when performing gonioscopy is Schwalbe’s line. This can be located at the termination of the peripheral corneal wedge (see Figure 6).

Figure 6. The corneal wedge as a means of locating angle structures

é

A gonioscopic view of the drainage angle at high m agnification (x 16 or x 25) with a bright, narrow, horizontally off-set beam allows a view of the "corneal wedge"; the junction o f transparent cornea and translucent sclera. The w edge (arrow) may have an acute or obtuse termination. This point is often seen as a slightly raised line (Schwalbe's line).

R eproduced from : Fundamentals of Clinical Ophthalmology: Glaucoma (Ed. Hitchings), BMJ Books, London 2000. Chapter 16. Classification and Clinical Features. PJ Foster and GJ Johnson.

Figure 7. Anatomy of the eye

eye muscle

zonula fibers anterior eye chamber

cornea

conjunctiva posterior eye chamber

eye muscle

retina vitreous body

ciliary body

retinal pigment epithelium sdera

optic nerve

A cross-section of the eye. Ocular dimensions are im portant in assessing risk from angle-closure. The axial depth of the anterior chamber, thickness of the lens and overall length of the globe (axial length) are the most com m only cited

dimensions.

Figure 8. Gonioscopic anatomy illustrating major anatomical landmarks

Pigment on Schwalbe's line Anterior trabecular meshwork Pigmented (posterior) trabecular meshwork Scleral spur Ciliary body band Peripheral iris

Lying immediately posterior to the corneal wedge is the trabecular meshwork (see

Figures 7 & 8). It is believed that the posterior half of the trabecular meshwork is responsible for the majority of aqueous drainage via the trabecular route. When assessing the degree of functional obstruction to drainage, apposition of the iris and posterior trabecular meshwork is of greatest relevance. During gonioscopic examination, it is important to avoid high levels of illumination. This may cause pupil constriction and deceptive widening of the angle. The room illumination should be minimal. The beam should be horizontally off-set when examining the superior and inferior angles, and vertically off-set (with a horizontally orientated beam) when examining the nasal and temporal angles. This helps to create a visible corneal wedge, and avoids troublesome reflection from the lens. Care should be taken to avoid shining the beam into the pupil, or inadvertent pressure with the gonioscope. Both may cause a significant change in configuration of the angle, which may take several minutes to reverse.

There are two widely used (and often confused) schemes for classifying and recording the gonioscopic appearance of the drainage angle, described by Scheie

and Shaffer (Table 2). Scheie’s scheme describes the angle structures that are visible, and the degree of pigmentation in the angle. The angle width is

graded “O” for wide open with the ciliary body being visible, and IV representing a state where no angle structures are visible. Pigmentation is graded “none” to IV. In contrast, the Shaffer system attempts to describe the angular width of the irido­ corneal recess, and uses Arabic numerals in the reverse order, 0 meaning an angle of 0°and 4 indicating 30-45°. It is difficult to accurately assess this angle and in practice most ophthalmologists use a combination of these two schemes, but usually record the findings according to Shaffer's convention.

Table 2. A comparison of the Scheie & Shaffer gonioscopic grading schemes

Scheie Description Risk of Shaffer Angle Description

Grade Closure Grade width

O W ide open Impossible 4 35-45° W ide open

1 SI. Narrowed Impossible 3 20-35° W ide open

II Angle apex not

visible

Possible 2 20° Narrow

III Posterior half of trabeculum not visible Probable 1 10° Extremely Narrow IV No structures seen Closed 0 0° Closed

*The two schemes are presented as approximately equivalent. This is a generalisation, but is valid in most circumstances.

In an account that gives the most detailed description of the gonioscopic anatomy of European people, Spaeth used a sophisticated grading scheme that describes several features of angle configuration Angular width was estimated as that between a tangent to iris about one third of the way from its insertion into the ciliary body, and the surface of the trabecular meshwork. The level of insertion of the iris was graded A to E representing “A”nterior to trabecular meshwork;

“B”ehind Schwalbe’s line; s“C”leral spur; “D”eep into the ciliary body, and "E ’xtremely deep. Using these grades, a further refinement to this scheme

records the point of appositional contact between the iris and trabecular meshwork in brackets, before the actual insertion of the iris. Thus an angle with an apparent iris insertion of grade B that, with indentation gonioscopy, has a true insertion of “D” grade would be recorded as “(B)D”. The profile of the iris in the sagittal plane was graded as either “s” steep or convex, “r” regular or flat and “q” queer or concave (see Figure 9).

Figure 9. Scheme for estimating gonioscopic angle width

This diagram illustrates Spaeth's method of grading angle width on gonioscopic exam ination. The observer attempts to estimate the angle in degrees between a line extended from the plane of the trabecular meshwork and a tangent to iris about one third of the way from its insertion into the ciliary body.

R eproduced from : Fundam entals of Clinical O phthalm ology: G laucom a (Ed. H itchings), BMJ Books, London 2000. C hapter 16. C lassification and Clinical Features. PJ Foster and GJ Johnson (re-draw n from Spaeth's original).

Using this system of grading, Oh et al (working in Spaeth’s unit in Philadelphia) studied the gonioscopic characteristics in 291 people of African, European and East Asian ancestry. There were approxim ately equal numbers in each ethnic group, with a sim ilar age distribution. The Europeans and Africans had a mean refraction approaching 0, with the East Asians having a mean refraction of minus 1.1 dioptres. The subjects were examined using a Zeiss four-m irror gonioscope. The width of the drainage angle was similar in all groups. The point of contact between the iris and the corneo-scleral surface (before and after indentation) was recorded. The authors concluded that the iris joined the scleral wall most

anteriorly in Asians (graded C: 27%, D: 69%, E: 4%), and most posteriorly in Europeans (graded C: 10%, D: 80%, E: 10%). Another finding was that Asians had a higher proportion of people with a concave iris configuration than Europeans

(13% vs. 2% respectively) This finding is somewhat counter-intuitive. If one expects Asians to have a higher rate of angle-closure, a higher rate of convex iris configuration would be expected. However, the subjects in this study in North America are almost certainly not representative of most people East Asia. They are likely to be far better educated, and consequently more myopic, than most people in rural China. It is therefore inappropriate to extrapolate these findings from America to Asia.

A feature of gonioscopic anatomy that has been linked with glaucoma is that of iris processes. In Europeans, these structures usually insert at the ciliary body,

although they may extend as far anteriorly as Schwalbe’s line. It has been observed that iris processes may be extensive and extend anteriorly onto the trabecular meshwork in people with dark brown irises and could be mistaken for

peripheral anterior synechiae by the unwary In a study of 340 eyes of 175 non- glaucomatous subjects, those with and without iris processes had similar

intraocular pressures. More than half of the eyes examined (57%) had iris

processes, with 30% (102 eyes) inserting anteriorly to scleral spur A series of overtly glaucomatous eyes was also examined and the proportion of people with and without dense iris processes was 1:4.1 in POAG, 3:4 in pigmentary glaucoma and 1:26 in other glaucomas, compared with 1:43 in the normal series

1.7.2.1.1 “Occludable” drainage angles

One objective in examining the drainage angle in cases of suspected primary- angle closure is to determine whether closure is indeed possible, and whether there is any evidence that it is occurring. In epidemiological research, the most widely used definition of an occludable angle is one in which the view of the posterior, (usually) pigmented trabecular meshwork is obscured throughout 270° or more of its circumference, without indentation or manipulation of the gonioscope

This definition has found wide usage more for the purpose of allowing comparison between studies than any deeply-held conviction that it is correct. Use of this definition in clinical practice would be a gross over-simplification. It is likely that wider angles may be occludable, and narrow angles may never become occluded

Pigmentation on the trabecular meshwork may give clues to the presence and clinical course of closure of the angle. In subjects with brown irises, iris pigment may be deposited on either Schwalbe’s line (in an interrupted linear pattern) or the surface of the trabecular meshwork (usually in round or irregular geographical blotches) when there is closure of the angle. Occasionally, patchy adhesions may be seen to divide when performing pressure gonioscopy. A fine dusting of

pigment on the surface of the inferior and nasal trabecular meshwork is a normal finding in persons over the age of 30, although deposition of pigment on the

superior trabecular meshwork may indicate intermittent closure of the angle An irregular, granular pigmentation of the meshwork is believed to be a sign of

significant ischaemic damage to the trabeculum. By alternately opening and closing the beam of the slit lamp, a dangerously narrow angle may be seen to close and open, demonstrating the potential for closure of the angle.

Although dynamic gonioscopy is held to be the cornerstone of an examination to confirm angle-closure, the static picture may not always be representative of the true in vivo configuration. The act of placing a gonioscope on the eye will

inevitably cause some distortion of the angle configuration if the lens is ill-fitting or, if there is inadvertent pressure. This distortion may be caused either by the

observer or the upper lid of the subject (especially if the palpebral fissure is small). Excessive tilting of the lens, in an attempt to see over the apex of a very convex iris, may also cause indentation of the cornea, leading to widening of the angle, and the mistaken impression that the angle is wider than it is. These factors have also been suggested as one explanation of why the superior angle usually

appears to be the narrowest quadrant Closing the eyes during gonioscopy invariably causes upward rotation of the globe (the Bell’s phenomenon). Pressure from the upper lid on the edge of the gonioscope may result in inadvertent

mechanical distortion of the cornea, making the drainage angle appear artificially narrower

1.7.2.2 Estimation of limbal chamber depth

While gonioscopy is considered the definitive method of assessing the

time consuming in a busy practice. Estimation of the limbal chamber depth was suggested as a non-invasive alternative to gonioscopy.

The original description of this technique was based on data from 5,436 patients, aged from the first to seventh decade, attending a practice in San Francisco for refraction. Approximately 400 of this number were examined by gonioscopy. It was stated that the four point LCD grading scheme was roughly equivalent to another five-point method of grading the gonioscopic estimate of the angular width of the peripheral anterior chamber There was rarely disagreement by more than one grade, and if disagreement did occur, the gonioscopic estimate was usually narrower than the LCD grade

Subsequently, there have been a few descriptions of the performance of this test in populations reported to have a high prevalence of primary angle-closure

glaucoma. Studies in Greenland, Southern India and Taiwan reported sub-optimal performance of the test in screening for primary angle-closure

The population distribution of LCD has subsequently been studied in Europe, North America and Asia. LCD was graded as a percentage fraction of peripheral corneal thickness (PCT) in the Framingham Eye Study. In this predominantly white North American population, 6% of people had LCD < 30% In Rotterdam, The Netherlands, only 2.2% of a group of 6,760 subjects aged 55 years and older were found to have LCD < 25% PCT Surprisingly, a study in Tibet found a similar figure; 2.4% of people aged 40 years and older had LCD < % PCT. The prevalence of primary angle-closure glaucoma in this east Asian population was reported to be very low. The efficacy of the technique as a screening tool was not evaluated in these studies.

Figure 10. A heavily pigmented drainage angle

A gonio-photograph in an eye with prim ary angle-closure after laser iridotom y show ing heavy pigm ent deposition. The irregular "high-tide" m ark is approxim ately along Schw albe's line

Figure 11. The van Herick technique of estimating limbal chamber depth

The van H erick technique is used to estim ate the depth of the anterior cham ber (the gap between the corneal endothelium and the iris) at the tem poral lim bus. The estim ate is expressed as a fraction o f the corneal thickness.