Respuestas de algunos estudiantes a la secuencia didáctica ESTUDIANTE No

1.7.6.1 Damage to the trabecular meshwork

There have been several descriptions of the pattern of angle-closure and the formation of synechial damage. Phillips gave anecdotal descriptions of his observations in European people Irido-corneal contact in the superior half of the angle was found in individuals without symptoms. It was stated that closure of the angle progresses to the nasal and temporal sectors as age (and lens size) increases. It was suggested that pathological angle-closure is an evolving process that starts with intermittent, appositional contact which gradually becomes

permanent with the formation of synechiae. Several possible explanations were cited for the finding of sectorial variations in angle-width:

1. Sectorial differences in the origin of iris from the ciliary body

2. Unequal differentiation of sectors of the angle during development 3. Orientation of the lens

4. Decentration of the pupil

5. Uneven flattening of the cornea by the eyelids

The latter is the most attractive idea. In a later, more thorough study, 20 subjects with PAC were examined. Inclusion criteria were: a vague history of transient blurring of vision; no corneal oedema; lOP greater than 21 mm Hg and less than 51 mm Hg; disc and field examination being normal or showing early

glaucomatous changes; gonioscopic examination showing a partially or completely occluded drainage angle and partial or complete reversal of angle closure with instillation of pilocarpine. Of these 20 patients, 19 met inclusion criteria for both eyes, although only one eye of each patient was used in the analysis. In every case, the area of angle-closure was continuous, and extended for at least 3 clock hours. The superior area from 11 to 1 o'clock was closed in all cases. The 6 o’clock position was closed in only one person. Areas that remained closed after instillation of pilocarpine hydrochloride (1 or 2%) were said to have

goniosynechiae, although dynamic gonioscopy was not used. The proportion of individuals with closure of the angle reduced in all sectors after pilocarpine was introduced

A similar study in Japan used dynamic gonioscopy to assess the morphology and distribution of peripheral anterior synechiae (PAS) in 171 eyes of 101 people with PAC. In this study, the inclusion criteria were a closed or partially closed drainage angle with an lOP > 21 mm Hg, or a narrow angle in the fellow eye once lOP had been effectively controlled. The eyes were sub-divided into those with signs and symptoms of sudden pressure elevation (“acute”), asymptomatic people with narrow angles and either PAS or lOP > 21 mm Hg (“chronic”), and “fellow eyes”. Pressure gonioscopy was carried out, with areas of PAS graded according to height (posterior, mid- or anterior trabecular meshwork) and width (narrow<15°, medium 15-30° and broad >30°). Clock hours 11-12 and 12-1 were termed superior, with 5-6 and 6-7 being inferior. The intervening 4 “hour” sectors were termed nasal and temporal. PAS were found in 87% of the “acute” group, 84% of the “chronic” group and 51% of the “fellow eyes”. The classification of PAS width was narrow; 44.7%, medium: 15.6%, broad: 39.7%. Interestingly the morphology of PAS varied with clinical type of PAC. In “chronic” cases, narrow PAS were most common (53%), whereas “acute” cases had more broad PAS (74%). This

difference was highly significant. Broad PAS were also more common in the fellow eye group (52%). PAS of all types were most common in the superior sector and least common in the inferior sector. Table 4 shows the distribution of height of PAS. No narrow PAS reached Schwalbe’s line. 94% of broad PAS reached the anterior or mid-trabecular meshwork. In the symptomatic “acute”

cases, there was a correlation between the width of PAS and the duration of symptoms.

Table 4. Height of peripheral anterior synechiae and symptoms of primary angle-closure

Height of synechiae on the trabecular meshwork

PAS level Posterior Middle Anterior

Acute 3.4 % 68.7 % 27.9 %

Fellow eyes 15.5% 80.3 % 4.2 %

Chronic 26.5 % 70.2 %

—T28---

3.3 %

As long ago as 1960, Gorin suggested two possible modes by which closure of angle may occur. Firstly that there was contact between Schwalbe's line and the peripheral iris. Synechiae then form in an anterior-to-posterior direction. Secondly, he suggested that peripheral anterior synechiae may form initially in the periphery of the angle. The first suggestion (initial contact between Schwalbe's line and the peripheral iris) has subsequently been verified by ultrasound biomicroscopy Pitch has subsequently called these two patterns “s” (for closure at Schwalbe's line) and “b” (from bottom of the angle) types respectively. I noue et al argue that their data support the case for the second mechanism (posterior-to-anterior

development of PAS), although it is difficult to understand how they can determine that high PAS have not developed by adhesion of the peripheral iris to Schwalbe's line. On balance, it seems likely that both mechanisms occur.

Pathological studies of primates show that lOP 15 mm Hg below ophthalmic artery perfusion pressure is sufficient to cause ischaemic necrosis of the iris. This was associated with formation of a fibrinous clot and fibro-vascular proliferation in the drainage angle. This presumably acts as a foundation for the formation of high, broad peripheral anterior synechiae

1.7.6.2 Corneal damage

Observational studies by Lowe suggested that symptomatic PAG may produce several changes in the cornea, beginning with pressure-related epithelial oedema.

followed by folds in Decemet’s membrane. Prolonged central oedema with Descemet’s folds was thought to occur if the endothelium was damaged. Later changes include vascularization, lipoid infiltration and band-type degeneration

In a quantitative study in China, corneal endothelia of 87 cases of unilateral glaucoma were studied with a specular microscope, with healthy fellow eyes used as controls. The endothelial cell density was found to be decreased in the majority of glaucomatous eyes. The reduction was 13% in eyes with symptomatic PAG (34 cases), by 5% in asymptomatic PAG (23 cases), and by 12% in glaucoma cyclitic syndrome (30 cases). In comparison, the mean fall in endothelial cell densities after filtering operations in 16 eyes was 10%, and 5% after Nd:YAG laser iridotomy

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1.7.6.3 Lens damage

The characteristic lens change in PAG is the formation of “glaucomflecken”. These were initially described by Vogt in 1930^^^. There is considerable variation in the formation and extent of these opacities. They form in the anterior

subcapsular region. It is believed that they are areas of denatured lens protein, and reflect a reduced ability of the terminal areas of transparent lens fibres to withstand physiological stress. With the passage of time they descend deeper into the lens as more fibres are formed on top. They do not form at the posterior pole of the lens, and are said to be restricted to the sutural areas of the lens. Other lens changes include fibrosis of the lens capsule, cortical opacities and dense nuclear sclerosis

1.7.6.4 Uveal and retinal damage

Studies of primates (owl monkeys) suggest that the first permanent lesions to occur with acute elevation of lOP are partial necrosis of the iris stroma and ciliary body, associated with microscopic lesions of the photoreceptors and retinal pigment epithelium around the optic disc and in the retinal periphery. These changes occurred at an lOP 15 mm Hg below perfusion pressure. At higher levels of pressure (lOP 5 mm Hg below perfusion pressure) damage to the retinal nerve- fibre layer and optic disc was observed, together with more diffuse retinal damage. It was suggested that the more pressure-sensitive nature of uveal tissue may

result in ciliary body shut-down, and halt any further pressure rise. The retina and optic nerve have a higher lOP threshold for damage, and hence their function is preserved in many cases of symptomatic lOP rises

Symptomatic cases of primary angle-closure glaucoma may be accompanied by substantial inflammation, and hypopyon formation has been reported

Iridoschisis is also reported to be associated with primary angle-closure Light and transmission electron microscopy show that during an acute attack of

symptomatic angle-closure, the anterior border layer of the iris became thickened. Following the symptomatic episode, the iris becomes structurally disrupted and the stromal cells degenerate markedly. In end-stage disease the stromal cells were atrophic.

1.7.6.5 Disc damage

Sudden rises in lOP to near perfusion pressure are associated with atrophy of retinal, pre- and retro-laminar ganglion cell fibres. Remarkably, the glial cells remain almost completely unaffected. The changes in the retro-laminar portion of the optic nerve are identical to an ascending optic neuropathy. Cavernous optic atrophy was not seen

1.7.6.6 Visual field loss

Descriptions of the characteristics of visual field loss in PAG are scarce and contradictory (probably as a result of inconsistency in nomenclature). One

description of subjects with primary and secondary angle-closure found that most (9/11) subjects with chronic disease had visual field loss in a nerve fibre bundle pattern. Conversely, 11/18 subjects who had suffered an acute, symptomatic attack of angle-closure had no impairment of the visual field. The remaining 7/18 showed typical nerve-fibre bundle visual field loss Another study of 25 cases of acute, unilateral angle-closure described a “significant” field defect in over half this group, usually characterised by constriction of the upper field (not felt to be

typically “glaucomatous” in nature). Ten of these 25 subjects did exhibit a blue- yellow dyschromatopsia In Japan, a study of 110 subjects with acute (42) and chronic (68) angle-closure using Goldmann kinetic perimetry concluded that the pattern of visual field loss in PACG was similar to that in POAG

In Singapore, the presenting features were studied retrospectively in 50 patients with primary angle-closure. Visual field loss was more common and severe in patients with "chronic" disease than in “acute” angle-closure. Field loss in chronic angle-closure glaucoma was more severe in eyes with higher intraocular pressure. Among 20 symptomatic cases, 15 had a full field, 3 had a nerve fibre bundle pattern of damage and 2 had gross field constriction. There were 30 subjects with “chronic” disease; 2 had full fields, 2 had generalised constriction (most marked on nasal side), large nasal sectorial defects were found in 5 people while gross loss with central or paracentral islands were present in 15. Six people had total field loss

1.7.6.7 Prognosis in angle-closure

In Israel, cases of symptomatic PAC presenting less than 24 hours after the onset of an attack were found to have a better long-term outcome (in terms of final lOP and need for continuing medical therapy) than those presenting later A study in England found no association between a similar delay in presentation and visual acuity

A retrospective study in Singapore looked at duration of symptoms prior to presentation, treatment required to control the attack, the presence of iris ischaemia and presenting lOP as predictors of the long-term likelihood of

developing glaucoma. Those patients who presented more than 24 hours after the onset of symptoms had a relative risk of 2.78 (01= 1.03 to 7.46) of developing glaucoma. Those who required an additional laser procedure to control the lOP at presentation and those who required an urgent trabeculectomy had relative risks of 3.63 (01= 1.49 to 8.89) and 4.83 (1.18 to 19.7) respectively of developing glaucoma