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Recently, new techniques have been introduced for performing glaucoma surgery without opening into the anterior chamber, thus avoiding the complications which are commonly associated with penetrating glaucoma surgery. These methods involve exposure of the canal of Schlemm under a deep scleral flap without actually entering the anterior chamber and sometimes insertion of a collagen sponge under the scleral flap.1-4
Essentially these methods stem from the seminal work of Grant who demonstrated that in normal enucleated human eyes 75% of the resistance to aqueous outflow was located in the trabecular meshwork.5 Following Grant's work, various attempts were made to redesign glaucoma surgery to remove resistance at the trabecular meshwork thus lowering intraocular pressure. It is instructive now to reappraise these attempts.
Redmond Smith6 introduced the concept of trabeculotomy as surgery for open angle glaucoma and this was further developed by Harms and Dannheim.7 Soon after Grant's work Dvorak-Theobald and Kirk8 pointed out that some cases of open angle glaucoma were due to obstruction of the scleral collectors and Krasnov introduced sinusotomy for use in glaucoma when trabecular function appeared adequate.9 Sinusotomy and trabeculotomy were even combined to produced a filtering bleb with an intact anterior chamber.10
In 1968 John Cairns introduced trabeculectomy having noted the lack of acceptance of trabeculotomy.11 Essentially this functions as a guarded full thickness sclerectomy, although Cairns originally postulated that removal of trabecular meshwork would allow free flow of fluid into the open lumen of the canal of Schlemm bypassing trabecular resistance. There is now little doubt that trabeculectomy has supplanted all other forms of glaucoma surgery.
This early experience is probably relevant to the recent innovations. The lack of randomised, prospective, case-control trials with adequate follow up for the new procedures has recently been criticised.12 13 This was remedied in theBJO last year for cases with a collagen implant14 although various criticisms have remained, including the risk of perforation of the trabecular meshwork and the frequent need for YAG laser trabecular puncture postoperatively. The criteria of success include a pressure of less than 21 mm Hg, hardly acceptable to most current workers in glaucoma for advanced cases and topical medication is often needed to achieve this.
In the light of the complexity of the procedures proposed, the lack of careful study of the trabecular meshwork is notable although some early studies have been reported. In the current issue of theBJO (p 1354), a careful study of the trabecular meshwork in glaucoma is presented stressing the variability of the tissue and that the anterior meshwork is extremely thin in older patients. This makes perforation of the mesh at operation likely even for experienced surgeons.
In view of these findings and remembering our previous experience with surgery of the trabecular mesh, it seems wise to withhold judgment on deep sclerectomy and viscocanalostomy until more extensive clinical appraisal is available.
Cover illustration: The pinhole camera
This primitive species of mollusc, theNautilus is probably over 150 million years old. The invertebrate has changed little over that time and still has a primitive eye, illustrating its successful visual adaptation to its environment. The eye is located behind the tentacles and a pinhole opening is its only external sign. Curiously, there is no lens, no cornea and, in fact, no dioptric element whatsoever. Furthermore, there is no obstruction to the incursion by seawater, thus allowing for a continual exchange with the external pelagic environment. There may be a mucous or gelatinous substance filling the cavity of the eye in life, although its composition is not known nor its presence even confirmed. It probably resembles a vitreous body, but surely must contain an admixture of seawater because of the open fistula.
The eyecup is approximately round with a pupil which has some contractile features to it and with a known pupillary diameter range of at least 0.7–2.25 mm. The weakly contractile pinhole leads into a surprisingly large cavity lined nearly 360° with cells, individually called retinula, each of which includes a rhabdom at its distal end. These cells are primitive photoreceptors. Other cells in the retina are supporting cells that are directly interspersed between the retinulae, sending processes distally that extend between the rhabdoms. Both types of cells contain pigment granules that can migrate within the cell and are probably used to control light stimulus to the retinula. The axons lead away from the proximal base of the retinula directly to the optic nerve without synapse. Anteriorly, near the pupil, there are pigmented cells without rhabdoms and mucous cells that probably secrete the mucoid-like contents of the chamber. Morphologically resembling an arrogant pugilist, the retinulae lead with their chins by extending the rhabdomal portion of the cell towards the inner cavity, allowing direct seawater contact with the photoreceptive element. In an evolutionary sense, this is a photoreceptive eyespot that has invaginated and developed supporting elements.
Such an eye is not subject to chromatic or spherical aberration, as there is no lens or cornea, but is subject to diffraction limitations of a small opening. Additionally, adequate sensitivity and resolution cannot be obtained simultaneously. As the pupil size becomes smaller the resolution improves up to the point of pupillary diffraction, but the sensitivity decreases dramatically. Not surprisingly, investigators believe the Nautilus actually prefers the light (sensitivity, as they seem to go towards the brighter of two light stimuli in experimental work) confirming that the visual system retains adequate sensitivity even at the expense of resolution.
The Nautilus lives in the Pacific ocean at depths of 150–600 metres along steep underwater cliffs where bioluminescence is a primary source of light. Maximum light transmission at that depth peaks at about 475 nm corresponding to the absorption spectrum of its visual pigment. Spectral sensitivity, as determined by the absorption spectrum of the extractable visual pigment found in the retina, has been determined to be maximal at about 470 nm.
Although Nautilus shows diurnal activity, they are most active as crepuscular or nocturnal creatures with circadian vertical migrations. These animals are benthic in habit and, given the dark sloping substrate on which they live, they may not know which direction is up. The animals are blessed with statocysts providing information on the direction of gravity as well as the depth, since they would probably implode due to the pressure of seawater when at depths over 800 metres. Strong currents characterise their environment and probably cause these animals to drift long distances, requiring vertical localisation skills.
Bioluminescence is important at these depths, and the fish and other animals found here can be considered point sources. Decaying food matter is the principal diet of the Nautilusand is often associated with bioluminescence. Other animals such as species of shrimp are also attracted to the decaying food matter and many of these shrimp also have bioluminescent characteristics that will attract the Nautilus to its food sources.Nautilus feed slowly and distance vision beyond a few metres is probably of little interest to the animal; they probably do not rely upon vision for protection either since they are encased in a hard outer shell.
Their primitive eye probably has not changed during the 150 million years they have been on earth and this helps us understand how visual systems have evolved. The vertebrates that inhabit the same environment have eyes that are at least superficially similar since both are “camera”-type eyes in contrast with the compound eyes of many other invertebrates, including insects and crustaceans. The “camera”-type eye is also seen in other cephalopods such as the octopus and squid, which are related to the Nautilus. These other cephalopods do have a lens and cornea as the eye has evolved to improve the focus of the image. These cephalopod “camera”-type eyes also show convergent evolution since no common ancestor to these invertebrates and vertebrates exist. They have achieved similar results for ocular morphology, but by very different mechanisms, and represent convergent evolution.—Ivan R Schwab,UC Davis Department of Ophthalmology, Sacramento, CA, USA ( ).
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