Old Theories and New Ideas on Ocular Function

Can the Choroid Affect ination in cadavers, to demonstrate that associated with its hyperproliferative Primate Eye Size? only the global layer of each rectus ex- property. ■ traocular muscle inserts on and rotates Earlier studies with chicks have shown the eye. The orbital layer instead inserts that choroid thickness is actively regu- on that muscle’s connective tissue pul- lated in response to retinal defocus and ley, translating it along the muscle axis. Is Glaucoma Only a may be involved in the regulation of eye This finding implies that pulleys, which Disease of the Eye? growth. In two articles in this issue, act as functional origins of the extraoc- Glaucoma is a disease of the optic nerve Troilo et al. (p. 1249) and Hung et al. (p. ular muscles, are under powerful active that also results in the degeneration of 1259) independently show that the pri- control. ■ mate choroid also undergoes changes ganglion cells within the retina. While in thickness associated with visual ex- glaucomatous changes within the retina perience and experimentally induced Is ‘Visual Acuity’ the Best and optic nerve are well-described, little changes in eye size and refraction. Measure of Visual is known about the degeneration that There is some evidence that these occurs postsynaptically within the LGN, changes, although much smaller than Function? a primary site of visual integration. This those seen in chick, might also partially Is visual acuity the most appropriate vi- study by Weber et al. (p. 1370) shows compensate for imposed retinal de- sual function to measure in determining that glaucoma has a significant effect on focus. ■ the difficulties that patients have with the size, density, and number of neu- their vision? For patients with macular rons in the LGN, as well as volume of disease, Hazel et al. (p. 1309) find that the nucleus itself. These data indicate Is There Conjunctiva- low-contrast tests explain most of the that strategies aimed at mitigating glau- Associated Lymphoid variance in self-reported problems with comatous neuropathy need to consider Tissue in the Human? reading, and that text-reading speed treatment of the entire central visual correlates highly with overall concern pathway, and not the eye alone. ■ Although the mucosal immune system is about vision. High-contrast distance vi- of increasing relevance, there is still little sual acuity is not the most relevant out- evidence for a respective tissue in the come measure for assessment of macu- eye, especially in the human. Knop and lar degeneration patients. Therefore, the Is Lens Yellowing a Knop (p. 1270), in a study on a number assumption that visual acuity is the most Function of the Age? of human autopsy tissues, provide evi- relevant measure of visual function dence for the existence of a conjunctiva- in other disease groups is question- There is much discussion in the current associated lymphoid tissue (CALT) in able. ■ literature concerning the regions of the normal individuals, its topographic anat- ultraviolet spectrum that are most dam- omy, its continuation in the lacrimal aging to the human lens. Of fundamen- drainage system, and its contribution to Abnormal Telomerase tal importance to addressing this issue is the secretory immune system. These Activity in Pterygia an accurate knowledge of the absorp- findings characterize the basis for immu- tion characteristics of the lens as a func- Telomerase is an enzyme that extends nologic phenomena on the ocular sur- tion of age. Gaillard et al. (p. 1454) re- the life span of cells by elongating telo- face and may enhance our understand- port a novel method, wherein whole meres at the chromosome ends. Telom- ing of physiologic homeostasis and lenses are sectioned into thin slices and erase activity is often detected in neo- pathologic alterations of the ocular sur- the absorption spectra of individual face. ■ plastic cells, but is very rare in somatic cells in adults. By using a technique slices are added to yield cumulative called the telomeric repeat amplification spectra along the visual axis. The spec- tral changes observed for a series of pri- Extraocular Muscles Do protocol (TRAP), Shimmura et al. (p. 1364) demonstrate the presence of te- mate lenses of different ages indicate More than Turn the Eye lomerase activity in the epithelium of that the age-related yellowing of lens Extraocular muscles consist of two lay- pterygia, but not in normal conjunctiva. protein is the result of a chronological ers: orbital and global. Demer et al. (p. This indicates that although a pterygium process (such as chemical or photo- 1280) used magnetic resonance imaging is not a neoplastic disease, abnormal chemical modification) rather than bio- in living people, and histological exam- telomerase activity in epithelial cells is logical aging (senescence). ■

Downloaded from iovs.arvojournals.org on 09/25/2021 Cataract: The Osmotic form CML and bind redox active copper. altered mERG responses, local wave- Theory Revisited The occurrence of this process in the forms do not appear to correlate with aging human lens may lead to a vicious local changes in the visual field. The Osmotic stress, due to large accumula- cycle consisting of glycoxidation, lipoxi- authors are not optimistic that a measure tion of sorbitol and galactitol in the lens, dation, and metal binding. ■ of the mERG will prove useful in detect- is thought to be the cause of diabetic ing either local or early glaucomatous and galactosemic cataract, respectively. damage. The multifocal VEP (mVEP) has However, it remains controversial as to A Cone Influence on also been suggested as an objective al- what extent cataract in these models is Subretinal Fibrosis? ternative to visual field perimetry. Hood due to a disruption of signal transduc- et al. (p. 1580) argue that intersubject Following retinal detachment, retinal tion pathways by myo-inositol, in addi- variability is too great for the standard Mu¨ller cells often extend processes into tion to altered osmolarity. This paper by mVEP to be useful as a quantitative mea- the subretinal space. In this condition, Jiang et al. (p. 1467) shows that increase sure of visual field defects. However, the known as subretinal fibrosis, the growth in myo-inositol, as a consequence of authors show that local monocular dam- of Mu¨ller cell processes into the subreti- overexpression of a sodium-dependent age to the ganglion cells can be quanti- nal space can completely inhibit the re- myo-inositol co-transporter in the lens, tatively measured by comparing the generation of photoreceptor outer seg- also leads to cataract. Further, sorbitol mVEP responses obtained by stimulat- ments following retinal reattachment and myo-inositol have an additive effect ing each eye separately. These interocu- surgery. Lewis and Fisher (p. 1542) have on cataract development. These results lar comparisons may provide an alterna- found that when Mu¨ller cells grow into are consistent with an osmotic theory of tive to standard perimetry. ■ cataract. ■ the subretinal space they are always ad- jacent to cone photoreceptors, suggest- ing that cones may produce factors that Copper, CML, and Cataract initiate the Mu¨ller cell outgrowths. Iden- Do Rods Determine Their tification here of a relationship between Visual Sensitivity? The crystallins of the aging human lens cones and Mu¨ller cells may aid in choos- What determines infants’ low scotopic become progressively pigmented, cross- ing strategies for inhibiting this re- visual sensitivity, the immature rods or linked, oxidized, and fragmented with sponse. ■ age, and it is generally thought that postreceptoral processing? Fulton and these modifications predispose the lens Hansen (p. 1588) used electroretino- toward formation of senile cataract. Sax- Is mERG and mVEP Useful graphic and psychophysical procedures ena et al. (p. 1473) have tested the hy- to study the course of development of ⑀ in Glaucoma? pothesis that N -carboxymethyl-L-lysine rod photoreceptor sensitivity and dark- (CML), a modification occurring in aging Because significant damage to ganglion adapted visual sensitivity. The shape of lens crystallins, can bind redox active cells can occur before changes are de- the logistic growth curve and the age at copper due to its EDTA-like structure. tected in visual fields, alternative mea- which sensitivity reaches 50% of the The authors find that copper content sures are being sought. Hood et al. (p. adult value is similar (10 to 13 weeks) and copper binding by CML-rich lens 1570) show that although the multifocal for the rods and peripheral visual sensi- crystallin extracts from senile cataracts ERG (mERG) is altered in some patients tivity. A parsimonious explanation is are increased, and that the crystallin with glaucoma, other patients with that, during development, rod photo- copper complexes can oxidize ascorbic clearly altered visual fields can have receptor sensitivity determines dark- acid. When crystallins are modified by mERG responses that appear normal. adapted, rod-mediated visual sensi- ascorbic acid degradation products, they Further, even in patients who exhibit tivity. ■

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