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The Mystery of Cotton-Wool Spots a Review of Recent and Historical Descriptions

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The Mystery of Cotton-Wool Spots a Review of Recent and Historical Descriptions June 24, 2008 EU RO PE AN JOUR NAL OF MED I CAL RE SEARCH 231 Eur J Med Res (2008) 13: 231-266 © I. Holzapfel Publishers 2008 Review THE MYSTERY OF COTTON-WOOL SPOTS A REVIEW OF RECENT AND HISTORICAL DESCRIPTIONS Dieter Schmidt Univ.-Augenklinik Freiburg, Germany Abstract amaurosis fugax attack. Therefore, every patient who Purpose: Cotton-wool spots (CWSs) lie superficially as notices floaters or a brief visual disturbance, possibly opaque swellings in the retina, with occurring as acute a significant predictor of disease, should be examined lesions. The occurrence of CWSs is a sign of serious ophthalmoscopically. vascular damage. The detection of CWSs at an early stage is neces- Methods: CWSs can usually be diagnosed by ophthal- sary, because they could be the first sign of a systemic moscopy. In the literature, there are reports of exami- disease. Even an isolated CWS should alert a clinician nations by fluorescein angiography, visual fields, or to arrange an extensive examination of the patient. optical coherence tomography (OCT). The occurrence of CWSs is usually a serious sign of Results: CWSs are non-specific, as they can occur in vascular damage (Andersen, 1978) [1]. different diseases involving the retinal vascular system. CWSs are localized accumulations of axoplasmic de- I. DEFINITION OF COTTON-WOOL SPOTS bris within adjacent bundles of ganglion cell axons. They occur after arteriolar occlusion at the borders of There are several terms in the literature for "cotton- large ischemic areas, and should not be regarded as wool spots" (CWSs) or "cotton-wool patches", such as retinal fiber layer infarcts (McLeod). The "principal "snow bank lesions" or "soft exudates". The latter is a constituent of the CWS" are cytoid bodies. Microa- misnomer as the patches are not exudates. They ap- neurysms may be present at the edge of some CWSs pear as whitish-grey, fluffy deposits with frayed edges, in different diseases with retinal ischemia. The pres- as if they had been "pulled from a roll of cotton" ence of many CWSs may be a significant predictor for (Matas 1977) [192]. They lie superficially as opaque the development of rubeosis iridis, for instance in a swellings in the retina; most occur as acute lesions. patient with central retinal vein occlusion. CWSs have disappeared in weeks or occasionally a few months in II. PATHOPHYSIOLOGY hypertensive patients. However, in diabetic patients, they may persist for as long as one or two years. 1. PHYSIOLOGY OF AXOPLASMIC TRANSPORT AND ITS CWSs have been the most frequent ocular findings DISTURBANCE in patients with Aids. Aids patients show a strong correlation between A constant flow of subcellular particles and mole- CWSs and multiple opportunistic infections. Aids pa- cules has been identified within ganglion-cell axons of tients with CWSs have shown a poorer prognosis nerve-cells, the "axoplasmic transport". This transport compared to those without CWSs. Leukocyte counts has been investigated with enzyme markers and au- were found to be significantly lower, and the propor- toradiography. Mitochondria, secretory vesicles, en- tion of patients with significant weight loss was zymes, and ribonucleic acid are among the conducted greater among Aids patients who revealed CWSs. cellular constituents of axoplasm. The axonal trans- Conclusion: CWSs delineate ischemic retina, which is port is bidirectional. Slow and rapid phases of axo- attributed to obstruction of axoplasmic transport. Is- plasmic flow have been distinguished (Kirkpatrick & chemia is the essential factor in the development of Stern 1973) [147]. CWSs should be regarded as lo- CWSs. Early detection of a CWS is necessary to evalu- calised accumulations of axoplasmic debris in the reti- ate a hitherto occult systemic disease with a vascular nal nerve-fiber layer. Slow and rapid phases of axo- component. plasmic flow have been distinguished in retinal gan- glion-cell axons. Retrograde axonal flow has been INTRODUCTION demonstrated between synaptic terminals in the lateral geniculate body and the inner retina. Orthograde and A cotton-wool spot (CWS) usually occurs as an retrograde axoplasmic transport in retinal ganglion-cell asymptomatic retinal sign. Only rarely, a patient may axons can be interrupted by axonal ischemia (McLeod notice blurred vision in conjunction with the occur- 1975) [199]. rence of CWS, or CWSs may appear together with an Quigley & Anderson (1976) [252] reported that or- thograde transport depends on oxygen, ATP energy, and temperature. Ischemia or mechanical pressure can Acknowledgements: We thank "Bildung on demand" (Prof. interrupt the axoplasmic transport. When intraocular Dr. J. Hirsch) for financial support of this article. pressure has experimentally raised to 30 mm Hg below 232 EUROPEAN JOURNAL OF MEDICAL RESEARCH June 24, 2008 mean arterial blood pressure in animals, transport only in the context of a restricted collateral microcir- blockage was detected within two hours. culation." CWSs are nothing more than "sentinels" of retinal nerve-fiber, and should not be regarded as reti- CWS reflects obstruction of axoplasmic flow nal fiber layer infarcts [207]. Duke-Elder (1967) [72] assumed that localised de- Jampol & Rabb (1981) [133] pointed out that the non- struction of several axons, as may be caused by a reti- perfused areas often correspond to obstructed precapil- nal infarct with interruption by anoxia, may cause lary arterioles. With inner retinal ischemia there was or- CWSs. The common factor would be ischemia and thograde or retrograde axoplasmic stasis or both, with an anoxia due to acute occlusion of a focal area of small accumulation of cytoplasmic debris in the nerve-fiber vessels. Local factors were enumerated such as an in- layer. farct, thrombus, micro-embolism or spasm, or a sud- den decrease in retinal arteriolar pressure. However, 2. TRACER STUDIES McLeod (1976a) [200] explained the pathophysiology of CWSs in greater detail. He emphasized that CWSs McEwen & Grafstein (1968) [197] administered la- represent the amount of axoplasmic debris. CWSs re- beled leucine into the eye of a goldfish and found that flect obstruction of orthograde or retrograde axoplas- radioactive protein rapidly accumulated in the collate- mic transport in the unmyelinated axons. Orthograde ral optic tectum. In the transported protein, the label and retrograde axoplasmic flow occur simultaneously, remained attached to leucine. This material reached but independently. CWSs occur after arteriolar occlu- the tectum six to 12 hours after the isotope injection, sion at the borders of large ischemic areas. They often revealing a rate of transport of at least 40 mm per day. define the borders of large ischemic areas, but do not McLeod et al. (1977) [202] studied amino-acid uptake represent the full extent of the ischemic retinal area, and transport in retinal ganglion cells by autoradiogra- for instance after an arteriolar occlusion. Their size de- phy following an intravitreal injection of [3H] leucine pends largely on the number of affected axons. They in animals. When leucine was injected at the same time are thus most dense in the temporal peripapillary re- as retinal laser coagulation with arteriolar occlusion, gion (McLeod, 1976b) [201]. on the peripheral side of two-day-old infarcts, label Experiments in pigs were carried out with argon became concentrated in the swollen axon-terminals. laser photocoagulation of the retina to occlude small However, on the disc side of infarcts, label was absent retinal arterioles. McLeod et al. (1977) [202] observed from the terminal. The accumulation of [3H] leucine ischemic necrosis of the inner retina. Swollen axon and organelles was attributed to interruption of or- terminals were packed with cytoplasmic organelles in thograde axoplasmic transport. the retinal nerve-fiber layer on the infarcts´ peripheral In an additional experiment, [3H] leucine was inject- border. CWSs should be regarded as localised accumu- ed into the vitreous two days prior to laser coagula- lations of axoplasmic debris in the retinal nerve-fiber tion. Conspicuously, label became distributed through- layer. out the axoplasm, and then accumulated in swollen Experiments in monkeys by Kishi et al. (1985) [148] axon terminals on both sides of infarcts. This obser- showed that peripapillary CWSs occurred as a result of vation showed that both orthograde and retrograde accumulation of axoplasmic components due to the axoplasmic flow were obstructed. Minckler & Bunt blockage of orthograde axonal transport within the is- (1977) [213] injected radioisotopes into the vitreous of chemic optic disc. Multifocal ischemia causes a spotty eyes with marked papilledema in seven monkeys with accumulation of axoplasmic organelles. ocular hypotony. Orthograde axoplasmic transport Graham (1990) [101] reported that CWSs indicate was blocked in swollen axons of the optic disc, shown blockage of a retinal arteriole, caused by abnormalities by autoradiography. Retrograde transport was studied in the vascular endothelium, blockage by abnormal ery- in the same eyes by horseradish peroxidase injection throcytes, or emboli. The CWS is produced by occlu- into the dorsal lateral geniculate nuclei or optic tracts. sion of the precapillary arterioles and reflects the break- The authors emphasized that axoplasmic transport in down of retrograde and orthograde axoplasmic flow. the optic nerve head is sensitive to alterations in in- The axoplasmic debris accumulates at the junction of traocular pressure. Tsukada & Chihara (1986) [326]
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