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66-692: NAION typically presents suddenly upon awakening. The affected person notes seeing poorly in one eye. Vision in that eye is obscured by a dark shadow, often involving just the upper or lower half of vision, usually the area closer to the nose. There is no pain. Within approximately six months following the infarct, visual acuity improves by three or more lines of vision on the Snellen Chart (the chart with smaller letters on each lower line) in 42.7% of patients, while in 12.4% of patients, vision worsens by three lines. Opposite eye involvement occurs in approximately 15% to 20% of patients with NAION within 5 years. It

132-406: A standard normal random variable . An alternative approach to inference for odds ratios looks at the distribution of the data conditionally on the marginal frequencies of X and Y . An advantage of this approach is that the sampling distribution of the odds ratio can be expressed exactly. Logistic regression is one way to generalize the odds ratio beyond two binary variables. Suppose we have

198-408: A binary response variable Y and a binary predictor variable X , and in addition we have other predictor variables Z 1 , ..., Z p that may or may not be binary. If we use multiple logistic regression to regress Y on X , Z 1 , ..., Z p , then the estimated coefficient β ^ x {\displaystyle {\hat {\beta }}_{x}} for X

264-431: A dose-dependent manner. A Cochrane Review on treatments for acute CRAO included one randomized clinical trial involving pentoxifylline, which showed that pentoxifylline use (three 600 mg tablets daily) was associated with improved retinal perfusion, but it was unclear if significant improvements in visual acuity were also observed. Similar findings have been shown using Doppler OCT imaging in patients with NAION, though

330-433: A few cases of NAION result in near total loss of vision, most cases of AAION result in nearly complete vision loss. Nonarteritic anterior ischemic optic neuropathy is an isolated white-matter stroke of the optic nerve (ON). NAION is the most common cause of sudden optic nerve-related vision loss, affecting more than 10,000 Americans every year, often bilaterally. No clinically effective treatments exist, largely because little

396-404: A modern direct ophthalmoscope gives a view of the optic disc using the principle of reversibility of light. A slit lamp biomicroscopic examination along with an appropriate aspheric focusing lens (+66D, +78D or +90D) is required for a detailed stereoscopic view of the optic disc and structures inside the eye. A biomicroscopic exam can indicate the health of the optic nerve. In particular,

462-492: A natural history study on NAION (Ophthalmology 2008;115: 298–305.) showed that visual acuity can improve up to 6 months and not after that. To minimize the risk of further visual loss in the fellow eye or the same eye, it is essential to reduce the risk factors. Common sense dictates trying to control the cardiovascular risk factors for many reasons, including protection from this happening to the second eye. Sudden vision loss should lead to an ophthalmological consultation. If NAION

528-542: A patient to develop NAION, the most common precipitating factor is marked fall of blood pressure during sleep (nocturnal arterial hypotension)- that is why at least 75% of the patients first discover visual loss first on waking from sleep. When other risk factors for NAION are present, taking blood pressure medications at night should be avoided as this can exacerbate nighttime hypotension. Beta blockers in particular are associated with increased incidence of NAION. These vascular risk factors lead to ischemia (poor blood supply) to

594-502: A portion of the optic disc. The disc then swells, and in a crowded optic disc, this leads to compression and more ischemia. Since both eyes tend to have a similar shape, the optometrist or ophthalmologist will look at the good eye to assess the anatomical predisposition. The unaffected eye has a 14.7% risk of NAION within five years. A number of uncontrolled single case or small number of patient reports have associated NAION with use of oral erectile dysfunction drugs. Since arteritic AION

660-399: A probability? It cannot literally mean to double the original probability value, because doubling 50% would yield 100%. Rather, it is the odds that are doubling: from 1:1 odds, to 2:1 odds. The new probabilities would be 66⅔% for heads and 33⅓% for tails. Suppose a radiation leak in a village of 1,000 people increased the incidence of a rare disease. The total number of people exposed to

726-418: A random sample would not include anybody with the disease, since only 2.6% of the population are diseased. Instead, one might use a case-control study in which all 26 diseased villagers are interviewed as well as a random sample of 26 who do not have the disease. The results might turn out as follows ("might", because this is a random sample): The odds in this sample of getting the disease given that someone

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792-420: A sample of 100 men, 90 drank wine in the previous week (so 10 did not), while in a sample of 80 women only 20 drank wine in the same period (so 60 did not). This forms the contingency table: The odds ratio (OR) can be directly calculated from this table as: Alternatively, the odds of a man drinking wine are 90 to 10, or 9:1, while the odds of a woman drinking wine are only 20 to 60, or 1:3 = 0.33. The odds ratio

858-529: A study in India reported visual improvement compared to placebo in patients with optic neuropathy. Of note, TNF may be important in the immune response to certain nematode parasitic infections, and so pentoxifylline should be used with caution for those living in areas of parasite infestation; however, it is probably safe to use in otherwise healthy individuals living in urban, temperate environments. Overall, routine use of pentoxifylline in ischemic optic neuropathies

924-451: A variety of shapes from a shallow indentation to a bean pot —this shape can be significant for diagnosis of some retinal diseases. The optic disc or optic nerve head is the point of exit for ganglion cell axons leaving the eye . Because there are no rods or cones overlying the optic disc, it corresponds to a small blind spot in each eye. Almost all eye structures can be examined with appropriate optical equipment and lenses. Using

990-494: Is D N / V N = 6 / 600 = .01 {\displaystyle D_{N}/V_{N}=6/600=.01} . One obvious way to compare the risks is to use the ratio of the two, the relative risk . The odds ratio is different. The odds of getting the disease if exposed is D E / H E = 20 / 380 ≈ .0526 , {\displaystyle D_{E}/H_{E}=20/380\approx .0526,} and

1056-429: Is a statistic that quantifies the strength of the association between two events, A and B. The odds ratio is defined as the ratio of the odds of event A taking place in the presence of B, and the odds of A in the absence of B. Due to symmetry , odds ratio reciprocally calculates the ratio of the odds of B occurring in the presence of A, and the odds of B in the absence of A. Two events are independent if and only if

1122-418: Is a predisposition in the form of a type of optic disc shape. The optic disc is where the axons from the retinal ganglion cells collect into the optic nerve. The optic nerve is the bundle of axons that carry the visual signals from the eye to the brain. This optic nerve must penetrate through the wall of the eye, and the hole to accommodate this is usually 20-30% larger than the nerve diameter. In some patients

1188-402: Is also the entry point for the major arteries that supply the retina with blood, and the exit point for the veins from the retina. The optic disc is located 3 to 4 mm to the nasal side of the fovea . It is a vertical oval, with average dimensions of 1.76mm horizontally by 1.92mm vertically. There is a central depression, of variable size, called the optic cup . This depression can be

1254-440: Is as an estimate of the odds ratio between Y and X when the values of Z 1 , ..., Z p are held fixed. If the data form a "population sample", then the cell probabilities p ^ i j {\displaystyle {\widehat {p\,}}_{ij}} are interpreted as the frequencies of each of the four groups in the population as defined by their X and Y values. In many settings it

1320-464: Is called the rare disease assumption ), then the OR is approximately equal to the corresponding RR. The OR plays an important role in the logistic model . If we flip an unbiased coin, the probability of getting heads and the probability of getting tails are equal — both are 50%. Imagine we get a biased coin that makes it two times more likely to get heads. But what does "twice as likely" mean in terms of

1386-448: Is combined, it will often be expressed as "pooled OR". As explained in the "Motivating Example" section , the relative risk is usually better than the odds ratio for understanding the relation between risk and some variable such as radiation or a new drug. That section also explains that if the rare disease assumption holds, the odds ratio is a good approximation to relative risk and that it has some advantages over relative risk. When

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1452-448: Is defined. It is undefined if p 2 q 1 equals zero, i.e., if p 2 equals zero or q 1 equals zero. The odds ratio can also be defined in terms of the joint probability distribution of two binary random variables . The joint distribution of binary random variables X and Y can be written where p 11 , p 10 , p 01 and p 00 are non-negative "cell probabilities" that sum to one. The odds for Y within

1518-414: Is due to temporal arteritis (also called giant-cell arteritis ), an inflammatory disease of medium-sized blood vessels (Chapel-Hill-Conference) that occurs especially with advancing age. In contrast, NAION results from the coincidence of cardiovascular risk factors in a patient with "crowded" optic discs. Non-arteritic AION is more common than AAION and usually occurs in slightly younger persons. While only

1584-411: Is exposed is 20/10 and the odds given that someone is not exposed is 6/16. The odds ratio is thus 20 / 10 6 / 16 ≈ 5.3 {\displaystyle {\frac {20/10}{6/16}}\approx 5.3} , quite close to the odds ratio calculated for the entire village. The relative risk, however, cannot be calculated, because it is the ratio of the risks of getting

1650-517: Is impractical to obtain a population sample, so a selected sample is used. For example, we may choose to sample units with X  = 1 with a given probability f , regardless of their frequency in the population (which would necessitate sampling units with X  = 0 with probability 1 −  f ). In this situation, our data would follow the following joint probabilities: The odds ratio p 11 p 00  /  p 01 p 10 for this distribution does not depend on

1716-470: Is known about its pathophysiology, and there are few histopathological studies of the acute condition. An exhaustive review article published in March 2009 described the latest information on arteritic and non-arteritic ischemic optic neuropathy, both anterior (A-AION and NA-AION ) and posterior (A-PION, NA-PION, and surgical). Once NAION happens, it was thought that there was no accepted treatment to reverse

1782-628: Is much research currently underway looking at ways to protect the nerve (neuroprotection) or even regenerate new fibers within the optic nerve. So far there is no evidence in human studies that the so-called neuroprotectors have any beneficial effect in NAION. However, there is a new current clinical trial for the treatment of NAION in the United States with plans to include sites in India, Israel, Germany and Australia (see NORDICclinicaltrials.com and https://clinicaltrials.gov/ ). This trial will test

1848-448: Is not always devastating as visual acuity may remain only moderately impaired. Furthermore, most cases of NAION involve the loss of a hemifield (either the upper or lower half of the visual field, but not both). A few cases of NAION involve near-total loss of vision. The mechanism of injury for NAION used to be quite controversial. However, experts in the field have come to a consensus that most cases involve two main risk factors. The first

1914-436: Is related to a conditional odds ratio. Specifically, at the population level so exp ⁡ ( β ^ x ) {\displaystyle \exp({\hat {\beta }}_{x})} is an estimate of this conditional odds ratio. The interpretation of exp ⁡ ( β ^ x ) {\displaystyle \exp({\hat {\beta }}_{x})}

1980-465: Is similar in presentation to non-arteritic AION, patients over the age of 50 diagnosed with NAION must be evaluated to exclude AAION (symptoms: painful jaw muscle spasms, scalp tenderness, unintentional weight loss, fatigue, myalgias and loss of appetite); NAION patients over the age of 75 should always be tested. The distinction between AAION and non-arteritic AION was made to highlight the different etiologies of anterior ischemic optic neuropathy. AAION

2046-488: Is suspected, then ideally a neuro-ophthalmologist's consultation should be obtained. A recent Cochrane Review sought to determine the extent of safety and efficacy of optic nerve decompression surgery for NAION, compared to other treatments, or no treatment. The one study included in the review found no improvements in visual acuity among patients who underwent surgery for NAION, and adverse events (pain, double vision) experienced by participants who underwent surgery. There

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2112-400: Is thus 9/0.33, or 27, showing that men are much more likely to drink wine than women. The detailed calculation is: This example also shows how odds ratios are sometimes sensitive in stating relative positions: in this sample men are (90/100)/(20/80) = 3.6 times as likely to have drunk wine than women, but have 27 times the odds. The logarithm of the odds ratio, the difference of the logits of

2178-416: The probabilities , tempers this effect, and also makes the measure symmetric with respect to the ordering of groups. For example, using natural logarithms , an odds ratio of 27/1 maps to 3.296, and an odds ratio of 1/27 maps to −3.296. Several approaches to statistical inference for odds ratios have been developed. One approach to inference uses large sample approximations to the sampling distribution of

2244-410: The OR equals 1, i.e., the odds of one event are the same in either the presence or absence of the other event. If the OR is greater than 1, then A and B are associated (correlated) in the sense that, compared to the absence of B, the presence of B raises the odds of A, and symmetrically the presence of A raises the odds of B. Conversely, if the OR is less than 1, then A and B are negatively correlated, and

2310-466: The basis of their systole - diastole variations, and reveal ocular hemodynamics in human eyes. Furthermore, the Doppler spectrum asymmetry reveals the local direction of blood flow with respect to the optical axis. This directional information is overlaid on standard grayscale blood flow images to depict flow in the central artery and vein. A systematic review of 106 studies and 16,260 eyes compared

2376-443: The complete eye examination by an eye care physician. Ophthalmologists and optometrists are able to provide this service. Blood flow in the retina and choroid in the optic disc region can be revealed non invasively by near-infrared laser Doppler imaging . Laser Doppler imaging can enable mapping of the local arterial resistivity index , and the possibility to perform unambiguous identification of retinal arteries and veins on

2442-456: The damage. However, a recent uncontrolled retrospective large study has shown that if patients are treated with large doses of corticosteroid therapy during the early stages of NAION, in eyes with initial visual acuity of 20/70 or worse, seen within 2 weeks of onset, there was visual acuity improvement in 70% in the treated group compared to 41% in the untreated group ( odds ratio of improvement: 3.39; 95% CI:1.62, 7.11; p < 0.001). That study and

2508-445: The denominators in the relative risk and odds ratio are almost the same ( 400 ≈ 380 {\displaystyle 400\approx 380} and 600 ≈ 594 ) {\displaystyle 600\approx 594)} . Relative risk is easier to understand than the odds ratio, but one reason to use odds ratio is that usually, data on the entire population is not available and random sampling must be used. In

2574-409: The disease and H N = 594 {\displaystyle H_{N}=594} stayed healthy. We can organize this in a contingency table : The risk of developing the disease given exposure is D E / V E = 20 / 400 = .05 {\displaystyle D_{E}/V_{E}=20/400=.05} and of developing the disease given non-exposure

2640-410: The disease and we would need V E {\displaystyle V_{E}} and V N {\displaystyle V_{N}} to figure those out. Because the study selected for people with the disease, half the people in the sample have the disease and it is known that that is more than the population-wide prevalence. It is standard in the medical literature to calculate

2706-417: The example above, if it were very costly to interview villagers and find out if they were exposed to the radiation, then the prevalence of radiation exposure would not be known, and neither would the values of V E {\displaystyle V_{E}} or V N {\displaystyle V_{N}} . One could take a random sample of fifty villagers, but quite possibly such

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2772-576: The eye care physician notes the colour, cupping size (as a cup-to-disc ratio ), sharpness of edge, swelling, hemorrhages, notching in the optic disc and any other unusual anomalies. It is useful for finding evidence corroborating the diagnosis of glaucoma and other optic neuropathies, optic neuritis , anterior ischemic optic neuropathy or papilledema (i.e. optic disc swelling produced by raised intracranial pressure ), and optic disc drusen . Women in an advanced stage of pregnancy with pre-eclampsia should be screened by an ophthalmoscopic examination of

2838-568: The eyes, including the optic disc. They quantify the nerve fiber layer of the disc and surrounding retina and statistically correlate the findings with a database of previously screened population of normals. They are useful for baseline and serial follow-up to monitor minute changes in optic disc morphology . Imaging will not provide conclusive evidence for clinical diagnosis however, and the evidence needs to be supplanted by serial physiological testing for functional changes. Such tests may include visual field charting and final clinical interpretation of

2904-640: The labeling of categories as zero and one is arbitrary, so there is nothing special about concordant versus discordant values in these applications. If we had calculated the odds ratio based on the conditional probabilities given Y , we would have obtained the same result Other measures of effect size for binary data such as the relative risk do not have this symmetry property. If X and Y are independent, their joint probabilities can be expressed in terms of their marginal probabilities p x  =  P ( X  = 1) and p y  =  P ( Y  = 1) , as follows In this case,

2970-456: The log odds ratio (the natural logarithm of the odds ratio). If we use the joint probability notation defined above, the population log odds ratio is If we observe data in the form of a contingency table then the probabilities in the joint distribution can be estimated as where ︿ p ij  =  n ij  /  n , with n  =  n 11  +  n 10  +  n 01  +  n 00 being

3036-641: The odds if not exposed is D N / H N = 6 / 594 ≈ .0101 . {\displaystyle D_{N}/H_{N}=6/594\approx .0101\,.} The odds ratio is the ratio of the two, As illustrated by this example, in a rare-disease case like this, the relative risk and the odds ratio are almost the same. By definition, rare disease implies that V E ≈ H E {\displaystyle V_{E}\approx H_{E}} and V N ≈ H N {\displaystyle V_{N}\approx H_{N}} . Thus,

3102-417: The odds ratio R differs from 1, then where p 1•  =  p 11  +  p 10 ,   p •1  =  p 11  +  p 01 , and In the case where R  = 1 , we have independence, so p 11  =  p 1• p •1 . Once we have p 11 , the other three cell probabilities can easily be recovered from the marginal probabilities. Suppose that in

3168-423: The odds ratio and then use the rare-disease assumption (which is usually reasonable) to claim that the relative risk is approximately equal to it. This not only allows for the use of case-control studies, but makes controlling for confounding variables such as weight or age using regression analysis easier and has the desirable properties discussed in other sections of this article of invariance and insensitivity to

3234-535: The odds ratio equals one, and conversely the odds ratio can only equal one if the joint probabilities can be factored in this way. Thus the odds ratio equals one if and only if X and Y are independent . The odds ratio is a function of the cell probabilities, and conversely, the cell probabilities can be recovered given knowledge of the odds ratio and the marginal probabilities P ( X  = 1) =  p 11  +  p 10 and P ( Y  = 1) =  p 11  +  p 01 . If

3300-450: The odds ratio is: where q x  = 1 −  p x . An odds ratio of 1 indicates that the condition or event under study is equally likely to occur in both groups. An odds ratio greater than 1 indicates that the condition or event is more likely to occur in the first group. And an odds ratio less than 1 indicates that the condition or event is less likely to occur in the first group. The odds ratio must be nonnegative if it

3366-522: The optic disc for early evidence of a rise in intracranial pressure . A normal optic disc is orange to pink in colour and may vary based on ethnicity. A pale disc is an optic disc which varies in colour from a pale pink or orange colour to white. A pale disc is an indication of a disease condition. Traditional colour-film camera images are the reference standard in imaging, requiring an expert ophthalmic photographer, ophthalmic technician, optometrist or ophthalmologist for taking standardised pictures of

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3432-406: The optic disc, it corresponds to a small blind spot in each eye. The ganglion cell axons form the optic nerve after they leave the eye. The optic disc represents the beginning of the optic nerve and is the point where the axons of retinal ganglion cells come together. The optic disc in a normal human eye carries 1–1.2 million afferent nerve fibers from the eye toward the brain. The optic disc

3498-442: The optic disc. Stereoscopic images offer an excellent investigative tool for serial follow-up of suspected changes in the hands of an expert optometrist or ophthalmologist. Automated techniques have also been developed to allow for more efficient and less expensive imaging. Heidelberg retinal tomography (HRT), scanning laser polarimetry and optical coherence tomography are computerised techniques for imaging various structures of

3564-490: The optic nerve is nearly as large as the opening in the back of the eye, and the optic disc appears "crowded" when seen by ophthalmoscopy . A crowded disc is also referred to as a "disc at risk". While a risk factor, the vast majority of individuals with crowded discs do not experience NAION. The second major risk factor involves more general cardiovascular risk factors. The most common are diabetes , hypertension and high cholesterol levels. While these factors predispose

3630-509: The parameter of greatest interest is actually the RR, which is the ratio of the probabilities analogous to the odds used in the OR. However, available data frequently do not allow for the computation of the RR or the ARR, but do allow for the computation of the OR, as in case-control studies , as explained below. On the other hand, if one of the properties (A or B) is sufficiently rare (in epidemiology this

3696-430: The performance of the imaging techniques, and found that all three imaging tests performed very similarly when detecting for glaucoma. The review found that in 1,000 patients subjected to imaging tests, with 200 having manifest glaucoma, the best imaging tests would miss 60 cases out of the 200 patients with glaucoma, and incorrectly refer 50 out of 800 patients without glaucoma. Odds ratio An odds ratio ( OR )

3762-406: The population log odds ratio is L  ± 1.96SE . This can be mapped to exp( L  − 1.96SE), exp( L  + 1.96SE) to obtain a 95% confidence interval for the odds ratio. If we wish to test the hypothesis that the population odds ratio equals one, the two-sided p-value is 2 P ( Z  < −| L |/SE) , where P denotes a probability, and Z denotes

3828-435: The presence of one event reduces the odds of the other event occurring. Note that the odds ratio is symmetric in the two events, and no causal direction is implied (correlation does not imply causation ): an OR greater than 1 does not establish that B causes A, or that A causes B. Two similar statistics that are often used to quantify associations are the relative risk (RR) and the absolute risk reduction (ARR). Often,

3894-541: The radiation was V E = 400 , {\displaystyle V_{E}=400,} out of which D E = 20 {\displaystyle D_{E}=20} developed the disease and H E = 380 {\displaystyle H_{E}=380} stayed healthy. The total number of people not exposed was V N = 600 , {\displaystyle V_{N}=600,} out of which D N = 6 {\displaystyle D_{N}=6} developed

3960-453: The results relative to what would have been obtained for a population sample. Due to the widespread use of logistic regression , the odds ratio is widely used in many fields of medical and social science research. The odds ratio is commonly used in survey research , in epidemiology , and to express the results of some clinical trials , such as in case-control studies . It is often abbreviated "OR" in reports. When data from multiple surveys

4026-400: The sum of all four cell counts. The sample log odds ratio is The distribution of the log odds ratio is approximately normal with: The standard error for the log odds ratio is approximately This is an asymptotic approximation, and will not give a meaningful result if any of the cell counts are very small. If L is the sample log odds ratio, an approximate 95% confidence interval for

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4092-451: The two subpopulations defined by X = 1 and X = 0 are defined in terms of the conditional probabilities given X , i.e. , P ( Y  | X ) : Thus the odds ratio is The simple expression on the right, above, is easy to remember as the product of the probabilities of the "concordant cells" ( X  =  Y ) divided by the product of the probabilities of the "discordant cells" ( X  ≠  Y ) . However in some applications

4158-465: The type of sampling . The odds ratio is the ratio of the odds of an event occurring in one group to the odds of it occurring in another group. The term is also used to refer to sample-based estimates of this ratio. These groups might be men and women, an experimental group and a control group , or any other dichotomous classification. If the probabilities of the event in each of the groups are p 1 (first group) and p 2 (second group), then

4224-789: The use of a synthetic siRNA that blocks caspase 2, an important enzyme in the apoptosis cycle. In addition to such research, patents have been applied for by Pfizer, The University of Southern California, Otsuka Pharmaceutical and other individual inventors for innovations related to the treatment of anterior ischemic optic neuropathy. In recent years, pentoxifylline has emerged as a potential treatment option for NAION and other diseases involving ocular ischemia. Pentoxifylline has been shown to reduce erythrocyte rigidity, resulting in decreased blood viscosity and increased flow velocity. Animal studies have demonstrated that pentoxifylline can inhibit TNF and, in turn, prevent retinal ganglion cell death and axonal degeneration associated with optic neuropathy in

4290-417: The value of f . This shows that the odds ratio (and consequently the log odds ratio) is invariant to non-random sampling based on one of the variables being studied. Note however that the standard error of the log odds ratio does depend on the value of f . This fact is exploited in two important situations: In both these settings, the odds ratio can be calculated from the selected sample, without biasing

4356-495: Was not supported given limited evidence, but the absence of major adverse effects and the absence of other proven therapies suggest a possible therapeutic role for pentoxifylline. It is estimated that the incidence of AION in the US is about 8,000 persons per year. Optic disc The optic disc or optic nerve head is the point of exit for ganglion cell axons leaving the eye . Because there are no rods or cones overlying

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