Podcast: Tests for imaging the optic nerve and its fibres for diagnosing glaucoma

The Cochrane Database of Systematic Reviews contains dozens of reviews of the accuracy of diagnostic tests and imaging, along with the thousands of reviews of the effects of treatments. In November 2015, these were added to by a new review of imaging devices for glaucoma and Gianni Virgili, from the University of Florence in Italy, tells us about the findings in this podcast.

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John: The Cochrane Database of Systematic Reviews contains dozens of reviews of the accuracy of diagnostic tests and imaging, along with the thousands of reviews of the effects of treatments. In November 2015, these were added to by a new review of imaging devices for glaucoma and Gianni Virgili, from the University of Florence in Italy, tells us about the findings in this Evidence Pod.

Gianni: Glaucoma is a progressive degenerative disease that affects the optic nerve and gradually damages the visual field. The course of the disease can be slowed or halted by reducing pressure inside the eye by using eye drops, laser treatment or surgery; and our review looks at the evidence for diagnosing glaucoma in people who are at risk. We assessed three imaging devices: confocal scanning laser ophthalmoscopy, scanning laser polarimetry and optical coherence tomography, or OCT; which can measure the structure of the optic nerve head, the thickness of the nerve's fibres, or both.
We found 106 studies, published up to February 2015, that had included more than 16,000 patients. Most of the studies investigated OCT, and we found that the tests were better at detecting more severe glaucoma compared to early glaucoma. The selection of two well-defined groups of healthy and glaucoma eyes in nearly all studies, rather than the use of these imaging tests in a series of patients at risk of glaucoma, may overestimate the accuracy of these devices.
The performance of the devices was variable across the studies, but overall they gave similar findings; which we can summarise as follows. In 1000 people, of whom 200 have manifest glaucoma, such as those who have already undergone some functional or anatomic testing by optometrists, the devices would miss about 60 out of the 200 patients with glaucoma, which is a sensitivity of 70%. The devices would incorrectly refer 50 out of the 800 patients without glaucoma, which is a specificity of 95%. If the prevalence were lower, say 5% instead of 20%, such as in 1000 people with a family history of glaucoma, the corresponding figures would mean that 15 of the 50 patients with manifest glaucoma would be missed, and 890 out of the 950 non-glaucomatous people would not need to be referred. Most studies reported the sensitivity for diagnosing glaucoma at high levels of specificity, as we report above, but did not give the cut-off values at which such accuracy measures were obtained.
In conclusion, we feel that our findings should be regarded as exploratory at this time, and we recommend that further diagnostic accuracy studies are carried out. These should include more information about the selection of patients and the consequences of false positives and false negatives in the setting in which the diagnostic test is done.

John: If you would like to read more about the studies that have been done and the three different types of imaging, you can find the full review on the internet. Go to Cochrane Library dot com and search for 'imaging of glaucoma'.

John: “The Cochrane Database of Systematic Reviews contains dozens of reviews of the accuracy of diagnostic tests and imaging, along with the thousands of reviews of the effects of treatments. In November 2015, these were added to by a new review of imaging devices for glaucoma and Gianni Virgili, from the University of Florence in Italy, tells us about the findings in this Evidence Pod.”

 

Gianni: “Glaucoma is a progressive degenerative disease that affects the optic nerve and gradually damages the visual field. The course of the disease can be slowed or halted by reducing pressure inside the eye by using eye drops, laser treatment or surgery; and our review looks at the evidence for diagnosing glaucoma in people who are at risk. We assessed three imaging devices: confocal scanning laser ophthalmoscopy, scanning laser polarimetry and optical coherence tomography, or OCT; which can measure the structure of the optic nerve head, the thickness of the nerve's fibres, or both.

 

We found 106 studies, published up to February 2015, that had included more than 16,000 patients. Most of the studies investigated OCT, and we found that the tests were better at detecting more severe glaucoma compared to early glaucoma. The selection of two well-defined groups of healthy and glaucoma eyes in nearly all studies, rather than the use of these imaging tests in a series of patients at risk of glaucoma, may overestimate the accuracy of these devices.

 

The performance of the devices was variable across the studies, but overall they gave similar findings; which we can summarise as follows.  In 1000 people, of whom 200 have manifest glaucoma, such as those who have already undergone some functional or anatomic testing by optometrists, the devices would miss about 60 out of the 200 patients with glaucoma, which is a sensitivity of 70%. The devices would incorrectly refer 50 out of the 800 patients without glaucoma, which is a specificity of 95%. If the prevalence were lower, say 5% instead of 20%, such as in 1000 people with a family history of glaucoma, the corresponding figures would mean that 15 of the 50 patients with manifest glaucoma would be missed, and 890 out of the 950 non-glaucomatous people would not need to be referred. Most studies reported the sensitivity for diagnosing glaucoma at high levels of specificity, as we report above, but did not give the cut-off values at which such accuracy measures were obtained.

 

In conclusion, we feel that our findings should be regarded as exploratory at this time, and we recommend that further diagnostic accuracy studies are carried out. These should include more information about the selection of patients and the consequences of false positives and false negatives in the setting in which the diagnostic test is done.”

                                                                                                                                             

John: “If you would like to read more about the studies that have been done and the three different types of imaging, you can find the full review on the internet. Go to Cochrane Library dot com and search for ‘imaging of glaucoma’.

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