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History, logic the best diagnostic tools in neuro-ophthalmology
Instruments and laboratory testing can provide support after a first diagnosis has been made using decision-tree analysis, clinician says.
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CONEGLIANO, Italy – Patient history and a systemized approach based on the tenets of logic are the best diagnostic tools in neuro-ophthalmology, according to Alfredo Sadun, MD, PhD, the Thornton Professor of Ophthalmology at the University of Southern California.
“Complicated instruments and laboratory testing don’t make a diagnosis in this branch of ophthalmology and may in fact leave one very confused,” he said during his master lecture at a meeting here. “Instead, listening carefully to the patient, formulating hypotheses and verifying their plausibility by asking specific questions will progressively get you to the heart of the problem.”
In all the other branches, ophthalmology is “a visual science in more senses than one,” he said. By observation of a retinal lesion, for instance, the specialist can determine what the retinal pathology is. But in neuro-ophthalmology the lesions are frequently obscure because they are not in the eye but in the brain, he said.
Dr. Sadun said that the best approach uses decision-tree analysis.
“This method provides a highly effective structure within which you can lay out options and investigate the possible outcomes of choosing those options. Ask question 1, and if the answer is 1a, go to 2. If it is 1b, go to 3 and so on. This form of approach, which may seem complicated at first because there are so many branches, after a while becomes a mental habit and is at the root of proper diagnosing,” he said.
Optic nerve vs. macular disorders
To explain his method, Dr. Sadun presented the example of a 26-year-old patient complaining of monocular visual loss for 4 days. No pain or correlated problems were present. Visual acuity was 20/20 in the right eye and 20/40 in the left eye, where the patient had a small central scotoma.
“Where is the root of the problem? In the optic nerve or in the retina? Do we go in for optic neuritis or central serous maculopathy? Objective testing won’t take us very far. Instead, we should start with the history. Examinations and tests will follow at a second stage,” he said.
| Fundus examination can reveal the presence of folminant optic disc edema (top), secondary optic atrophy (center) or severe optic atrophy (bottom). Images: Sadun A Sectoral optic disc atrophy Anatrophic disc may have fewer countable vessels than a normal disc. Sectoral optic disc edema Signs of disc edema include blurring of the disc margins and filling of the cup. |
The first two options of the decision tree in this case are the optic nerve and the macula, he said. Starting with the optic nerve, Dr. Sadun explained, four variables should be investigated: the characterization of the problem, the presence or absence of pain, the time and duration of symptoms, and the presence or absence of additional neurological problems.
“When the problem lies in the optic nerve, vision is characterized by darkness, clouding and smudging. Colors are perceived but are less vibrant, opaque and darker. If the problem is the optic disc, there will be characteristic altitudinal visual field defects,” he said.
Time, he said, is an important factor. “In a transient obscuration of vision (TOV), a duration of 3 seconds rather than 3 minutes or 30 minutes may be determinant in differential diagnosis. We also want to know if the visual loss was acute or progressive; a sudden, very acute visual loss is typical of anterior ischemic optic neuropathy (AION), while a less acute loss followed by an almost complete recovery is more typical of optic neuritis. If the problem is chronic and progressive, it is likely to be correlated to compression of the optic nerve by a tumor,” he said.
But the history of a macular problem is entirely different, Dr. Sadun said. Vision is blurred and often hyperopic, and the most characteristic sign is distortion. “I tell my patients to look at the door, and say if they see it straight or crooked,” he said.
In addition, it is important to verify if there is a history of trauma or systemic disorders, he said.
Dr. Sadun said that age is also a determining factor. Younger patients are more apt to have optic neuritis, while elderly patients are more likely to suffer from AION or macular problems, he said.
Pain is another discriminating factor, Dr. Sadun said. Optic nerve problems might generate pain with eye movements, but macular disorders do not usually cause pain, he said.
Diagnostic examinations
Once a first diagnosis has been made, a targeted eye examination can be performed, Dr. Sadun said.
“For the optic nerve, what we call the optic nerve triad (afferent pupillary defect, color and brightness) will provide us with most of the information, but also visual acuity and fundus appearance are important diagnostic means,” Dr. Sadun pointed out.
He said that for the optic nerve triad, simple testing methods work just as well as the more sophisticated ones. For example, just by moving a light source rapidly from the right to left eye, a different response in the pupil of each eye can be evaluated.
Ishihara plates can be used to assess color perception, he said, but asking the patients whether a color they see is the same with both eyes may be sufficient in many cases. With monocular optic neuritis, for example, the patient will see the color red with both eyes, but the normal eye perceives the color more vividly, he said.
Brightness can be measured with specially designed spectacles, but the patient can simply be asked to look at light sources with alternating eyes and grade the perceived light intensity.
Fundus examination will evaluate the presence of edema, temporal, diffuse or sectorial optic atrophy (pallor), and damage to the nerve fiber layer, he said.
According to Dr. Sadun, the Kestembaum rule can provide further useful information.
“Optic nerve disorders don’t change the number of blood vessels, but do change their size, and therefore the number of countable vessels may vary. With a normal nerve, you’ll be able to count about 12 vessels crossing the disc margins, which will be increased to 15 or 16 in case of hyperemia or decreased to six or seven when the nerve is atrophic,” he pointed out.
The outcome of the diagnostic examinations will add new elements to differentiate a diagnosis of an optic nerve disorder from that of a macular disorder, Dr. Sadun said. The afferent pupillary defect is typically present with optic nerve problems, and both brightness and color vision are markedly reduced with optic nerve problems but only slightly with macular diseases, he said.
Special testing
Dr. Sadun performs special tests as a last step, including visual field, contrast sensitivity and MRI testing.
“Visual field testing helps a lot because the optic fibers enter the nerve from two arched bundles. If there is a problem with the optic nerve, there are always modifications of this characteristic arrangement of nerve fibers,” he said.
Contrast sensitivity can be evaluated in many ways, and is an important element in differential diagnosis, he said. A marked contrast sensitivity loss in the high spatial frequencies with good contrast in the low frequencies is typical of optic neuritis. In general, optic nerve disorders cause the greatest losses between 6 and 12 cycles/degree and macular disorders around 18 cycles/ degree, he said.
An MRI should always be performed in patients with suspected optic neuritis to predict the risk of developing multiple sclerosis, which affects 20% to 40% of patients with optic neuritis, he said.
“As far as treatment is concerned, clinical trials have taught us what not to do,” he said.
Steroids, such as prednisone, which have long been used for optic neuritis, have been shown to be contraindicated, while encouraging results seem to be obtained with the use of interferon, he said.
With AION, on the other hand, optic nerve sheath decompression (ONSD) has been shown to be ineffective in restoring visual acuity, he said. A clinical trial compared the results of 199 patients who underwent ONSD with those of 125 control patients. After 6 months an improvement of three or more lines had occurred in 33% of ONSD patients and in 43% of controls. “ONSD simply doesn’t work for this disease,” Dr. Sadun concluded.
For Your Information:
- Alfredo Sadun, MD, PhD, can be reached at the Keck School of Medicine, University of Southern California, Doheny Eye Institute, 1450 San Pablo St., DEI 5802, Los Angeles, CA 90033-4671; 323-442-6417; fax: 323-442-6407; e-mail: asadun@usc.edu.
- Michela Cimberle is an OSN Correspondent based in Asolo, Italy.