Examination of the optic nerve at the slit-lamp biomicroscope with a handheld lens
Recent technological advances in the field of glaucoma relating to optic nerve imaging can help assist ophthalmologists in diagnosing and monitoring glaucoma in their patients. However, proper clinical evaluation of the optic nerve remains the mainstay in the evaluation and diagnosis of glaucoma. Examination and documentation of the optic nerve can be effectively achieved through a systematic approach as described in the following steps:
Methods of Examination
Examination of the optic nerve includes a combination of methods ranging from direct assessment by the ophthalmologist to the information provided by ancillary testing equipment.
Direct Ophthalmoscopy. Examination of the optic nerve can be performed with a handheld direct ophthalmoscope. Direct ophthalmoscopy provides an upright, magnified (approximately 15x's), monoscopic view of the optic nerve. The structure of the exam is similar to the one described below with the 7 Steps. Depending on the specific patient exam, certain features of the ophthalmoscope should be adjusted. For example, the size of the pupil influences the optimal aperture selected. Ideally, the aperture should approximate the size of the pupil as this will allow sufficient light to enter eye to provide enough illumination of the fundus. The selection of an aperture larger than the pupil introduces more light than can enter the eye and creates secondary problems such as glare and constriction of the pupil. Additonally, the use of the red-free light can help identify hemorrhages such as optic nerve head (Drance) hemorrhages.
Slit-lamp Exam. Slit-lamp examination of the optic nerve is ideal and uses hand-held lenses to magnify and control the view of the exam. This method provides a stereoscopic, inverted view of the optic nerve. Detailed information regarding this exam is given in the 7 Steps below.
Optic Disc Stereophotography. Monitoring progression of optic nerve changes can best be performed with optic disc stereophotography. This method of comparing prior with current optic nerve images allows the examiner to evalute for both gross and fine changes. For example, examination using this method allows for changes seen with optic nerve head hemorrhages that may lead to notching, or monitoring progressiving neuroretinal rim loss by following barring of vessels.
GDx. The GDx involves scanning laser polarimetry that uses polarized light and bifirengence to monitor retinal nerve fiber layer.
OCT. The OCT (Optical Coherence Tomography) can be used to evaluate the optic nerve based on low coherence interferometry.
HRT. The HRT (Heidelberg Retina Tomograph) is a confocal scanning laser that uses multiple cross section images to create and analyze three dimensional structures such as the optic nerve.
See Eyewiki article "Optic Nerve and Retinal Nerve Fiber Imaging"
Step 1: Cup
Slit-lamp measurements of the vertical and horizontal cup-to-disc ratios should be determined. Asymmetry of 0.2 or more between the two optic nerves raises suspicion for possible glaucoma. Care must be taken to note whether the cup was identified by change in color or contour (see Step 2). Visibility of lamina cribosa fenestrations in the cup is known as the laminar dot sign.
Step 2: Color
The pallor associated with glaucomatous changes in the cup chronologically follows the thinning of the neuroretinal rim and its color. Therefore, close attention should be given to the areas where vessels appear to be kinking in addition to that of pallor as the former would more accurate in identifying the cup. This discrepancy between the cup and pallor becomes more apparent when saucerization is present. Saucerization occurs when a shallow cup leads to the cup margin with the remaining central pallor intact.
Step 3: Contour
The contour of the neuroretinal rim may reveal subtle features consistent with glaucomatous change. Focal notching, or thinning, of the neuroretinal rim may exist. This occurs particularly after resolution of splinter hemorrhages. Focal notching will lead to localized loss of retinal nerve fiber that can be demonstrated using the red-free filter on the slit-lamp or be directly measured for with ancillary testing such as the GDx. This loss in the retinal nerve fiber may proceed changes in the optic nerve located if discovered early in the disease process. Eventual progression of focal notching leads to a sharpened rim where sections of the neuroretinal rim are virtually non-existent. In contrast to these localized areas, concentric atrophy results in a generalized loss of tissue. Besides glaucoma, normal age-related changes in the retinal nerve fiber do occur and can mimic similar atrophy.
Step 4: ISNT Rule
The four quadrants of the neuroretinal rim follow a characteristic pattern of thickness in normal individuals as described by the ISNT rule. The ISNT rule represents the thickness of these quadrants in decreasing order: inferior (I), superior (S), nasal (N), and temporal (T). Any violation of the ISNT rule raises suspicion of glaucomatous changes. This is especially important if thinning is noted inferiorly and superiorly as these areas are usually the first to change in glaucoma.
Step 5: Disc Size
The dimensions of the disc should be measured along its long axis making sure to account for the magnification factor of the lens being used—1.1 and 1.3 for the 78 and 90 diopter lenses, respectively. Other methods for determing size includes using the 5 degree aperture on the direct ophthalmoscope that closely approximates the size of a normal disc. Also, confocal scanning laser ophthlamoscopes can provide detailed measurements as part of their outpatient during examination. Care needs to be taken not to include the zones around the disc in the measurement such as the scleral lip or the zones of alpha and beta atrophy (see Step 7).
Step 6: Vessel Caliber
Vessel tortuosity denotes chronic central vessel occlusion with presence of collateral vasculature. Sharply bending bayoneting vessels can be located at sharpened rims. Baring of circumlinear vessels that once resided at the rim margin can be seen moving away from the border. Presence of splinter hemorrhages can signify signal glaucomatous changes and usually leave behind focal notching of the rim once they resolve approximately 15 to 18 months after initially presenting. These hemorrhages can be easily overlooked if the examiner does not take care to look for them. They rarely occur in non-glaucomatous eyes and should be noted if discovered especially as they last for a brief period of time (usually between 2 to 4 months). They commonly exist in the superotemporal or inferotemporal rim. In particular, patient with normal tension glaucoma are at increased risk (between 3 to 5 times more likely) to have them. However, care needs to be taken if disc hemorrhages are noted as the differential includes other causes such as posterior vitreous detachment, trauma, hypertension, or use of anticoagulation medications like coumadin.
Step 7: Peripapillary Atrophy (PPA)
Evaluation of the peripapillary architecture and related atrophy does aid in glaucoma assessment. Two distinct zones of atrophy—zone alpha and beta—can surround the optic disc. The inner zone beta represents loss of retinal pigment epithelium and choriocapillaris leaving intact choroid vasculature. Zone beta atrophy is more often seen in glaucomatous nerves. Its size significantly correlates with other factors used to determine the severity of glaucoma such as perimetric defects and help differentiate between classes of chronic open angle glaucoma. These areas of atrophy are predominated located temporally but can encircle in the optic nerve. Zone alpha atrophy is more common in normal eyes and is located on the outer surface of zone beta if present. It contains areas of hyper or hypopigmentation with thinning of the choriocapillaris. The difference in location between the two types of atrophy translates to differing visual field defects. Of note, care must be taken when examining patients that are myopic or have tilted optic nerves. In these individuals, scleral crescents, in particually inferiorly, can be present and must not be confused with zone alpha or beta atrophy.
Variability in Examination
Examination of the optic nerve demonstrates both interobserver and intraobserver variability.
Interobserver Variability. Interobserver variability occurs when two or more observes vary in their evaluation on one common entity. When evaluating the optic nerve (and the progression of glaucoma), different clinicians will give varying measurements in their examination. This leads to different examination and ultimately practice patterns as these subjective parameters are used in clincial decision making.
Intraobserver Variability. Intraobserver variability arises from the variation of evaluation from one observer about the same entity. Similarly to interobserver variability, intraobserver variability when evaluating the optic nerve results in different assessments of glaucomatous nerves even though the judgement comes from the same observer. These varying assessments of the optic nerve result in different practice patterns as clinicial decisions are based on these findings.
Repetition and adherence to these (or similar steps) can dramatically aid any ophthalmologist. This is especially important when facing the common dilemma of diagnosing glaucoma or monitoring its progression in cases that are not obvious.
(Optic Nerve Photos: Courtesy of Sarwat Salim, MD,FACS, University of Tennessee and George Spaeth, MD, Wills Eye Hospital)
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