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Show Journal of Clinical Neuro- ophthalmology 9( 2): 79-- 82. 1989. © 1989 Raven Press. Ltd., New York Echographic Correlation of Optic Nerve Sheath Size and Cerebrospinal Fluid Pressure Steven Galetta, M. D., Sandra Frazier Byrne, and J. Lawton Smith, M. D. A 23- year- old obese woman presented with papilledema. Computed tomography showed no intracranial mass lesions and lumbar puncture revealed an increased opening pressure, confirming the diagnosis of pseudotumor cerebri. Standardized echography of the optic nerves was performed immediately before and after lumbar puncture. A marked reduction of cerebrospinal fluid pressure correlated with a decrease in the subarachnoid fluid of the optic nerve sheath. Key Words: Optic nerve- Cerebrospinal fluid pressure- Echographv- Papilledema- Pseudotumor cerebrio From the Department of Neurology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania ( S. C.), and Bascom Palmer Eye Institute, Department of Ophthalmology, University of Miami School of Medicine, Miami, Florida ( S. F. B., J. L. S.). Address correspondence and reprint requests to Dr. S: Caletta at Department of Neurology, Hospital of the UniversIty of Pennsylvania, 3400 Spruce Street, PhIladelphIa, PA 19104, U. S. A. 79 Standardized echography, with the use of the 30° test ( 1), has become an important dynamic imaging technique to detect the changes of optic nerve anatomy underlying various pathologic processes ( e. g., optic neuritis, papilledema, meningioma, glioma, and optic atrophy). To our knowledge, this is the first study to correlate the echographic diameter of the optic nerve sheath with the absolute level of cerebrospinal fluid pressure. MATERIALS AND METHODS On June 16, 1988, a 23- year- old obese woman presented to her optometrist to obtain new contact lenses. She had been completely asymptomatic, but funduscopic examination revealed bilateral swollen discs. There was no relevant history of medication or vitamin intake. The patient was immediately referred to the triage clinic at the Bascom Palmer Eye Institute for evaluation, with a diagnosis of papilledema. On that day, the following diagnostic investigation was performed: complete neuroophthalmologic examination, formal perimetry, fundus photos, computed tomography of the head with contrast, and lumbar puncture. Echograms of the optic nerves were obtained by one of us ( S. F. B.) just before and then again immediately after the spinal tap. The technique for studying the optic nerve diameter using standardized A- scan has been described in detail elsewhere ( 1,2). In brief, a small pencil- sized A- scan probe is placed near the equator temporally as the patient fixates in primary gaze. Maximal diameter of the optic nerve is obtained both anteriorly ( just behind the globe) and posteriorly ( near the orbital apex). The patient 80 S. GALETTA ET AL. FIG. 1. Fundus photo of left eye showing mild optic disc elevation. then refixates with gaze directed at least 300 toward the probe; the optic nerve sheath thickness is then remeasured. A reduction in nerve sheath diameter of at least 10% proves the presence of increased subarachnoid fluid. Contact B- scan is useful to document elevation of the optic disc as well as the gross diameter of the anterior portion of the retrobulbar optic nerve. Examination revealed a best corrected visual acuity of 20/ 20 in both eyes. The pupillary, motility, visual field, and slit lamp examinations were normal. On funduscopy, there was mild swelling of both optic discs without the presence of spontaneous venous pulsations ( Fig. 1). Complete neurologic examination was otherwise normal. Computed tomography of the head with contrast showed no mass lesions. Contact B- scan examination showed bilateral optic disc elevations ( Fig. 2). Standardized A- scan echography of the optic nerves revealed an anterior optic nerve diameter of 4.6 mm on the right and 4.1 mm on the left ,~ ,.. , ) f; ) in the primary position. In eccentric gaze ( 30 0 test), both optic nerves measured 3.1 mm, documenting the presence of compressive fluid within the optic nerve sheaths ( Fig. 3). Studies conducted in our Echography Department ( 3) have shown the normal optic nerve to average 2.5 mm in diameter with a range of 2.2- 3.3 mm. The patient immediately underwent lumbar puncture; an opening pressure of 320 mm Hg was recorded. Thirty- five milliliters of clear colorless fluid was collected and the closing pressure was 110 mm Hg. Within minutes, the diameters of the optic nerve sheaths were remeasured echographically. Anterior optic nerve thickness had decreased to 2.6 mm on the right and 2.8 mm on the left and there was no change in size of the nerve sheaths in eccentric gaze ( Table 1; Fig. 3). The gross diameter of the optic nerves was also shown to be smaller on B- scan examination following the lumbar puncture ( Fig. 4). The remainder of the cerebrospinal fluid examination was unremarkable, showing no cells, protein level of 27 mg/ dl, and glucose concentration of 60 mg/ dl. The patient was observed for a period of 2 h and sent home on acetazolamide therapy. The patient returned for follow- up examination 1 month later and was doing well. Repeat examination with standardized echography showed the optic nerve diameters to be slightly less wide than the measurements taken prior to the lumbar puncture. The 30 0 test was positive. DISCUSSION Standardized echography has become an invaluable method of imaging the intraorbital optic nerve. A major advantage of the technique is its FIG. 2. Left: Axial contact B- scan showing patient's elevated left optic disc ( arrow). Right: Axial B- scan of normal flat disc ( arrow) in another patient. STANDARDIZED ECHOGRAPHY OF OPTIC NERVES 81 PRE POST FIG. 3. Standardized A- scans of left optic nerve with patient fixating in primary gaze ( left) and 30° gaze ( right). Echograms on top were obtained prior to lumbar puncture and those on bottom were obtained just after procedure. Arrows, optic nerve sheaths. TABLE 1. Optic nerve diameters ( in millimeters) Standardized A- scan measurements ( in both primary and 30° gaze) of optic nerve diameters before and after lumbar puncture ( LP). dynamic nature and ability to prove the presence of increased subarachnoid fluid in the optic nerve sheath. This is accomplished by performance of the 30° test; a positive test occurs when there is a ~ 10% reduction of optic nerve diameter upon remeasurement in lateral gaze. Increased subarachnoid fluid may be seen in a variety of conditions such as increased intracranial pressure, optic neuritis, arachnoid cyst of the optic nerve, optic nerve trauma, and anterior to an orbital mass or one located within the cavernous sinus. By introducing rubber balloons into the subarachnoid space of rhesus monkeys, Hayreh ( 4) has studied the pathogenesis of optic disc edema secondary to increased intracranial pressure. He was able to establish the presence of a constant but variable communication between the subarachnoid space of the optic nerve sheath and the intracranial cavity. This trabecular meshwork was most narrow and variable near the optic canal, Prior to LP Following LP 4.6 2.6 00. 3.1 2.6 4.1 2.8 O. S. 3.1 2.8 making this area critical in the transmission of increased cerebrospinal fluid pressure and probably responsible for the asymmetry of papilledema observed in certain cases. Surgical decompression of the optic nerve sheath has rarely lowered lumbar spinal fluid pressure or relieved papilledema of the contralateral eye ( 4,5). Davidson ( 6) postulated that the unilateral resolution of papilledema following optic nerve decompression was the result of fibroblast proliferation and local obliteration of the subarachnoid space. This local mechanism is unlikely to explain the resolution of optic disc swelling seen with acetazolamide therapy, lumbar peritoneal shunting, and serial lumbar punctures. Our patient showed a remarkable reduction in subarachnoid fluid volume corresponding with a significant decrease in cerebrospinal fluid pressure. We have established in this patient that, with a cerebrospinal fluid pressure of 320 mm Hg, the size of the anterior optic nerve sheaths was 4.6 mm on the right and 4.1 mm of the left with compressible fluid in eccentric gaze. When the pressure was lowered to 110 mm Hg, optic nerve sheath thickness decreased to 2.6 mm on the right and 2.8 mm on the left without compressible fluid. These findings raise the possibility of a linear relationship between optic nerve width and cerebrospinal fluid pressure. Alternatively, there may be a correlation between the amount of compressible fluid in the JClill Neuro- ophthalmol, Vol. 9, No. 2, 1989 82 S. CALETTA ET AL. FIG. 4. Contact B- scans of left optic nerve obtained just prior to lumbar puncture ( top) and immediately following procedure ( bottom). Note much small diameter of optic nerve pattern following the procedure. Arrows, optic nerve. sheath and the spinal fluid pressure. In this regard, we have observed several pseudotumor cerebri patients treated with acetazolamide ( 7) that show a progressive decline in sheath fluid when examined with repeat echography. The reduction of the cerebrospinal fluid pressure transmitted into the optic nerve sheath may be the most common and basic mechanism by which papilledema is relieved. Further research will be necessary to determine whether echography of the optic nerve can be used as a noninvasive method of estimating cerebrospinal fluid pressure or monitoring the effects of therapy. Perhaps a graph of cerebrospinal fluid pressure versus optic nerve sheath size can be established in the future. REFERENCES 1. Ossoinig KC, Cennamo G, Byrne SF. Echographic differential diagnosis of optic nerve lesions. Doc Ophthalmol Proc Ser 1981; 29: 327- 32. 2. Byrne SF. The echographic measurement and differential diagnosis of optic nerve lesions ( review). In: Ossoinig KC, ed. Ophthalmic echography. The Netherlands: Dr. Junk, 1987; 571- 85. 3. Gans MS, Byrne SF, Glaser JS. Standardized A- scan echography in optic nerve disease. Arch Ophthalmol 1987; 105: I232~. 4. Hayreh 55. Pathogenesis of edema of the optic disc. Doc Ophthalmol 1968; 24: 289- 411. 5. Keltner J, Albert OM, Lubow M, et a!. Optic nerve decompression. Arch OphthalmoI1977; 95: 97- 104. 6. Davidson 51. The surgical relief of papilledema. In: Cant JS, ed. The optic nerve. London: Henry Kimpton, 1982: 174- 9. 7. Tomsak RL, Niffenegger AS, Remler BF. Treatment of pseudotumor cerebri with Diamox ( acetazolamide). JClin Ncuro- ophthalmo[ 1988; 8: 93- 8. |
