Journal of Neuro-Ophthalmology, September 1995, Volume 15, Issue 3

Optic Nerve Compression from a Basal Encephalocele

A woman developed headaches, transient visual obscurations, anosmia, and decreased visual acuity. Ocular examination showed bilateral pulsatile proptosis and disc edema with choroidal folds. Standardized ophthalmic echography showed absence of bony orbital roofs, prominent dural pulsations, direct apposition of brain parenchyma and orbital tissues, and echographic signs suggesting bilateral optic nerve compression. CT and MRI showed a large defect in the floor of the anterior cranial fossa. The cribriform plate, both orbital roofs, and sphenoid bones were displaced by a large basal encephalocele. Clinical improvement followed reconstruction of the anterior cranial fossa and decompression of both optic nerves.

journal of Neuro- Oyhthatmohsy 150): 161- 165, 1995. €> 1995 Li ppincotl- Raven Publishers, Philadelphia Optic Nerve Compression from a Basal Encephalocele Gerard L. Hershewe, D. O., James J. Corbet!, M. D., Karl C. Ossoinig, M. D., and H. Stanley Thompson, M. D. A woman developed headaches, transient visual obscu­rations, anosmia, and decreased visual acuity. Ocular examination showed bilateral pulsatile proptosis and disc edema with choroidal folds. Standardized ophthal­mic echography showed absence of bony orbital roofs, prominent dura! pulsations, direct apposition of brain parenchyma and orbital tissues, and echographic signs suggesting bilateral optic nerve compression. CT and MRI showed a large defect in the floor of the anterior cranial fossa. The cribriform plate, both orbital roofs, and sphenoid bones were displaced by a large basal en­cephalocele. Clinical improvement followed reconstruc­tion of the anterior cranial fossa and decompression of both optic nerves. Frontal basal encephaJoceles are congenital mal­formations in which brain parenchyma herniates through a bony defect in the cribriform plate and sphenoid bone. These disorders may be associated with midline facial defects and anomalous optic discs ( 1). Basal encephaloceles may produce uni­lateral or bilateral proptosis and visual loss due to compressive optic neuropathy ( 2). We report a case with bilateral pulsatile proptosis and com­pressive optic neuropathy. Proptosis and visual acuity improved following reconstructive surgery. Manuscript received January 12, 1995. Prom the Division of Neurology and Neuro- Ophthalmology ( G. L. H.), Department of Medicine, University of Nevada School of Medicine, Reno, Nevada; Department of Neurology ( J. J. C.), University of Mississippi Medical Center, Jackson, Mis­sissippi; and Echography and Neuro- Ophthalmology Services ( K. C. O., H. S. T.), Department of Ophthalmology, University of Iowa Hospitals and Clinics, Iowa City, Iowa, U. S. A. Address correspondence and reprint requests to Dr. Gerard L. Hershewe, University of Nevada School of Medicine, Neuro-ophthalmology Clinic, 350 West 6th St., Reno, NV 89503, U. S. A. CASE REPORT A 35- year- old right- handed woman had a year­long history of intermittent headaches which she attributed to " sinus congestion." The constant dis­comfort across her forehead was aggravated by bending over. Her nose felt " plugged up," espe­cially in the mornings. She had brief, transient monocular visual obscurations in her right eye last­ing only seconds. On one occasion after she had been lying down, she stood up abruptly and be­came completely blind for about 10 s. During the preceding year, she had noticed intermittent bulg­ing of her eyes. During the last two months she had developed progressive proptosis associated with worsening vision. She felt that she had lost her sense of smell, but she insisted that her ability to taste her food was unaffected. Examination During the initial examination, the patient's vi­sual acuity was 20/ 40 OD and 20/ 30 OS. Color vi­sion was only slightly impaired ( 9/ 14 AO color plates in each eye). Foveal flicker fusion was defi­nitely impaired ( RE 18 Hz, LE 20 Hz; the normal 161 162 G. L. HERSHEWE ET AL. range with this equipment is 27- 32 Hz). The pupils were equal in size and reacted well to light. There was a small ( 0.3 log unit) relative afferent pupillary defect OD. Fundus examination showed moderate right disc swelling with choroidal folds, and slight left disc swelling with choroidal folds. There was bilateral pulsatile exophthalmos ( Fig. 1) with Her-tel readings of 24 mm OD and 25 mm OS ( base 100). Horizontal versions were mildly restricted in both directions. Vertical gaze was normal. Gold-mann visual fields showed bilateral superior arcu­ate scotomas with enlargement of the blind spot. Speech was fluent, and insight, judgment, and comprehension were all normal. She was unable to recognize the odor of oil of cloves or of pepper­mint. The remainder of the neurological examina­tion was normal. Axial and coronal Tl- weighted MRI showed downward herniation of brain parenchyma through the base of the skull into both orbits ( Fig. 2). The CT scan showed absence of the cribriform plate and superior orbital roof and only remnants of the lesser wing of the sphenoid. There was her­niation of brain parenchyma through the floor of the anterior cranial fossa with lateral displacement of both optic nerves ( Fig. 3). Standardized A- scan and B- scan echography ( 3,4) showed the absence of the bony roof in both orbits, mostly between 10: 00 and 2: 00 o'clock OD, and from 10: 00 to 1: 30 o'clock OS ( Fig. 4). Brain and dura were herniated through the bony defects and were lying on top of the thinned levator and superior rectus muscles, below the level of the su­perior orbital rims. All these structures had prom­inent pulse- synchronous movements visible in real- time echography. Both optic nerves were sur­rounded by excessive subarachnoid fluid in their most anterior portions ( Fig. 5). Alternating right FIG. 1. The patient was proptotic but did not have any midline facial anomalies suggestive of an encephalo-cele such as hypertelorism or a median cleft lip. FIG. 2. This T1- weighted axial MRI scan shows the large midline frontal encephalocele displacing both orbits laterally. and left prolonged maximal horizontal gaze ( so-called " optic nerve exercise") was kept up for at least 3 min ( 4), and during this time the sheath distension decreased only minimally. Normally, all the subarachnoid fluid surrounding the optic nerves is expressed from subarachnoid space dur­ing this maneuver. The fact that no fluid could be expressed from the optic nerve sheaths is an echo-graphic sign suggesting severe optic nerve com­pression ( 4), A bifrontal craniotomy disclosed that both or­bital roofs were missing. The sphenoid wing was completely absent on the right side and a small FIG. 3. This coronal CT image shows the downward herniation of brain tissue into both orbits and ethmoi­dal sinuses. / Neiii- o- OfilillsiiIiimi Vol. 15. No. ,1, 1995 OPTIC NERVE COMPRESSION 163 FIG. 4. Standardized echograms from the right superior orbit and the overlying brain of the patient. Top left: A- scan echogram obtatned by placing the probe on the right globe behind the limbus at 8: 00 and by aiming the sound beam superiorly into the brain tissues through the globe and the adjacent thin, high reflective, superior orbital soft tissues ( thinned superior rectus and levator muscles, left arrow); the brain tissues have irregular acoustic structure with low reflectivity: low spikes from the brain matter and single higher spikes from the large surfaces and vessels ( between the two arrows). Bottom left: A- scan echogram obtained by placing the probe on the upper lid at 12: 00 and directing the beam through the upper lid and adjacent dura ( high spikes on the left) into brain tissue ( irregularly distributed, mostly low spikes throughout the rest of the echogram). Top right: B- scan echogram obtained by placing the probe on the globe across the 6: 00 meridian behind the limbus and by aiming a " transverse" acoustic section ( transverse to the 12: 00 meridian) steeply up into the brain through the globe and superior orbit. The upper end of this B- scan echogram corresponds to the nasal end of this acoustic section. The arrows indicate the curved lines representing the inferior gyrate surfaces of the brain, which are separated from the dura by nonreflective subarachnoid fluid. The weak signals at the far right of the echogram are artifacts ( reverberation signals). Bottom right: Longitudinal B- scan echogram obtained in a direction perpendicular to the top B- scan echogram by aligning the acoustic section along the 12: 00 meridian from anteriorly ( upper end of echogram) to posteriorly. remnant of the sphenoid wing remained on the left. There was a herniation of the gyrus rectus through a large defect in the cribriform plate. Dexon mesh filled with methylacrylic was used to make a new floor for the anterior cranial fossa to keep the frontal lobes out of the orbits. Following surgery, the patient's headaches and puisatile pro-ptosis improved. DISCUSSION An encephalocele is a herniation of brain tissue through a defect in the bony coverings of the skull. / Neuro- Ofihlhithinil, Vol. 15, No. 3, 1995 164 G. L. HERSHEWE ET AL. ORBIT G= 65.& IB F M 0- 20148 RCN 3d RRACH I NfW13> 90J 1 Vel.= J55& r/ s 1 J i d 0131= 7.07 im ul_ FIG. 5. Standardized echograms showing the images of the right optic nerve and its sheaths, which are distended with subarachnoid fluid. Top left: A- scan measurement of the maximum arachnoidal diameter ( distance between the two arrows corresponding to 8,12 mm) with the patient looking straight ahead. Arrows point to electronic gates, which were placed over the peaks of the temporal ( left in the echogram) and nasal ( right in the echogram) arachnoidal spikes [ corresponding to thearachnoid/ ftuid interface ( i. e., the inner dural surface)]. These spikes had been maximized in height, indicating perpendicular sound beam incidence at these interfaces. Note the steep descent and ascent of these signals, indicating the smoothness of the arachnoidal surface. The low spikes between the arrows come from the optic nerve, which, in this section, was not reached by a perpendicular beam. Bottom left: A- scan measurement of the maximum arachnoidal diameter ( 7.07 mm) during maximal abduction of the right eye ( 30 degree test) showing only a minor decrease from the straight ahead measurement ( presumably because the fluid was distributed over a longer distance within the stretched optic nerve sheaths). Right: Axial B- scan echograms obtained in two planes perpendicular to each other ( transverse on top and longitudinal at bottom). The optic discs are visibly elevated due to the papilledema. The increased " sheathing sign," the sepa­ration of optic nerve ( piai) and sheath ( arachnoidal) surfaces by the nonreflective fluid, is clearly noticeable. The bottom B- scan shows the " flying bat" configuration typical of such a condition: The head of the bat is represented by the elevated disc; the two wings correspond with the fluid- filled spaces beneath the arachnoidal/ dural sheaths, which are more distended in their anterior portion as seen above and below the optic nerve pattern in this echogram. The term cranial meningocele is reserved for the forms in which the herniated sac contains only the meninges and cerebrospinal fluid ( 5). The incidence of encephaloceles varies from 0.3 to 0.7 cases/ 1,000 births and 16% are familial ( 6). According to the location of the bony delect, en­cephaloceles are broadly classified as calvarial, frontal, and basal ( 7). Basal encephaloceles have / Nenro- Oiilillmhwl, Vol. 15, No. 3. 1935 OPTIC NERVE COMPRESS/ ON 165 been further subdivided into the following groups: transphenoidal, transethmoidal, sphenoethmoi­dal, frontosphenoidal, and sphenoorbital ( 8). In children, a basal herniation is not visible ex­ternally and may remain clinically undetected within the nasal cavity, the epipharynx, or the or­bit. A soft tissue mass present in the nasal cavity can be misinterpreted as a nasal polyp ( 9). The herniated mass may enlarge and become symp­tomatic with crying, jugular compression, or a Val­salva maneuver. In adults, encephaloceles may be discovered during an investigation for hypothalamic dysfunc­tion ( diabetes insipidus, hypogonadism) or visual disturbance ( optic atrophy, bitemporal hemiano-pia) ( 10,11). Bilateral pulsatile proptosis may result if the herniating mass extends into the orbit. The differential diagnosis of pusatile exophthalmos in­cludes neurofibromatososis ( 12), acquired carotid-cavernous fistula, congenital arteriovenous malfor­mations, venous varicocele, encephalocele ( 13), and trauma ( 14). Orbital encephaloceles frequently cause unilateral exophthalmos ( 15,16). Various optic nerve abnormalities have been re­ported associated with basal encephaloceles, in­cluding pale discs, colobomas of the nerve head, optic disc pits ( 17), morning glory disc ( 18), and megalopapilla and other disc anomalies ( 19). The presence of such optic nerve anomalies, coupled with midline facial defects such as ocular hypertel­orism and median cleft lip, should alert the clini­cian to suspect the presence of a basal encephalo­cele ( 11). CT will define the bony abnormalities, although MRI provides more detailed assessment of associ­ated brain anomalies, aberrant vascular structures, and occipital encephaloceles ( 20,21). In this case, standardized echography ( 3,4), besides showing the missing orbital roof and the real- time pulsa­tions of the dura and adjacent tissues, helped to underline the optic nerve compression. Unilateral visual loss due to optic nerve com­pression without associated midline facial defects and without pulsatile proptosis has been reported in one case ( 2). The patient, a 23- year- old male, had sudden loss of vision OD, a soft tissue density in the right ethmoidal sinus, and a bony defect in the cribriform plate that was surgically repaired. Following surgery, the visual acuity recovered from 20/ 70 to 20/ 30 and the visual field improved. Clinical suspicion of a basal encephalocele may be confirmed by a variety of imaging techniques, and these images are an essential part of a preoperative evaluation. Acknowledgments: Wc would like to thank Dr, Bc-hrouz Rassekh for providing the clinical details of this patient's operative and postoperative course. REFERENCES 1. Suwanwela C, Hongsaprabhas C. Fronto- Ethmoidal En-ceplialomeiiingocele. J Nenrosing 1966; 25; 172- 82. 2. Monica M, Connolly E, Kali! R. Basal encephalocele pre­senting with monocular visual loss. / Clin Nairu- ophfhatmot 1934; 4: 9- 13. 3. Ossoinig KC Standardized echography; Basic principles, clinical applications and results. Ophthalmic ultrasonography: compara­tive techniques, international ophthalmology clinics. Boston; Lit­tle, Brown, 1979: 127- 210, 4. Ossoinig KC. Standardized echography of the optic nerve ( 1st SIDUO Jules Francois Memorial Lecture). Doc Ophthal­mol Proc Set- 1993; 5: 3- 99. 5. Okazaki H. Fundamentals of neuropathology. 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