Compression From Above: A Case Series of Compressive Optic Neuropathy Secondary to Idiopathic Gyrus Rectus Herniation

Clinical Correspondence Section Editors: Robert Avery, DO Karl C. Golnik, MD Caroline Froment, MD, PhD An-Guor Wang, MD Compression From Above: A Case Series of Compressive Optic Neuropathy Secondary to Idiopathic Gyrus Rectus Herniation Seema Emami, MD, Laila Al-Shafai, MD, Edward Margolin, MD W e report 2 unique cases of compressive optic neuropathy caused by herniation of normal frontal lobe tissue onto the prechiasmatic optic nerve, highlighting the potential of ectopic normal tissue to produce vision loss. CASE 1 A 69-year-old man reported 1 year of progressively blurred vision in his left eye (LE). Visual acuity was 20/ 30 and 20/25 with brisk left relative afferent pupillary defect (RAPD). There was obvious left optic nerve pallor on ophthalmoscopy. Peripapillary ocular coherence tomography (OCT) was normal on the right eye but demonstrated diffuse thinning of the peripapillary neuroretinal fiber layer (RNFL) in LE with corresponding diffuse thinning of macular ganglion cell complex. Humphrey visual field (24-2 algorithm) was severely constricted on the left eye. Contrast-enhanced MRI of the brain demonstrated inferior herniation of the left gyrus rectus with resultant compression of the left prechiasmal optic nerve (Fig. 1). The left optic nerve demonstrated reduced caliber without abnormal signal or enhancement. The brain parenchyma and meninges were radiographically normal, and follow-up neuroimaging 1 year later was unchanged. CASE 2 A 55-year-old woman complained of worsening blurry vision in LE over past year. She had a history of pituitary microadenoma treated with cabergoline and dyslipidemia. Three years ago, she was found to have an abnormal visual field in the LE on routine optometric examination and eventually diagnosed with probable nonarteritic ischemic Departments of Ophthalmology and Vision Sciences (SE, EM) and Medical Imaging (LA-S), University of Toronto, Toronto, Canada; and Department of Medicine, Division of Neurology, University of Toronto (EM), Toronto, Canada. The authors report no conflict of interest. Address correspondence to Edward Margolin, MD, Department of Ophthalmology and Department of Medicine, University of Toronto, 801 Eglinton Avenue West Suite 301, Toronto, ON M5N 1E3, Canada; E-mail: [e-mail] e466 optic neuropathy (NAION). Now, visual acuity measured 20/25 in each eye with subtle left RAPD. Ophthalmoscopy showed pallor of the left optic nerve (Fig. 1B). Ancillary testing demonstrated incongruous temporal defect with superimposed inferior nerve fiber bundle defect in the LE with few depressed spots temporally in the right eye (Humphrey visual field 24-2 algorithm; Fig. 1D), with corresponding thinning of peripapillary RNFL and macular ganglion cell complex on OCT in the LE. Contrastenhanced brain MRI demonstrated low-lying left gyrus rectus causing mass effect on prechiasmal left optic nerve associated with diffuse left optic nerve atrophy and focal atrophy on the left side of optic chiasm (Fig. 2). Both frontal lobes demonstrated normal radiographic characteristics, and small pituitary microadenoma was stable without mass effect on adjacent structures. DISCUSSION Compressive optic neuropathies are characterized by mass effect by lesions on the anterior visual pathway. The gyrus rectus forms the inferomedial aspect of the frontal lobe and is separated from the anterior optic chiasm by the chiasmal cistern. There are very few reports of gyrus rectus–related optic neuropathy, all demonstrating important differences from our cases. Smith et al described a 51-year-old woman with transient unilateral blindness associated with retinal vein engorgement, arteriolar attenuation, and loss of pupillary light reflex during one observed episode. Her symptoms were attributed to compression of the optic nerve by ipsilateral low-lying gyrus rectus and resolved after surgical resection of gyrus rectus (1). However, transient nature of visual loss with alterations of inner retinal blood flow suggested possible episodic compression of ophthalmic artery and its distal branches as a cause of symptoms in this case. In contrast, our patients reported subacute vision loss with optic nerve head pallor, typical of compressive optic neuropathy. More recently, Gilani et al (2) reported a man with previously unexplained unilateral blindness since birth. Neuroimaging revealed inferior herniation of the ipsilateral gyrus rectus compressing the optic nerve, which was inferred to be the Emami et al: J Neuro-Ophthalmol 2022; 42: e466-e468 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Clinical Correspondence FIG. 1. T2 coronal fat-saturated sequence. A. At the level of intracanalicular segments of the optic nerves demonstrating high signal on left side (short white arrow) indicating abnormal nerve without enhancement (not shown). B. At the level of prechiasmatic optic nerves demonstrating atrophy of the left nerve (long black arrow) and abutting it superiorly is left gyrus rectus (long white arrow) producing mass effect on the nerve. C. At the level of optic chiasm demonstrating atrophy of the left nerve (short black arrow), which is abutted superiorly by left gyrus rectus (long white arrow) also producing mass effect on chiasm. The gyrus rectus had no focal or diffuse signal abnormality or abnormal enhancement (not shown). cause of visual loss. In our series, visual loss occurred in adulthood, suggesting an active process of tissue herniation, whereas the case of congenital vision loss may represent abnormal brain development or childhood insult. Finally, a Japanese case series suggested an association between gyrus rectus compression and vision loss (3). However, optic neuropathy in this series was imprecisely defined as optic disc atrophy accompanied by a visual field defect, which is difficult to differentiate from glaucoma, particularly given that bilateral involvement occurred in 73% of patients. Moreover, the authors reported optic nerve compression by the gyrus rectus in one-third of control patients, a finding that has not been replicated elsewhere in the literature (3). Conclusions from this study are further limited by the use of older MRI protocols with reduced resolution, precluding reliable analysis of signal abnormalities within the optic pathway or brain parenchyma. The mechanism of idiopathic gyrus rectus herniation is not understood likely owing to the rarity of this condition. Indeed, its incidence is unknown in the general population. Idiopathic herniation of other brain structures, including the cuneate gyrus and middle ear meningoencephaloceles, have been described and are hypothesized to arise from defects in surrounding dura mater and thin temporal bone, respectively (4,5). Drawing from this literature, we hypothesize that focal defects in the arachnoid matter enveloping the ventral gyrus rectus surface may allow inferior herniation of otherwise normal brain tissue. Pathology review of herniated tissue and their surrounding meninges may permit more definitive understanding. In our series, gradual visual loss, optic disc pallor, and constricted visual fields were in keeping with compressive optic neuropathy due to herniated gyrus rectus tissue pressing on prechiasmatic optic nerve. The degree of compression was not trivial because it has produced noticeable degree of optic nerve atrophy on neuroimaging. The differential diagnosis for patients with slowly progressive visual loss is not long, and compressive optic neuropathy tops the list, although infiltrative, inflammatory, infectious, hereditary, and glaucomatous optic neuropathy were considered in both patients. Both patients were well, did not have optic nerve cupping characteristic of glaucoma, and instead demonstrated pallor of the neuroretinal rim; visual loss was insidious and unilateral, making hereditary causes exceedingly unlikely, optic nerve did not demonstrate enhancement on imaging, and testing for myelin-oligodendrocyte and neuromyelitis optica antibodies was negative in both patients. FIG. 2. A. T2 coronal fat-saturated sequence at the level of posterior intraorbital optic nerves demonstrating high signal and atrophy of left nerve (short white arrow) indicating chronic optic neuropathy. B. T1 coronal sequence at the level of prechiasmatic optic nerves demonstrating subtle atrophy of the left optic nerve (long white dotted arrow) and abutting it superiorly is left gyrus rectus (long white arrow) with mass effect on the nerve. C. T1 coronal sequence at the level of optic chiasm demonstrating slight atrophy of the left optic nerve (short white dotted arrow) and abutting it superiorly is left gyrus rectus (long white arrow) exerting mass effect on chiasm. The gyrus rectus had no focal or diffuse signal abnormality or abnormal enhancement (not shown). Emami et al: J Neuro-Ophthalmol 2022; 42: e466-e468 e467 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Clinical Correspondence In case 1, compression affected prechiasmatic optic nerve resulting in unilateral vision loss. In case 2, the initial misdiagnosis of NAION likely represented acute onset of compressive optic neuropathy because visual fields at presentation demonstrated temporal visual field defect likely secondary to chiasmal compression by gyrus rectus. We elected to observe both patients given chronic visual field loss, presence of optic nerve pallor suggesting axonal atrophy, and preserved central visual acuity. Although surgical decompression of compressive optic neuropathy secondary to prolapsing gyrus rectus has been reported (1,3), the risk of surgical intervention in both patients who had relatively preserved central visual acuity would have been much higher than its potential benefits. This series adds to the limited literature on compressive optic neuropathy caused by herniation of normal gyrus rectus tissue. We emphasize the importance of close neuroradiologic examination of the entire anterior visual pathway with attention to the prechiasmatic optic nerve and optic chiasm, particularly in cases of unexplained optic neuropathy. Early identification of occult compressive optic neuropathy may allow timely intervention and prevention of vision loss. e468 STATEMENT OF AUTHORSHIP Category 1: a. Conception and design: E. Margolin, S. Emami, and L. AlShafai; b. Acquisition of data: E. Margolin, S. Emami, and L. AlShafai; c. Analysis and interpretation of data: E. Margolin, S. Emami, and L. AlShafai. Category 2: a. Drafting the manuscript: E. Margolin, S. Emami, and L. AlShafai; b. Revising it for intellectual content: E. Margolin, S. Emami, and L. AlShafai. Category 3: a. Final approval of the completed manuscript: E. Margolin, S. Emami, and L. AlShafai. REFERENCES 1. Smith J, Jack MM, Peterson JC, Chamoun RB. Herniated gyrus rectus causing idiopathic compression of the optic chiasm. Clin Neurol Neurosurg. 2017;153:79–81. 2. Gilani K, Maralani PJ, Sundaram AN. Congenital herniation of the gyrus rectus resulting in compressive optic neuropathy. Can J Neurol Sci. 2020;6:1–2. 3. Nishioka T, Okumura R, Ishikawa M, Kondo A, Masai H, Ueki M. Prolapsing gyrus rectus as a cause of progressive optic neuropathy. Neurol Med Chir (Tokyo). 2000;40:301–309. 4. Duarte MP, Maldjian TC, Tenner M, Adam R. Magnetic resonance imaging of idiopathic herniation of the cuneus gyrus. J Neuroimag. 2007;17:353–354. 5. Heffner DK. Brain in the middle ear or nasal cavity: heterotopia or encephalocele? Ann Diagn Pathol. 2004;8:252–257. Emami et al: J Neuro-Ophthalmol 2022; 42: e466-e468 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited.