ABSTRACT

A 57-year-old man, who had developed bilateral symmetric optic atrophy and cupping due to methanol intoxication nine years ago, noticed slowly progressive visual impairment in the left eye (OS). In ophthalmic examination, visual acuity was 20/60 in the right eye (OD) and ‘hand motion’ in OS. Intraocular pressure was 20 in OD and 28mmHg in OS. Anterior segment biomicroscopy showed bilateral pseudoexfoliation, more intense in OS. Ophthalmoscopy revealed bilaterally deep and wide cupping with cup-to-disc ratios of 8/10 in OD and approximately 10/10 in OS. Based on these findings, the diagnosis of left pseudoexfoliative glaucoma, in addition to bilateral toxic optic atrophy and cupping, was made. Pathological optic-disc cupping may be caused by many conditions other than glaucoma. In these cases, glaucoma may aggravate the both cupping and atrophy. (Turk J Ophthalmol 2011; 41: 256-9)

Introduction

Optic-disc cupping is most often caused by glaucoma, but may be seen in many infrequent neuro-ophthalmic and oncologic pathologies that are not associated with elevated intraocular pressure (IOP), such as congenital optic-disc anomalies (e.g. optic-disc coloboma, pits, hypoplasia, optic nerve head drusen and tilted disc), optic neuropathies (e.g. ischemic,  traumatic, hereditary and toxic) and compressing orbital, cerebral or optic nerve tumors.1-5 Although seldom seen, methanol poisoning must be also considered in the group of toxic optic neuropathies causing non-glaucomatous optic-disc cupping with optic atrophy.6-8

In this article, we  present a patient with toxic optic atrophy and cupping caused by methanol poisoning and pseudoexfoliative glaucoma which was added to this pathology.

Case Report

A 57-year-old man who had been diagnosed with bilateral advanced glaucoma was referred for glaucoma consultation. He noticed increasing visual impairment in his left eye over several months. In ophthalmic examination, visual acuities were 20/60 in the right (OD) and ‘hand motion’ in the left eye (OS). Pupillary light reflex was sluggish in both eyes and there was a relative afferent pupillary defect (RAPD ) in OS. Anterior segment biomicroscopy showed bilateral pseudoexfoliation on the anterior lens surface and pupillary margin, more intense in OS. IOPs measured by Goldmann applanation tonometry were 20mmHg in OD and 28 mmHg in OS. Central corneal thickness was 534 µm in OD and 542 µm in OS. On funduscopy, there was bilateral striking cupping with cup-to-disc ratios of 8/10 in OD and approximately 10/10 in OS. The neuroretinal rim was pale and the brightness of the retinal nerve fiber layer (RNFL) was absent in both eyes (Figure 1).

Formal automated visual fields (Humphrey SITA-Standart 30-2) demonstrated total visual field defect in both eyes. So, a central 10-2 perimetry was performed using Humphrey Matrix FDT (Zeiss, Dublin, CA, USA). This test revealed diffuse visual field loss, severely in OS (Figure 2). In the optic disc topography obtained by the Heidelberg Retinal Tomograph (HRT II, Heidelberg Engineering GmbH, Germany), bilateral advanced neuroretinal rim damage was detected and the patient was diagnosed with bilateral
glaucoma according to the Moorfields classification (Figure 3). The mean peripapillary RNFL thickness measured by a spectral optic coherence tomography device (spectral OCT/SLO, OTI Inc., Toronto, Canada) was 64 µm in OD and 39µm in OS. The physiological ''double-hump'' appearance of the peripapillary RNFL was deteriorated bilaterally (Figure 4).

Despite these findings which may be associated with bilateral advanced glaucoma, we thought that the cupping, especially in OD, was not glaucomatous, based on the signs such as the pallor of rims, horizontal elongation of cup-disc ratio, RAPD and severely impaired visual acuity. Detailed investigation revealed that the patient had methanol intoxication caused by counterfeit raki (a Turkish alcoholic drink) consumption nine years ago.  According to patient’s records of examination performed 8 months after the intoxication, he had bilateral symmetric optic atrophy and cupping:  the C/D ratio was 0.8, IOP 12 mmHg and his visual acuity was 20/60 bilaterally. Based on these findings, the patient was diagnosed with bilateral optic atrophy and cupping due to methanol intoxication, and pseudoexfoliative glaucoma in OS.

Discussion

Methyl alcohol is used as a solvent-indicator and a reactant in ethyl alcohol denaturizing processes in chemical industry. The substances containing methyl alcohol include adhesives, paint thinners, glass cleaner fluids, copier fluids, carburetor fluids and antifreezes. Because of its cheaper cost in our country, it is used in the production of counterfeit raki, and methyl alcohol poisonings often arise with oral intake of them. Its chemical structure, odor and taste are similar to ethyl alcohol. Not methanol itself but the metabolites like formaldehyde and formic acid have toxic effects in the body.9 Formic acid causes cell death by inhibiting the cytochrome oxidase complex in the mitochondrial respiratory chain.10

In animal studies, cell damage due to methyl alcohol intoxication has been shown in the white matter of the brain and in the optic nerve axons at the optic disc level.11,12  In the acute period, fundus examination shows an optic disc hyperemia similar to the papilloedema and sometimes peripapillary retinal edema. While fundus findings regress in a two-month process, a permanent optic atrophy appears, therefore, the visual prognosis is poor.13 On the other hand, necrosis in the putamen and subcortical areas of the central nervous system has been reported.14  It has been considered that methanol damage happens both in the optic disc and in the section of the optic nerve immediately behind the lamina cribrosa, due to high arterial flow. Retrograde retinal ganglion cell damage occurs because of axonal necrosis.9,11,12

In the literature, there are only three publications about the optic disc cupping caused by methanol intoxication. In the first of them,  Benton and Calhoun reported that optic disc cupping similar to glaucoma could also be seen rarely with optic atrophy in their large series of methanol intoxication.15  Stelmach and O'day presented a young female patient with methanol poisoning owing to counterfeit alcohol intake and observed that symmetric disc cupping could also occur with optic atrophy and some visual functions could be maintained by treatment.16  Also, Sharma et al.6 reported a 31-year-old male patient with methanol intoxication because of counterfeit alcohol consumption and noted that his visual acuity was decreased to light perception, and then a symmetric and deep cupping with optic atrophy emerged after a period of  8 months. In these studies cited above, there is no any theory about the mechanism by which the optic disc cupping occurs following the optic atrophy. Sharma et al. briefly connected this advanced cupping to the acute and excessive loss of ganglion cells.6 But most of the patients with methanol poisoning already have acute and excessive loss of ganglion cells; however, disc cupping is seen rarely. Consequently, it is not possible to explain the mechanism of the cupping with this theory.

In conclusion, although optic-disc cupping is one of the essential findings in glaucoma, it may also be seen in many less-common neuro-ophthalmic and oncological pathologies that are not associated with elevated intraocular pressure. So, we have to consider the distinctive signs of glaucomatous cupping in the patient with optic-disc cupping. Furthermore, the patients with toxic optic atrophy and non-glaucomatous cupping should be enrolled to a periodic follow-up program if they have the risk factors such as pseudoexfoliation, family history of glaucoma, age, etc.,  because they can completely lose their retinal ganglion cells, which have already been damaged in small number due to insidiously developing glaucoma, as seen in our case.