When ‘glaucomatous fields’ are not glaucoma: bilateral calcarine fissure strokes masquerading as glaucoma in a normal tension glaucoma suspect

Abstract

A 78-year-old man with vascular risk factors and a family history of glaucoma presents with bilateral superior arcuate visual field loss. MRI brain was reported normal. Intraocular pressure (IOP) and optical coherence tomography of the retinal nerve fibre layer (RNFL) were within normal limits. A tentative diagnosis of normal tension glaucoma was made. Over the next 5 years, IOP remained stable without treatment, serial visual fields noted repeatable bilateral superior depressions with normal RNFL. Referral to a glaucoma subspecialist and subsequently neuro-ophthalmologist prompted repeat MRIs, which demonstrated mild small vessel ischaemia. Standard visual evoked potentials (VEPs) were normal. Multifocal VEPs identified poor response across the entire visual field in both eyes. The combination of visual defects, unremarkable RNFL and reduced multifocal VEPs raised suspicion of bilateral inferior calcarine fissure change. Retrospective review of MRI’s in a multidisciplinary meeting confirmed extensive microvascular changes with bilateral inferior calcarine fissure ischaemia.

Background

We report the clinical course of a patient with bilateral calcarine fissure strokes, initially managed as normal tension glaucoma (NTG) suspect. This case stresses the importance of considering other causes of visual field defects when assessing a patient with ‘typical’ glaucomatous field findings with no corresponding disc vulnerabilities. This case also highlights the importance of the need for a multidisciplinary approach both clinically and radiologically in patients with clinical inconsistencies.

Case presentation

A 78-year-old Caucasian man was referred for ongoing management of NTG. The diagnosis of NTG was initially made by the referring ophthalmologist on the basis of visual fields noting bilateral superior arcuate visual field loss (figure 1). MRI brain (figure 2) reported normal and a positive family history of primary open angle glaucoma (POAG) (patient’s mother, who was treated with a single-agent glaucoma medication). Intraocular pressure (IOP) untreated and optical coherence tomography (OCT) of the retinal nerve fibre layer (RNFL) were within normal limits.

Figure 1.

Visual fields at the initial referral of the right (top) and left (bottom) eyes.

Figure 2.

MRI brain T2-axial (initial MRI at presentation).

At the time of referral, the patient reported poor vision and balance that had been unchanged for many years. There were no associated memory difficulties, positive visual phenomena or difficulties with spatial perception or facial recognition.

Past medical history was significant for a number of vascular risk factors including hypertension, hypercholesterolaemia, paroxysmal atrial fibrillation and previous coronary artery bypass grafting. He was warfarinised (target INR 2.5–3.5) for a metallic mitral valve replacement 8 years earlier. His INR control was excellent, and readings had been therapeutic (range 2.6–3.8) for all recorded pathology in the previous 3 years. He had also undergone transurethral prostatic resection for prostate cancer and had been diagnosed with a large vessel vasculitis that was managed with prednisolone and methotrexate. He remained on low-dose prednisolone (5 mg daily) as prophylaxis. The patient’s other medications included carvedilol, ramipril, digoxin, frusemide, atorvastatin and supplements of calcium, vitamin D, potassium, magnesium and folate.

Examination revealed 6/5–1 corrected vision bilaterally and normal IOPs in both eyes (OD 9 and OS 11). There was no relative afferent pupil defect. Optic discs were both small with no pallor and a cup–disc ratio of 0.5 without significant notching (figure 3). Fundus examination was otherwise unremarkable. Cranial nerve examination was normal.

Figure 3.

Optic disc photographs of the patient’s right (top) and left (bottom) eyes.

He had a reduction in colour vision with no congenital colour vision defect pattern on Ishihara testing, but very slow recognition of numbers with 3–4 errors in each eye. Temporal artery pulses were palpable and not tender. Central corneal thicknesses were 576 nm and 574 nm on the right and left respectively.

Investigations

Multiple visual field testing with good reliability indices over the next year demonstrated repeatable bilateral superior depressions (figure 4). Spectral-domain OCT of the RNFL was within normal limits. Repeat MRI, undertaken 5 years after his initial unremarkable imaging, failed to report a cause for the patient’s symptoms. Non-specific high T2 signal foci of white matter in keeping with mild small vessel ischaemia were noted.

Figure 4.

Visual fields 12 months later of the right (top) and left (bottom) eyes.

Treatment

The patient was commenced on brimonidine tartrate/timolol maleate 0.2/0.5% twice daily in his right eye and referred to his general practitioner for a vascular workup.

Outcome and follow-up

There remained a concern that his bilateral altitudinal changes on visual field testing did not match his disc findings and he was referred to a glaucoma specialist who determined that it was unlikely that his field loss was due to glaucoma. A third MRI was organised, which noted chronic small vessel ischaemia.

He was subsequently referred to a neuro-ophthalmologist. Standard visual evoked potentials (VEPs) were normal. The pattern electroretinography was normal. Multifocal VEPs showed a poor response across the entire visual field in both right and left eyes. It was felt that the combination of visual defects, unremarkable nerve findings and reduced multifocal VEPs were more in keeping with bilateral inferior calcarine fissure change.

His neuroimaging was reviewed at a multidisciplinary team meeting in a tertiary hospital with neuroradiologists, ophthalmologists and neurologists present. An addendum to the last MRI confirmed extensive microvascular changes with bilateral inferior calcarine fissure ischaemia, more apparent on the left than the right, and involving the occipital radiations (figure 5). Furthermore, review of previous MRI noted retrospective identification of the bilateral calcarine fissure infarcts, even on initial MRI reviewed on printed film (figure 2). The patient was referred to a stroke specialist for consolidation of his vascular risk factors and ongoing management. Further imaging 2 years later noted the old bilateral infarcts associated with established postinfarction gliosis (figure 6).

Figure 5.

MRI brain T2-axial (5 years after initial presentation) noting bilateral calcarine fissure strokes (arrows) and evidence of gliosis.

Figure 6.

MRI brain T2-axial flair (7 years after initial presentation) demonstrating old bilateral calcarine fissure strokes (arrows) with associated gliosis.

Discussion

Altitudinal field defects describe selective visual field loss either above or below the horizontal meridian. They are commonly associated with intraretinal or optic nerve lesions including retinal hemibranch artery or vein occlusion, anterior ischaemic optic neuropathy and advanced glaucoma.¹ Rarely, patients with bilateral striatal lesions may present with altitudinal homonymous visual field defects.^{2 3}

The primary visual cortex is located within the occipital lobe and is divided into superior and inferior banks by the calcarine fissure. The horizontal meridian is buried along the sulcus separating each bank.⁴ Foveal representation is posterior at the occipital pole while the far peripheral field is located anteriorly on the medial occipital surface.

The occipital cortex has a well-described retinotopic arrangement. Focal destruction of the occipital cortex typically produces highly congruous homonymous hemianopias or quadrantanopias, with or without macular sparing. Severity is dependent on the location and size of the occlusion along with the availability of collateral blood flow. The superior bank of the calcarine fissure receives visual input from the inferior visual field while the inferior bank contains representation of the superior visual field.⁴ Separate vasculatures to the upper and lower banks of the calcarine fissure allow the banks to be involved separately in thromboembolic disease.⁵

A lesion of the lower bank of the calcarine cortex will cause a homonymous contralateral superior quadrantanopia respecting both the horizontal and vertical meridians. Homonymous contralateral inferior quadrantanopias result from lesions of the superior bank.⁵ As was the case with our patient, superior altitudinal visual field defects may thus be caused by bilateral infarction of the inferior calcarine banks. Strokes limited to the striate cortex typically produce isolated visual field deficits without other neurological signs and symptoms.³ Quadrantanopia resulting from an optic radiation lesion is often clinically differentiated from a striatal quadrantanopia by the presence of hemiparesis, neglect, amnesia or dysphasia.⁵

While ischaemia is the most common cause of visual cortex dysfunction, other aetiologies including neoplasm, demyelination, infection, trauma, haemorrhage and neurodegenerative may also be responsible. Right and left striatal cortices are apposed on their medial occipital surface and focal midline lesions may affect both striatal hemispheres concurrently.⁵

Occasionally, other signs of higher cortical dysfunction including alexia or hemiachromatopsia may be present with striatal lesions. When infarction is limited to the lower banks of the calcarine fissures bilaterally, prosopagnosia and defective colour vision may be present.⁶ These patients have difficulty in recognising, matching or naming colours accurately. It is possible that our patient has prosopagnosia leading to his difficulty with colour vision testing.

Although our patient was initially referred as an NTG suspect, he had clinical inconsistencies suggesting an alternative diagnosis. NTG is a common form of primary open-angle glaucoma and is defined as glaucomatous optic neuropathy with a statistically normal IOP.^{7 8} NTG is a diagnosis of exclusion and signs of optic nerve vulnerability should be present without other explanation for disc abnormality and field loss.

Patients with NTG display vulnerability of the optic nerve head to lower IOPs and benefit from lowering of IOP.⁹ Vascular dysregulation and disruption to perfusion of the optic nerve head may also play a role in the pathogenesis in a subset of NGT patients.⁸ Distinguishing phenotypic features of NTG includes drance haemorrhages as the disc margin, acquired pits of the optic nerve and characteristic disc cupping.

Differential diagnosis of NTG includes POAG with large diurnal fluctuations, primary open angle glaucoma with thin central corneal thickness, the use of systemic beta-blockers concealing the true IOP, vasospasm/migraine or intermittent angle-closure glaucoma.¹⁰ Some patients may have had prior corticosteroid-induced glaucoma or an isolated episodic event (eg, cardiogenic shock) resulting in non-progressive optic nerve head cupping.

Although neuroimaging is not mandatory in the assessment of all patients with NTG, the American Academy of Ophthalmology suggests that consultation with a neuro-ophthalmologist and neuroimaging may be necessary if the optic nerve demonstrates more pallor than cupping, if patients are younger than 65 years, if there is rapid progression of optic nerve damage or there is marked asymmetry between the optic nerves, or if perimetry suggests that damage is further posterior in the visual pathway.¹¹

Management of posterior cerebral artery strokes follows the principles of other cerebral injuries including maximisation of function and the prevention of recurrent ischaemic events. Patients should be referred to a general physician, cardiologist or neurologist for stroke workup and minimisation of risk factors including control of blood pressure and serum cholesterol levels, arrhythmia management and assessment of carotid disease. This was delayed in this patient given his misdiagnosis as NTG.

Our patient had many thrombotic (hypertension, vasculitis and hypercholesterolaemia) and embolic (metallic prosthetic mitral valve and paroxysmal atrial fibrillation) risk factors for cerebral ischaemia and his referral for three MRI scans over 5 years reflects the high degree of suspicion among his treating clinicians. MRI is superior to CT in detecting small ischaemic lesions occurring after a minor stroke.¹² Both CT and MRI with coronal sections through the occipital lobes may miss small lesions, particularly at the occipital pole and during the first 24 hours of ischaemic stroke.⁵

This case highlights the value of a thorough investigation and the importance of a multi disciplinary approach when the diagnosis is uncertain and the index of clinical suspicion remains high.

Learning points.

• Normal tension glaucoma (NTG) is a diagnosis of exclusion and cases of apparent NTG should be thoroughly investigated and referral to ophthalmology subspecialists should be considered where the diagnosis is uncertain and clinical signs are incongruent.

• Other causes of visual field defects should be considered when assessing a patient with ‘typical’ glaucomatous field findings without corresponding optic disc signs.

• This case highlights the importance of multidisciplinary involvement in both clinical and radiological diagnostic challenges.