OPINION: Dr Tu Tran FRANZCO MBBS MPH discusses false positive errors in glaucoma detection

As a part of our series on glaucoma, Dr Tu Tran FRANZCO MBBS MPH discusses the role of false positive referrals in glacuoma detection and gives an example of this from her clinic.

Technological advancements such as OCT are continually improving our diagnosis and management of disease yet come with drawbacks; the risk of misinterpreting the data due to false positive and false negative errors.

False positive errors occur when an individual without the disease is “identified” by a diagnostic test as having the disease.

False negative errors occur when the test result incorrectly fails to indicate the presence of the disease.

A false positive error can lead to unnecessary treatment. A false negative error can lead to undiagnosed disease and adverse patient outcome with no treatment initiated.

The following case is an example of a false positive glaucoma patient.

This case involves a 47-year-old male referred for glaucoma assessment. The referral states:

“Please assess this gentleman with possible normal tension glaucoma. He has left nerve fibre layer defect, OCT abnormality and matching visual field loss.”

Refer to optometrist OCT and VF below:

He had an episode of visual disturbance in right eye 6 years ago and was investigated for possible transient ischaemic attack (TIA). MRI, carotid duplex, blood tests were normal. His vision returned to normal after a few hours. Two years later, he was diagnosed with multiple sclerosis after an episode of optic neuritis. MRI head reported multiple demyelinated plaques and optic neuritis.

He has been under the care of a Neurologist. He is not on any medication. Unfortunately, there were no previous optic disc photos, OCT or visual fields for comparison.

On presentation, he is asymptomatic.

Visual acuity is 6/6 right 6/7.5 left

Ishihara – all colour plates correctly identified bilaterally

There is mild red desaturation in left. There is no relative afferent pupillary defect.

Central corneal thickness (CCT) measures right 542um, left 548um. His intraocular pressures are 11mHg right and 12 mmHg left

Gonioscopy reveals trabecular meshwork in all quadrants.

Fundoscopy shows normal sized optic discs. Left optic disc has superior nerve fibre layer defect, CDR 0.7. Right optic disc is normal with healthy neuroretinal rim.

His left OCT shows superior RNFL defect. He has matching left inferior visual field defect.

Refer to disc photos, OCT and VF below:

The detection of retinal nerve fibre layer (RNFL) defect is a significant finding in this case. Retinal nerve fibre layer defects are not seen in normal eyes. These defects have been shown to be among the earliest signs of glaucomatous damage and can precede visual field defects. They are especially helpful for early glaucoma diagnosis and in eyes with small optic discs. The localised wedge-shaped defect of the RNFL is usually seen in association with notching of the neuroretinal rim, vertical enlargement of the cup or following disc haemorrhages.

Retinal nerve fibre defects occur in approximately 20% of all glaucoma eyes. They are not pathognomic of glaucoma. Retinal nerve fibre layer defects can occur in other diseases such as optic disc drusen, ischaemic optic neuropathy, chronic papilloedema and optic neuritis due to multiple sclerosis.

This is a false positive case of glaucoma. This case describes a middle-aged patient with an isolated arcuate defect within the nerve fibre layer and visual field. Although RNFL defect raises suspicion for glaucoma and prompts careful follow-up, other possible systemic reasons for localised retinal ischemia must also be considered. In this case he has underlying optic neuritis. By recognising this is a case of optic neuritis we can avoid unnecessary glaucoma treatment.

Nevertheless, it is important to follow him up regularly as it may be possible to have subsequent glaucoma or subclinical optic neuritis. It is important to obtain baseline tests including optic disc photos, OCT and visual fields and monitor for any changes regularly.

False positive errors highlight possible disease, which deserve further comprehensive testing/ follow up. False negative errors are more concerning as the tests do not detect disease and these patients may fail to be referred for treatment. The ability to recognise these errors is important and can be minimised with education (individual or group sessions), training and feedback.  Educational events with your local ophthalmologists represent ideal opportunities to discuss diagnostic and management cases; highlighting these errors for better detection and facilitating more focused referrals. These sessions enable channels of communication and collaboration between optometrists and ophthalmologists.