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Chronic open-angle glaucoma (COAG) is an umbrella diagnosis encompassing chronic progressive optic neuropathies with distinctive changes at the optic nerve head and retinal nerve fibre layer that lead to progressive death of retinal ganglion cells, visual field defects and resultant disability.1 It is described as primary: unattributable to pre-existing ocular disease or congenital abnormality. Does the National Institute of Health and Care Excellence (NICE) guidance lead to our patients receiving appropriate treatment?
(a) Who are we treating?
Voltaire, an 18th century philosopher, famously wrote:
“Doctors are men(sic) who prescribe medicines of which they know little, to cure diseases of which they know less, in human beings of whom they know nothing.”
Practising modern 21st century medicine, we are fortunate that critically analysed, robust clinical trials are screened by thorough independent peer review to create highly scientific knowledge of medicines and diseases. This is used to produce algorithms to guide clinical practice for the benefit of the general population. Unfortunately, these guidelines often adopt a one-size-fits-all approach and clinicians find themselves using the same protocols to treat patients who are crudely sorted into broad groups. Consequently the subtle differences of the individual patients in front of us in the clinic are rarely considered and the variable social implications of many of our treatments are rarely appreciated.
The complete guidance ‘Glaucoma: Diagnosis and Management of COAG and ocular hypertension (OHT)’ highlights the importance of a patient-centred approach with informed decision making. But the sense of tailoring a bespoke treatment pathway for each patient is quickly lost on first glance of the widely circulated guideline protocol for treating glaucoma, provided by NICE (table 1).2 Treatment is advised until a certain age depending on the corneal thickness. This is because the financial outlay, exposure to potential side effects and impact on daily life of indefinite ocular hypotensive drops is unnecessary if someone is unlikely to develop any disability due to optic nerve dysfunction in their lifetime. However, by offering such crude age restrictions, sweeping judgments are made and the life expectancy is assumed before anything about the patient's general health, physical fitness or life priorities can be taken into account.
The protocol explains that the risk to someone with normal vision within their lifetime is negligible once they have reached a certain age threshold.3 However the age threshold provided by NICE is calculated for the average patient. The vast majority of patients will either be able to safely discontinue treatment significantly earlier or may need to remain on treatment for significantly longer to prevent any vision deterioration. Therefore, rather than depending on an almost arbitrary algorithm published in 2009, each patient should be individually considered with reference to current literature. Other important factors to consider when calculating a target intraocular pressure (IOP) include level of glaucoma damage and rate of progression of glaucoma.4
(b) Why is central corneal thickness of primary significance in the decision: to treat, or not to treat?
It is well recognised that there are several factors which can affect IOP measurements, the most commonly cited being corneal thickness.5 To date, the most accurate method of measuring IOP is the Goldmann applanation tonometer (GAT) and appropriately this is the reference standard.6 However, this method assumes a central corneal thickness (CCT) of 555 μm. Since the IOP will likely be higher (or lower) than the GAT reading if the CCT is less (or more) than 555 μm, respectively, the treatment protocol separates patients into pathways depending on whether their IOPs are being underestimated, correctly estimated or overestimated.
The IOP reading is susceptible to a plethora of additional artefacts (eg, diurnal variation, corneal curvature and hydration, patient holding breath, wearing a tight collar or having the eyelids squeezed).7 This is by no means an exhaustive list, although it seems that the CCT is the only factor taken into account, for the simple reason that its effect is the most widely studied. CCT is relevant for diagnosing OHT. CCT remains a potential mechanical risk factor, but has little bearing on management once glaucoma is diagnosed. Furthermore, engineering models suggest that the material properties of the cornea (ie, tensile modulus, inherent stiffness and/or viscoelastic properties) likely dwarf the effect of CCT on GAT measurements.8 Thus, there is no commercially available correction algorithm to adjust for GAT-derived pressure for differences in CCT.9
Other potential limitations of GAT should not be ignored. For example, the SEAGIG guidelines suggest that GAT calibration errors of up to ±2 mm Hg are acceptable.10 Is this really tolerable when we are treating the ranges as shown in the NICE table, which are as specific as 4–7 mm Hg?
Is ongoing research likely to provide relevant evidence?
These concerns ought to be addressed before placing resolute faith in guidelines. We need innovation to provide a more accurate means of measuring the true IOP and further research to identify ways of avoiding various sources of error. Further research could investigate a population with OHT/COAG and varying CCT. Other methods of IOP measurement which are almost as reliable but less sensitive to CCT can be used such as dynamic contour tonometry.11 If IOPs measured by dynamic contour tonometry are treated accordingly, this could be compared with patients whose treatment results from IOPs measured by GAT, and the long-term outcomes of disability could be assessed and compared.
What should we do in the light of the uncertainty?
Although there are shortcomings, NICE offers guidance with best intentions, perhaps simplified for practicality. In theory, it should provide adequate treatment for the average patient. However, many patients will have target IOPs lower or higher than suggested by the NICE table. Many recorded IOPs will be different from the patient's true IOP. It is important for a clinician to remember that ‘guidelines’ are just that. They should guide but not be prescriptive. As in all aspects of medicine, the clinician should exercise their right to deviate and use their own judgement if this is likely to benefit the individual patient.
Contributors DA: Wrote the article. NK: Contributed to edit. RZ: Provided the idea behind the article.
Competing interests None.
Provenance and peer review Not commissioned; internally peer reviewed.
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