Suspicious discs, thin corneas and one unreliable field in a diabetic patient
A 54-year-old Kuwaiti woman with type 2 diabetes for 12 years (HbA1c 8.9%) attends a community optometry clinic after the national diabetic retinopathy screening service noted 'suspicious discs'. She feels well, has no visual complaints and reads 6/6 in each eye with her existing varifocals. Non-contact tonometry reads 24 mmHg right and 23 mmHg left; Goldmann applanation on the same visit reads 22 and 21 mmHg. Central corneal thickness is 498 µm right and 505 µm left. Gonioscopy shows open angles with grade 2 pigment in both eyes. The right disc has a vertical cup-to-disc ratio of 0.72 with inferior rim thinning and the left 0.60; there is no disc haemorrhage or peripapillary atrophy. A single 24-2 SITA-Faster field on the right eye shows fixation losses 6/12, false-positive responses 22% and false-negative 9%, with a pattern deviation map the machine flags as borderline. Her brother has glaucoma and her mother lost vision in one eye late in life. She asks why she needs 'more tests when the pressure is nearly normal'.
1. Interpret the findings and estimate her risk. What does the thin cornea do to your IOP interpretation, and why can you not exclude glaucoma from this one field test?
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The numbers.
- Goldmann 22/21 mmHg with CCT 498/505 µm. Thin corneas make applanation under-read; using a rough clinical rule of 0.4–0.5 mmHg per 10 µm from a 545 µm reference, her true pressure is closer to 24 mmHg in each eye. That is not 'normal'.
- Discs: vertical CDR 0.72 OD with focal inferior rim thinning, 0.60 OS. An asymmetry of 0.12 with focal thinning is a structural red flag; focal rather than concentric cupping is what matters.
- Risk factors: age 54, pressure in the mid-20s once corrected, thin CCT, CDR ≥ 0.7, a first-degree relative with glaucoma (brother; mother with late-life visual loss), and 12 years of poorly controlled diabetes. On a risk calculator this places her in a clearly elevated band - high enough to justify full glaucoma assessment rather than observation.
The field. One 24-2 SITA-Faster with 6/12 fixation losses and 22% false positives is unreliable. A high false-positive rate flattens the hill of vision and can either hide a real defect or produce a borderline trace. Glaucoma is diagnosed on reproducible structure–function damage, so a single unreliable field can neither confirm nor exclude it, and a 'borderline' printout should never be filed as normal.
The diabetic caveat. Diabetes can thin the retinal nerve fibre layer independently of glaucoma, so an OCT result on its own will not settle the diagnosis - disc appearance, fields and pressures have to agree.
2. Set out the investigation plan for the next three months, justifying each test and the order, including what you send to secondary care and what you repeat yourself.
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Repeat myself, properly, in the community clinic.
- Visual fields - at least two further 24-2 tests per eye, with coaching on fixation and on not pressing the button for the sound alone. If reliability stays poor, shorten the strategy. Pattern and progression only mean anything across repeated tests.
- Diurnal IOP - four Goldmann readings across clinic hours, since one mid-morning value misrepresents a fluctuating pressure.
- Gonioscopy repeated and documented - open with grade 2 pigment; look again specifically for pseudoexfoliation, pigment dispersion and any synechiae.
- Dilated fundus examination and fundus photography - for disc documentation and because her diabetic retinopathy status must be known; ask specifically about macular oedema, since 6/6 acuity does not exclude central thickening.
- Record the pachymetry on every referral document.
Send to secondary care, soon but not as an emergency.
- OCT RNFL plus macular ganglion cell complex on one platform - this is the baseline that will decide the diagnosis and later show progression, and it must be captured before any treatment is started.
- Glaucoma clinic risk stratification - is this ocular hypertension, pre-perimetric glaucoma, or a normal-tension picture, and is treatment indicated?
- Stereoscopic optic disc photography for the record.
What I would not do. Repeat the same unreliable field unchanged, or start drops 'to see whether they help' - that destroys the baseline.
3. She wants to be monitored in the community and asks whether she can start 'the pressure drops' now. How do you respond, and what does the public-health and ethical dimension look like in Kuwait?
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'Can I start the drops now?' No - and the reason is clinical, not bureaucratic. In the MOH pathway, IOP-lowering therapy is initiated by the ophthalmologist once the diagnosis and baseline imaging are established. A prostaglandin analogue started today would lower the pressure and remove the very measurement the glaucoma clinic needs, and would commit her to lifelong therapy with periorbital pigmentation, lash growth and conjunctival hyperaemia before anyone has confirmed glaucoma rather than ocular hypertension.
'Why more tests when the pressure is nearly normal?' Because pressure is a risk factor, not the disease. Glaucoma can damage the nerve within the statistically normal range, pressure fluctuates across the day, applanation underestimates pressure in a thin cornea, and the diagnosis rests on nerve damage that the tests are designed to detect and then to follow.
Ethical and public-health dimension.
- Consent and autonomy. She is entitled to the reasoning, to the uncertainty, and to decline. Document the discussion and give her a written plan with dates.
- Access and equity. Diabetic screening found her; the referral pathway must not lose her. Give a named appointment, not 'return if worse'.
- Adherence in Kuwait. Raise Ramadan and fasting early. Many patients stop drops while fasting; a once-daily evening prostaglandin or a fixed combination is easier to sustain than twice-daily timolol. Flag this to the prescribing ophthalmologist rather than letting her decide alone.
- Primary prevention. HbA1c 8.9% and blood pressure drive both her microvascular and her optic nerve risk; reinforcing control and keeping her retinopathy screening interval are part of the optometrist's role.
- Scope of practice. Detection, risk stratification and referral are mine; diagnosis and initiation of therapy are not. Saying so clearly protects the patient and the clinician.