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In his review of carbon monoxide poisoning (June 2001 JRSM, pp. 270-272) Dr Blumenthal draws attention to the confusing array of non-specific symptoms and signs which may characterize this potentially fatal condition. It should be noted, however, that the often neglected examination of ophthalmoscopy may reveal valuable clues to the diagnosis. Blumenthal comments that retinal haemorrhages are rarely seen. This may reflect more on the thoroughness of examination than on the actual prevalence of this sign. Ophthalmoscopy is many a physician's ‘blind spot’, and poor equipment and the avoidance of mydriatics in an emergency may exacerbate this. Indeed in one case-series retinal haemorrhages were found in all patients with carbon monoxide exposure of more than 12 hours (comprising about half of the patients in that series)1.
Case studies and series suggest that the retinal haemorrhages may occur superficially or deeper in the nerve fibre layer (flame haemorrhages), with a tendency to be peripapillary. Venous changes include engorgement and tortuosity, whilst oedema of the optic disc may also be noted. In general terms these changes reflect the extent of the hypoxic insult to the retina2.
Electrodiagnostic tests can detect subtle changes in visual function associated with carbon monoxide poisoning. These changes are typical of optic neuropathy and suggest that this toxic neuropathy shares aetiological mechanisms with tobacco amblyopia3. Interestingly, smokers appear to be particularly vulnerable to additional environmental carbon monoxide, with adverse effects on dark adaptation and light sensitivity4. Whilst such electrodiagnostic tests are not available in an emergency, careful ophthalmoscopy may greatly assist diagnosis of this insidious and life-threatening condition.