Why we get a blue stain

Sir,

We wish to add our comments to the recent review article describing the use of trypan blue (TB) in anterior segment surgery.¹ To understand its potential uses, we need some knowledge of its biochemistry. TB (C₃₄H₂₃N₆O₁₄S₄Na₄) is a negatively charged, water soluble bis-axo dye that is used in cell biology to confirm cell viability. The selectively permeable cell membrane of viable cells does not allow this ‘foreign' chemical to enter and hence appear colourless while the uptake in dead cells results in a blue discolouration. Similarly, TB will delineate collagen type IV-rich membranes. Both these properties are utilized in ophthalmology, the former in highlighting viable cells after conjunctival brush cytology, the latter in staining the anterior lens capsule (as well as ILM and ERM tissue).² Permanent discolouration is avoided by dilution with balanced salt solution. TB also stains poorly hydrated tissues better than well-hydrated ones explaining the contrast seen between the acellular lens capsule and the water-rich cells of the cortical lens.

Unmentioned risks include potential teratogenicity (has been associated with encephalocoele formation in animal models) and carcinogenicity (lymphoma in the rat model); both models, however, used subcutaneous injections of TB. Similarly, ocular abnormalities have been described in the pregnant murine model.³ As a cautionary measure, the risk benefit ratio should be properly assessed before proceeding with TB-assisted ophthalmic surgery in pregnant or nursing women.

The authors declare no conflict of interest.