Hydrogen ion concentration of tear fluid in newborn infants

The neonatal tear film

The importance of the tear film for the integrity of the ocular surface is well established. Full-term neonates produce tears normally, but low spontaneous blink rates during early life raises important questions regarding tear dynamics and stability. Although an afferent neural pathway that could potentially detect tear break-up is in place at birth, there is indirect evidence that the neonatal tear film is adapted to resist evaporation-mediated tear thinning. This adaptation presumably prevents drying of the ocular surface during long inter-blink periods. However, low rates of tear turnover may have important implications for the defence of the eye against potential pathogens.

Demonstration of increasing standard pH value of lacrimal fluid with increase of flow rate

PURPOSE

To test the hypothesis that pH value of human tears is affected by flow rate.

METHODS

After stimulation with a drop of 70% alcohol into the ipsilateral nasal cavity tears from the left eyes of 30 healthy volunteers were collected with glass capillaries while flow rate of tear secretion was registered. The pCO(2), actual and standard pH were determined in the collected samples with the Astrup method: Standard pH is defined as the pH of the sample that has been equilibrated at 37 degrees C with a gas mixture having pCO(2)=40 mmHg.

RESULTS

The flow rate correlated positively and significantly with both the standard pH value (r=0.503, p<0.01) and the pCO(2) (r=0.517, p<0.01) while there was no correlation between the actual pH and the flow rate.

CONCLUSION

Loss of CO(2) from the sample is a cause of serious uncertainty in tear pH determination. A positive flow rate-pH relationship was found for human tears when the uncertainty was removed by determination of the standard pH.

Fluorophotometric measurement of pH of human tears in vivo

PURPOSE

To measure the pH in the precorneal tear film of humans in vivo using a pH-sensitive fluorescent dye, bis-carboxyethyl-carboxyfluorescein (BCECF).

METHODS

The measurement was initiated by instilling 1 microliter of 2 mM BCECF solution into the subject's eye. The pH was calculated by measuring the ratio of fluorescent intensities at two excitation wavelengths (490/430 ratio), which was dependent on pH, but independent of the dye concentration and other variables. The tears of the same subject were then collected and loaded on to a micro pH-meter to ensure the accuracy of the measurement.

RESULTS

The mean pH values of 40 eyes from 20 healthy volunteers was 7.50 (SD +/- 0.23), which corresponded well with those measured by the micro pH-meter.

CONCLUSIONS

The method described was useful in measuring the pH of the precorneal tear film of humans with minimal invasion.

Tear fluid pH in normals, contact lens wearers, and pathological cases

The pH of the conjunctival fluid was measured with a micro-glass-electrode in the inferior conjunctival fornix of 200 persons. Among 41 binocularly normal persons the pH was found to be 6.93 +/- 0.24 (mean +/- SD) of the primarily examined eye, independent of sex and age. The conjunctival fluid was significantly more acid in contact lens wearers (6.66 +/- 0.28, N = 26), but became normalized after removal of the lens (6.49 +/- 0.32----7.09 +/- 0.19, N = 8, P less than 0.001. The conjunctival fluid was significantly more alkaline in patients with lacrimal stenosis (7.13 +/- 0.28, N = 11), keratitis (7.14 +/- 0.28, N = 10), especially mycotic keratitis and postoperatively (7.15 +/- 0.22, N = 10, the first 24 h after corneal transplantation). There was, perhaps, a tendency towards alkalisation in eyes with keratoconjunctivitis sicca, whereas the pH was normal in the other pathological states, or the number of cases representing these may be too small.

pH changes of the tear fluid in the conjunctival sac during postoperative inflammation of the human eye

Conjunctival fluid pH was measured pre- and post-operatively in 16 patients suffering from senile cataract. We found a significant increase (P less than 0.05) from 7.26 +/- 0.23 (mean +/- SD) on the day before operation to 7.50 +/- 0.23 (mean +/- SD) on the first post-operative day in the conjunctival fluid of the treated eye. Thereafter a gradual decrease towards pre-operative level was observed. These changes were not observed in the healthy fellow eye (P greater than 0.05). No significant pre-operative difference was shown between the two eyes. It is reasonable to assume that the composition of the transudate/exudate from the conjunctival vessels might be responsible for the changes observed.