To function, the tear system requires the joint participation of the tear secretion system and the drainage system, and they need to reach a certain state of balance. When one side changes and breaks the balance, the other side will also change in varying degrees [8, 9]. The TMH can reflect the amount of tears stored in the conjunctival sac to some extent. When the tear secretion volume is too high and the drainage is insufficient, the tear storage is on the high side; conversely, when the tear secretion is insufficient and the drainage is suitable, the tear storage is low; finally, when the tear secretion is insufficient and the drainage is insufficient, the tear storage may be normal [10, 11, 12]. SIt is a typical quantitative method for determining the composition of tear fluid samples and can reflect the basic secretion of tears, and SIt is also a common detection index for patients with dry eyes [13, 14].
In patients with lacrimal duct obstruction, the function of tear drainage is basically "lost" [15, 16]. When such patients undergo endoscopic dacryocystorhinostomy, it can be inferred that due to the "recovery" of drainage function, the TMH of these patients should all show a decreasing trend. However, the results of this study showed that the 120 sets of TMH data showed three trends; however, the overall trend gradually decreased, and the difference was statistically significant (P < 0.05). Each group of data was analysed independently and evaluate the individual the changes in trend. The statistical results show that only 45.00% (54/120 eyes) of the affected TMH showed a gradually decreasing trend; 28.33% (34/120 eyes) of the affected TMH showed an upward trend; 26.67% (32/120 eyes) of the affected TMH decreased and then increased, which is "contradictory" with the original theoretical inference.
Since the tear drainage system is changed by EN-DCR, theoretically, SIt, as an indicator of the tear secretion system, should not be changed by the operation. However, the SIt data of 120 eyes in this study showed an increasing trend, and the difference was statistically significant (P < 0.05). As shown in Fig. 3, when the preoperative SIt increased to a certain length, the SIt values showed a trend of "coincidence" preoperatively, 1 month postoperatively, and 3 months postoperatively. The preoperative SIt values were divided: "< 10 mm/5 min" and "≥10 mm/5 min". The results showed that when the preoperative SIt was < 10 mm/5 min, the results decreased gradually, and the difference was statistically significant (P < 0.05). However, there was no significant difference when the Sit was ≥ 10 mm/ 5 min preoperatively (P > 0.05). Combined with the domestic and international research reports on the lacrimal system in recent years, the following analysis can be made.
Tears are absorbed by the lacrimal passage and transported to the surrounding cavernous body, which is innervated by the autonomic nerve and regulates tear drainage [17]. Under normal conditions, tears are continuously absorbed into the surrounding cavernous vessels. These blood vessels are connected to the blood vessels of the outer eye and can be used as a feedback signal for tear secretion. If tears are not absorbed, tear secretion will stop and dry eyes will occur [3, 18]. The mechanism of implanting a lacrimal suppository or other methods of blocking the lacrimal passage in the treatment of xerophthalmia is to completely block the absorption of tears, resulting in the effect of "emptiness" in the lacrimal duct system. This effect is a strong stimulation signal produced by tears at the initial stage, but this stimulation effect weakens with time [8, 19]. In normal individuals, there is an automatic balancing mechanism that can restore the secretion and drainage of tears to a equilibrium state in a certain period of time, but this mechanism is missing in some people [20].
At present, many studies have confirmed the value of TMH in the diagnosis of dry eyes, and this parameter can reflect the amount of tear secretion to some extent. Although there are some differences in the distribution range of TMH values among different populations, patients can be considered to have dry eyes when the TMH is less than 0.2 mm [21, 22, 23, 24]. In group A, the percentage of eyes with a TMH less than 0.2 mm was 61.76% (21/34) preoperatively, 17.65% (6/34) at 1 month postoperatively, and 0 at 3 months postoperatively (Fig. 4). After receiving EN-DCR, tears began to be absorbed and drained, feedback signals were sent out to stimulate tear secretion, and enhanced tear secretion relieved dry eyes to a certain extent.
In group B, the percentage of 32 eyes with a TMH less than 0.2 mm was 3.13% (1/32) preoperatively, 59.36% (19/32) at 1 month postoperatively, and 9.36% (3/32) at 3 months postoperatively (Fig. 4). In most eyes, the TMH was higher than 0.2 mm or even higher preoperatively and at 3 months postoperatively, so there was no sufficient evidence to indicate that most patients had severe dry eyes. However, more than half of the eyes had a TMH less than 0.2 mm at 1 month postoperatively, indicating that they had dry eyes. This can be explained by the fact that the affected eyes have partial of the function of tear secretion preoperatively, and after receiving EN-DCR, the absorption and drainage of tears gradually recover; however, the secretion function is not significantly enhanced at this time, so there is a trend that the height of the tear meniscus decreases for a short period of time. When the height of the tear meniscus decreases to a certain threshold, the automatic balancing mechanism is activated, the secretion function is gradually strengthened, and the equilibrium state is finally reached. Therefore, these patients show a changing trend of decreasing first and then increasing..
The change trend of 52 eyes in group C was consistent with the initial theoretical inference. The mean preoperative TMH was 0.78 ± 0.30 mm, and the minimum TMH was 0.26 mm. At 3 months postoperatively, the minimum TMH was 0.06 mm, and the proportion of eyes with a TMH less than 0.2 mm was 13.46% (7/52) (Fig. 4). The TMH and SIt values of the affected eyes were higher preoperatively. After receiving EN-DCR, tears are continuously absorbed and drained. It can be considered that because the TMH does not decrease to a certain threshold, there is no stimulus for the secretion of more tears. This can also be explained by the fact that the automatic balancing mechanisms of some affected eyes in group C are defective or deficient [25].
When the SIt was < 10 mm/5 min preoperatively, the tears were in a low secretion state [26, 27]. After EN-DCR, under the stimulation of the successful absorption and drainage of tears, the tear secretion system "activated" the regulation mechanism, reversed the secretion of more tears, and gradually restored the normal secretion function. When the SIt was ≥ 10 mm/5 min preoperatively, the tear secretion function was basically normal, and even if the state of tear drainage changed, the tear secretion system still experienced no obvious changes. Therefore, it can be considered that the effect of EN-DCR on the tear secretion system is only shown in those with an abnormal tear secretion system.
Many patients with an obstruction of the lacrimal duct but no symptoms of lacrimal discharge have been reported clinically. Some of these patients have dry eyes at the beginning and can still show no overflow of tears even if they have an obstruction of the lacrimal duct in the later stage; for others that have an obstruction of the lacrimal duct, the tears are not absorbed, the feedback signal is terminated, the secretion of tears is stopped, and finally there is no overflow of tears [28, 29].
In patients with an obstruction of the lacrimal duct, due to the "lack" of a lacrimal drainage system preoperatively, the TMH depends on the amount of tear secretion, and there is a high correlation between TMH and SIt length. When patients received EN-DCR, the lacrimal drainage system gradually recovered, and the correlation between TMH and SIt values gradually weakened. At 3 months postoperatively, there was almost no correlation between the two parameters. The reasons for the final lack of correlation between TMH and SIt values can be analysed as follows: the lacrimal secretion system and tear drainage system have basically returned to a balance, but there are great differences in tear secretion function and tear drainage function in this population, and the SIt values mainly depend on the tear secretion function, while the TMH is affected by both of functions; this makes the TMH and SIt values show no correlation at the last timepoint.
The automatic balancing mechanism and feedback regulation of the lacrimal system mostly depend on the cavernous structure of the lacrimal duct, which exists in the lacrimal sac and nasolacrimal duct. To make the lacrimal system function effectively and stably, the lacrimal drainage system needs to maintain a normal structure [3, 8]. However, EN-DCR, as a diversion, impairs part of the function of the cavernous body of the nasolacrimal duct. Whether the loss of this part of the cavernous body will have a certain impact on the lacrimal system remains to be further studied and confirmed [30].
There are still some shortcomings in this study. (1) The number of cases is insufficient, and the results can be confirmed by a larger sample of patients in the future. (2) The follow-up time was insufficient, and the final measurement of TMH and the SIt values occurred at 3 months postoperatively, which could not reflect the long-term effect of EN-DCR on the lacrimal system. (3) The inconsistencies in patient eye habits postoperatively may have had a certain impact on the results of the study.