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DISCUSSION
The Pathogenesis of the Dry Eye ZuGuo Liu gives it a more comprehensive summary: Any element that causes the abnormality of ocular surface, innervation of the ocular surface (afferent nerve and efferent nerve), the lacrimal gland, and the feedback loop of ocular surface epithelial cells in any links or more links will cause dry eye syndrome [1].
The Innervation of Cornea and the Lacrimal Gland Corneal sensation is provided by the long ciliary nerves of the ophthalmic division of the fifth (trigeminal) cranial nerve. The long ciliary nerves enter the limbus predominantly at the nine oclock and three oclock positions with little innervation entering the eye superiorly and inferiorly. The nerves bifurcate 23 times then they are forming a dense plexus in the subBowman's layer, which densely innervates the central cornea. From there, nerve fibers perforate Bowmans layer and eventually form a dense neural plexus just beneath the basal epithelial cell layer, which is characterized by tortuous and thin beaded nerve fibers interconnected by numerous nerve elements; nerve fibers from this plexus are known to be responsible for the innervation of the epithelium. Feedback loop from cornea to lacrimal gland: The sensory nerves of cornea, conjunctiva, uvea, nasal mucosa, facial skin or lacrimal gland → pontine main sensory nucleus and spinal nucleus → tear nucleus →parasympathetic nerve fibers → the lacrimal gland [2].
Possible Mechanisms of Dry Eye Syndrome after LASIK
A The decline of corneal sensation is the major factor for dry eye syndrome. On the one hand, the creation of the flap and subsequent laser ablation sever most corneal nerves except at the hinge, which produces a decrease of corneal sensation that could cause decreased tear production based on an inhibited feedback loop from cornea to lacrimal gland; on the other hand, because there is a positive correlation between corneal sensation and the blink rate[3], when the corneal sensation declines, the blink rate will decrease following it. The blink is the basis for the reconstruction of tear film: When blinking, the tear mucin, liquid water layer and lipid layer will be distributed evenly on the corneal surface to complete the reconstruction of the tear film. If the blink rate reduces, the evaporation of the tear will enhance, and mucin could not be distributed evenly, so the liquid water layer and lipid layer could not be attached properly, and tear film could not be reconstructed.
B The tissues of ocular surface are damaged: During the surgery, the negative pressure suction will damage the bulbar conjunctiva while the microkeratome injuries the corneal epithelium mechanically, and a nonhealing epithelial defect can occur due to the loss of direct neurotrophic effects.The mucin can not be adsorpted to corneal surface due to the damage of the microvilli and fine relief of the epithelial cells. Part of the conjunctival goblet cells are destroyed, so the mucin secretion reduces, which is not conducive to the formation and maintenance of the tear film. The match between eyelid and corneal surface changes and leads to a decline of tear film stability. The damages of the ocular surface make the reduction of the corneal sensation more severely, which leads to less reduction of tear secretion while their interactions form a vicious cycle.
C Application of glucocorticoid, antibiotics, artificial tear eye drops, which have preservative in them, can change the permeability of cell membranes and result in ocular surface epithelium stripping, which affects the structure and stability of tear film directly. The destruction of the corneal epithelial microvilli and the reduction of the density of conjunctival goblet cells can affect tear film stability indirectly at the same time. in addition, the eye drops have a similar effect as detergents to the lipid layer of tear film, so they can speed up the evaporation of the tear [47] .
D Many factors can affect the tear mucin adhering to the ocular epithelial surface,such as decline of the regularity of corneal surface, the immune inflammatory response caused by the inflammatory mediators and immune mediators, wound healing and partial surgical incision uplift, which leads to a unstable tear film [8].
E Other factors: Patients with contact lenses for a long time before the surgery, are up to a certain age or have to be engaged in computer work all day long.
Sum Up Many researches showed that the dry eye syndrome disappeared before the corneal sensation returned to preoperative level or at the same time with it[9]. But in this study, although corneal sensation returns to its former level at three months (P> 0.05), patients' symptoms are even more severe. Compared with preoperative value, the score of dry eye syndrome has a significant difference at one month (P<0.05), and an extremely significant difference at three months (P<0.01). The indicators (tears flow, TBUT, corneal fluorescein staining) have improved gradually, but three months after the surgery, they did not restore to the preoperative levels. The main reasons may be: A: Flap creation in LASIK severs all superficial corneal nerves except at the hinge, which provides a route for undisturbed innervation. Therefore, the location of the hinge may be planned to afford the cornea the best sensory advantage. Use of a vertically hinged flap improves corneal sensation in the nasal quadrant and may theoretically decrease the chance of dryeye symptoms [910]. In our study, two major regional distribution of ciliary nerves were cut off in eyes with superior hinge flaps, so the decrease of corneal sensation were more serious, leading to the dry eye more easily to happen. B: The impact of the eye drops: When dropping the eye drops into the eye, with the stimulus of them, much tear will be secreted in a short period of time, so the drug concentration will decline rapidly. eye drops can be excluded from the ocular surface in a few minutes. But after LASIK, the decline of corneal sensation leads to a reduction of tear secretion, and the preservatives of the drugs can not be fully diluted in the eye, so they will stay longer in the ocular surface, which will more easily lead to the ocular surface damage. The ocular surface epithelial defects increase the toxicity of drugs, making corneal epithelial difficult to heal. More than ninetyfive percent of patients used artificial tears as long as they felt their eyes discomfortable, so that the actual frequency of they using artificial tears was more than six times a day, due to which not only normal tear was washed away, tear evaporation was speeded up [11], but also as a feedback effect, the normal tear secretion was inhibited. C: The change of habit that patients use their eyes: In the early stage after surgery, because of the psychological attention, patients seldom use their eyes, but as time goes by, patients psychological relax and needs of learning or work, they use their eyes more and more, especially some students or computer workers. Then they use artificial tears to alleviate the discomfort caused by excessive use of the eyes, and the ocular surface became drier by using the eye drops too frequently. D: Patients have a certain residual degree after the surgery so that the two eyes are probably at different refractive state. Some of the eyes are farsightedness, and their eyes are easily fatigued, and sometimes the patients can not distinguish whether the discomfort is caused by refractional difference from by dry eye. E: After LASIK, patients dont wear eyeglasses anymore, which will make tear evaporation speed up at certain extent. F: The deposition of drugs stay in the conjunctiva, affecting the storage of normal tear and the accuracy of Schirmers basic tear secretion test.
Therefore, in order to improve the quality of patients life after LASIK, alleviate the discomfort of dry eye syndrome, as surgeons, we should note those: A: Choose a suitable method to make the corneal flap, which can reserve the nerve growing channels as many as possible. B: During the surgery, we should minimize the ocular epithelial mechanical injury as possible as you can. C: Prevent and control the inflammation actively after the surgery. Using the artificial tear drops conventionally can enhance the viscosity of ocular surface, improve stability of the tear film, ease the patients dry eye symptoms, and promote the reparation of corneal epithelium. D: Choose eye drops without preservatives or with low toxicity or decomposable preservatives. E: Give patients the instruction of not using their eyes too much at the early stage after LASIK to avoid eyes fatigue, and minimizing the frequency of using eye drops as possible as they can. F: We can block their lacrimal punctum for the patients who have obvious dry eye symptoms preoperative, or a nonhealing epithelial defect, or their dry eye symptoms persistent and obvious.
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