A careful history of the patient is necessary to determine eligibility prior to fitting the patient with EW silicone hydrogel contact lenses. Patients should provide detailed information regarding their past ocular and general health. Motivational factors and compliance with instructions need to be assessed and addressed when the patient history is recorded. EW contra indications include: • diabetes • compromised immune system • conditions that require steroid or other systemic medication • Severe seasonal allergies • Previous reactions to solutions or to hydrogel contact lenses. Caution is advised with patients who have had previous episodes of CLARE or CLPU. The risk of recurrence needs to be taken in account in these cases.
The PoLTF is the tear layer between ocular surface and contact lens back surface. It is made up of a thin aqueous phase (1µm) and of a compressed mucin phase (30-40µm). The mucin phase is of unknown thickness but it is probably at least marginally thinner than the pre-ocular tear film. The maintenance of the integrity of the mucin layer is essential to safe contact lens wear. The maintenance of the aqueous phase is essential to controlling the viscosity of the PoLTF and facilitating the elimination of back surface debris. The closed eye wearing period with it’s associated aqueous depletion and reduced lens movement produces the most challenging phase in EW. The period immediately upon eye opening is therefore the key recovery phase when PoLTF aqueous replenishment takes place. Regarding silicone hydrogels, the following clinical practice is recommended: • Follow-up visits should take place in the morning and include the evaluation of the PoLTF and the assessment of lens movement. • Patients should be instructed to carry out self-evaluation of lens binding upon waking. • Non-viscous eye drops such as saline (preferably single dose eye drops) should be dispensed for use upon waking. Also see Lubricating drops.
The PrLTF is the tear layer on top of the contact lens front surface. It is alternately in contact with the air and tarsal conjunctiva. It is made up of an outer lipid layer and a base aqueous layer. The aqueous layer is usually of reduced thickness (3-4µm) compared to the pre-ocular aqueous layer (7µm).
Preservatives are compounds that are present in contact lens care solutions whose function is to kill or prevent the growth of microorganisms.
The push-up test is a two-step assessment of the lens dynamic performance to determine lens tightness: 1. The resistance to decentration is evaluated. For this the lens is gently dislodged by digital upward manipulation of the lower eyelid border against the lower edge of the contact lens. 2. When the pressure of the lower eyelid is removed type and speed of the recentration movement are judged.