X-linked retinoschisis (XLRS) is a rare hereditary eye disease that causes irreversible vision loss in boys and young men. This disease occurs in 1 in 10,000-30,000. This inherited condition is caused by pathogenic variants in a single gene, namely the Retinoschisin 1 (RS1). This gene encodes the retinoschisin protein. Pathological variants of retinoschisin lead to loss of retinal integrity, resulting in the characteristic cystoid fluid collections (CFC). From a young age, XLRS patients experience a gradual deterioration of vision. In middle-aged patients however, XLRS may be associated with macular atrophy because of the confluence of the cystoid lesions. No permanent treatment is yet available for XLRS patients. Currently, two different phase I/II studies are investigating the safety and effectivity of subretinal gene therapy. To create optimal retinal condition before gene therapy, CFC, a hallmark of XLRS, should not be present. Topical and oral carbonic anhydrase II inhibitors are used to combat CFC. This drug is still off-label prescribed for various hereditary retinal dystrophies. Consequently, there is no treatment regimen for prescribing acetazolamide to XLRS patients. A thorough understanding of the safety and efficacy of acetazolamide in reducing the central foveal thickness in XLRS patients is required before applying future gene therapy.

The proposed study is a investigator-initiated, single-center, prospective, experimental study consisting of seven visits at 2, 4, 12, 16, 20 and 32 weeks after the baseline evaluation visit. During each visit, participants will perform several ophthalmological measurements. In this study, participants with XLRS will be randomized into either a treatment or control group. The null-hypothesis of this study is that acetazolamide effectively reduces the central foveal thickness in patients with XLRS and significantly improves their visual function. The alternative hypothesis is that acetazolamide reduces not effectively the central foveal thickness in patients with XLRS and has no significant impact on their visual function. Treatment success will be based not only on anatomical improvement, but also on functional endpoints, which are most important from a patient's perspective. The study will last 32 weeks per participant. Each participant will come physically for seven visits. The whole study will last for max. 24 months. The examinations and number of visits are reduced to a minimum. In contrast to clinical care, the participants receive examinations that consist of a more extensive measurement of visual acuity, microperimetry and a questionnaire. These extra examinations are required to evaluate the functional vision-related endpoints of the study.

Inclusion Criteria:

• Patients with XLRS with cystoid fluid collections involving the fovea confirmed on SD- OCT
• are willing to undergo ophthalmic examinations at seven separate occasions;
• have no visual dysfunction that is also significantly associated with other ocular diseases besides XLRS (e.g., glaucoma, perforating trauma);
• have no known (non-)ocular disease/disorder which may influence the results of the measurements.

Exclusion Criteria:

• Severe hepatic impairment
• Severe renal insufficiency
• Sodium and Potassium Depletion
• Addison's disease
• Hyperchloremic Acidosis
• Cor pulmonale
• Chronic non-congestive angle-closure glaucoma
• Usage of acetazolamide
• Known allergy or intolerance for ocular anesthetic eye drops oxybuprocaine 0.4% or mydriatics tropicamide 0.5% and/or phenylephrine 5%;