Efficacy and Safety Study of Combigan and 0.5% Timoptic in Normal Tension Glaucoma

Inclusion Criteria: * over 18 years old * baseline IOP under 22 mmHg,both anterior chamber angle \> Grade 3\~4 by Shaffer grading system Exclusion Criteria: * IOP over 22 mmHg by Goldmann applanation tonometer * other types of glaucoma except open angle * other IOP lowering treatment * chronic or recurrent Hx. of ocular inflammation * using contact lens * any other ocular disease that could affect visual field examination (diabetic retinopathy, macular disease, uveitis, scleritis, optic neuritis, anterior ischemic optic neuropathy) * intraocular or glaucoma surgery within 6 months * Hx. of allergic reaction to timolol or brimonidine * bronchial asthma * moderate to severe chronic obstructive pulmonary disease * heart failure * 2\~3 degree A-V block, * MAO inhibitor use * anti depressant use * untreated pheochromocytoma * pregnant