This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Glaucoma, Subject name: Ophthalmology
A 70-year-old patient presents with progressive deterioration of vision. On examination, the pupillary reaction is sluggish and the lOP is normal. Fundoscopy shows a large and deep cup. Visual field reveals paracentral scotoma. What is the probable diagnosis 
Select and explain. 
A. Normal tension glaucoma, B. Absolute glaucoma, C. Primary angle closure glaucoma, D. Neovascular glaucoma
A: Normal tension glaucoma
Answer: A: Normal tension glaucoma 
Explanation: Normal Tension Glaucoma An intraocular pressure of less than 21 mm Hg is recorded in about 15% of patients with all the other features of POAG. This condition is labelled as Normal tension glaucoma. Characterstically the angle of anterior chamber is open on gonioscopy and there is no secondary cause for glaucomatous disc changes. NTG is varient of POAG which accounts for 16% of all cases of POAG. Etiopathogenesis : It is believed to result from chronic low vascular perfusion, which makes the optic nerve head susceptible to normal IOP. This view is suppoed by following association which are more common in NTG than in POAG: 1) Raynauld phenomenon i.e., peripheral vascular spasm on cooling. 2) Migraine, 3) Nocturnal systemic hypotension and overeated systemic hypeension. 4)Reduced blood flow velocity in the ophthalmic aery. Ref:- A K KHURANA; pg num:-224