NORMAL TENSION GLAUCOMA
The normal tension glaucoma (NTG), means as low tension glaucoma when typical glaucomatous disc changes with or without visual field defects are associated with an intraocular pressure (IOP) constantly below 21 mm of Hg.
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 and its prevalence above the age of 40 years is 0.2%.
Etiopathogenesis
It is believed to result from chronic low vascular perfusion, which makes the optic nerve head susceptible to normal IOP. This view is supported by following association which are more common in NTG than in POAG :
*� Raynauld phenomenon i.e., peripheral vascular spasm on cooling, Migraine, Nocturnal systemic hypotension and over treated systemic hypertension.
*� Reduced blood flow velocity in the ophthalmic artery (as revealed on transcranial Doppler ultrasonography).
Clinical features
As described in definition the clinical features of NTG (disc changes and visual field defects) are similar to POAG, but the IOP is consistantaly below 21mm Hg.
Other characterstic features of NTG are some associations mentioned in the etiopathogenesis.
Differential diagnosis
1. POAG. In early stages POAG may present with normal IOP because of a wide diurnal variation.
Diurnal variation test usually depicts IOP higher than 21 mm of Hg at some hours of the day in patients with POAG.
2. Congentical optic disc anomalies such as large optic disc pits or colobomas may be mistaken for acquired glaucomatous damage.
A careful examination should help in differentiation.
Treatment
1. Medical treatment to lower IOP. The aim of the treatment is to lower IOP by 30% i.e., to achieve IOP levels of about 12-14 mm of Hg. Some important facts about medical treatment of NTG are:
*� Betaxolol may be considered the drug of choice because in addition to lowering IOP it also increases optic nerve blood flow.
*� Other beta blockers and adrenergic drugs (such as dipiverafrine) should better be avoided (as these cause nocturnal systemic hypotension and are likely to affect adversely the optic nerve perfusion).
*� Drugs with neuroprotective effect like brimonidine may be preferred.
*� Prostaglandin analogues, e.g., latanoprost tend to have a greater ocular hypotensive effect in eyes with normal IOP.
2. Trabeculectomy may be considered when progressive field loss occurs despite IOP in lower teens.
3. Systemic calcium channel blockers (e.g. ,nifedipine) may be useful in patients with confirmed peripheral vasospasm.
4. Monitoring of systemic blood pressure should be done for 24 hours. If nocturnal dip is detected, it may be necessary to avoid night dose of anti-hypertensive medication.
OPTOMETRY-SHARP VISION
Optometrist
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