Normal pressure glaucoma – NPG
Normal-tension glaucoma (NTG) is a progressive degeneration of the optic nerve when intraocular pressure remains within the normal range (generally between 9 and 21 mmHg), unlike classic glaucoma. It is also called low-tension or normal-tension glaucoma. Its origin is uncertain; it is either a distinct form of primary open-angle glaucoma (POAG) or a separate optic nerve degeneration from POAG. It results in a more central and aggressive loss of visual field than in classic glaucoma.
What is normal pressure glaucoma?
Normal pressure glaucoma is a degeneration of the optic nerve, caused by hypersensitivity to eye pressures that are otherwise within normal limits.
The optic nerve "ages" too quickly (definition of glaucoma) and without any identifiable tension factor.
This leads to a loss of visual field and sometimes vision, which can even result in blindness.
A reminder about intraocular pressure and glaucoma
Intraocular pressure standards are set between 9 and 21 mmHg (millimeters of mercury)
In the event of increased intraocular pressure, the optic nerve can be damaged in an accelerated manner: this is glaucoma.
Fortunately, not everyone with high eye pressure will develop glaucoma.
Conversely, some patients will develop glaucoma even though their intraocular pressure is low.
How is it different from primary open-angle glaucoma?
Primary open-angle glaucoma and normal-tension glaucoma are two optic nerve degenerations that lead to visual field loss. However, certain characteristics are more frequently observed in patients with normal-tension glaucoma than in patients with the classic form of glaucoma (primary open-angle glaucoma).
Visual field defects are more often central in GPN.
Optic nerve hemorrhages are more frequent.
Patients are more frequently affected by migraines or sleep disorders (obstructive sleep apnea syndrome – OSA)
Causes
The pathophysiology (origin) of normal-tension glaucoma is still poorly understood. The two most frequently proposed hypotheses are:
A structural weakness in the tissues of the optic nerve
A defect in arterial blood supply to the optic fibers
Research is ongoing to understand why optical fibers are damaged when intraocular pressure is strictly normal.
Normal-tension glaucoma is more difficult to diagnose than primary open-angle glaucoma. This is because ophthalmologists are not alerted by a sudden increase in intraocular pressure.
The diagnostic assessment is, however, the same. The ophthalmologist notes normal intraocular pressure, but upon examination of the optic nerve, he observes that it presents a hemorrhage or is abnormally cavitated (hollow), indicating a loss of optic fibers.
If there is any diagnostic uncertainty, the ophthalmologist will supplement the examinations with optical coherence tomography (OCT). This test counts the optic nerve fibers and compares the patient to a database. A visual field test may also be performed to detect any vision loss.
Finally, a brain MRI, a sleep study (polysomnography) and a cardiovascular assessment are frequently requested, in order to rule out optic nerve compression, apneas and narrowing of the neck vessels which may explain a lack of blood flow to the optic nerve.
Normal pressure glaucoma is often asymptomatic for a long time. Your vision will remain normal for a long time, while the optic nerve deteriorates.
You will only experience losses in the visual field and decreased vision from a very late stage and poor prognosis.
That is why regular screening by an ophthalmologist is essential.
Normal-tension glaucoma is treated similarly to primary open-angle glaucoma. Treatment aims to artificially lower intraocular pressure.
Hypotensive eye drops – With prostaglandin analogues (Monoprost®) as the first line of treatment.
SLT laser – Less effective than in classic glaucoma.
Filtering surgery – In order to smooth out eye strain throughout the day.
Cardiovascular risk factors must also be addressed:
A sleep apnea syndrome is treated with a device.
In cases of carotid artery stenosis (neck arteries), surgical intervention should be considered.
In cases of nocturnal hypotension, blood pressure treatments may be adjusted.
-
Normal-tension glaucoma has a less favorable prognosis than primary open-angle glaucoma. This is because its mechanism is less well understood and diagnosis is often delayed due to the absence of ocular hypertension.
-
Regular visual field testing and OCT scans are necessary. Strictly follow the follow-up schedule recommended by your ophthalmologist to improve your vision prognosis.