Normal tension glaucoma

Normal Tension Glaucoma (NTG), also known as Low-Tension Glaucoma or Normal-Pressure Glaucoma, is a condition where the optic nerve is damaged despite intraocular pressure (IOP) being within the normal range. Unlike other forms of glaucoma, where high eye pressure is a clear risk factor, NTG challenges our understanding of glaucoma's pathophysiology.

Etiology

The exact cause of NTG remains unclear. However, several theories suggest that factors other than IOP, such as reduced blood flow to the optic nerve and a highly sensitive optic nerve that can be damaged at lower pressures, play a role. Risk factors include a family history of NTG, migraine, systemic heart disease such as ischemic heart disease, and Japanese ethnicity.

Pathophysiology

In NTG, optic nerve damage occurs despite normal IOP levels, which are typically between 12-22 mm Hg. This suggests that factors other than IOP contribute to the disease's progression. Theories include vascular dysregulation leading to insufficient blood supply to the optic nerve and a predisposition of the optic nerve to damage at lower pressures.

Symptoms

NTG progresses silently, often without symptoms until the late stages. When symptoms do appear, they may include gradual loss of peripheral vision, which can advance to tunnel vision and, eventually, to blindness if untreated.

Diagnosis

Diagnosis of NTG involves a comprehensive eye examination, including measuring IOP, assessing the damage to the optic nerve through ophthalmoscopy, and testing the visual field. Since IOP is not elevated, diagnosis relies heavily on observing the optic nerve's condition and visual field testing.

Treatment

Treatment aims to lower the IOP further to slow the progression of optic nerve damage. Options include medications, laser therapy, and surgery. Medications, such as prostaglandin analogs, beta-blockers, and carbonic anhydrase inhibitors, are often the first line of treatment. Laser therapy and surgery are considered when medication fails to halt disease progression.

Prognosis

The prognosis for NTG varies. With early detection and treatment, individuals can manage the condition effectively, preserving their vision. However, without treatment, NTG can lead to significant vision loss and blindness.

Epidemiology

NTG is more common in individuals of Japanese descent and tends to affect both eyes. It is more prevalent in women and the elderly. The global prevalence of NTG is not well-documented, but it is a significant cause of blindness worldwide.

Research Directions

Current research on NTG focuses on understanding its pathophysiology, identifying genetic markers, and developing new treatment options. Studies on vascular factors and neuroprotective treatments are particularly promising.