Ophthalmology – Normal-Tension Glaucoma

Basics

Description

Normal-tension glaucoma (NTG) is a form of primary open-angle glaucoma in which characteristic glaucomatous optic nerve damage and visual field loss occur despite measured intraocular pressure (IOP) remaining within the statistically normal range.

Typical features include:

• Open anterior chamber angles
• Glaucomatous optic nerve cupping
• Retinal nerve fiber layer loss
• Corresponding visual field defects
• No consistently documented untreated IOP above the normal statistical range

NTG is therefore a diagnosis of exclusion.

Important Concept

A “normal” IOP does not mean the pressure is safe for that particular optic nerve.

Some optic nerves may be damaged at relatively low IOPs because of:

• Structural susceptibility
• Vascular dysregulation
• Reduced ocular perfusion
• Thin corneas causing IOP underestimation
• Nocturnal or undetected IOP peaks

Epidemiology

The prevalence varies greatly between populations because of differences in:

• Diagnostic criteria
• IOP measurement methods
• Visual field definitions
• Population characteristics

NTG constitutes a substantial proportion of open-angle glaucoma, particularly in some Asian populations.

It is likely underdiagnosed because IOP may appear “normal” during routine examination.

Risk Factors

Important associated risk factors include:

• Increasing age
• Family history of glaucoma
• Female sex in some studies
• Migraine
• Raynaud phenomenon
• Obstructive sleep apnea
• Systemic hypotension
• Nocturnal blood pressure dipping
• Vascular dysregulation
• Thin central cornea
• Disc hemorrhage

Genetics

NTG is genetically heterogeneous.

Reported genes and loci include:

• OPTN – optineurin
• TBK1 in selected familial cases
• Other glaucoma-associated loci

Most cases are multifactorial rather than caused by a single mutation.

Genetic testing is not routinely required for typical NTG.

Pathophysiology

The final pathway is:

Retinal ganglion cell loss → optic nerve axonal loss → visual field loss

Multiple mechanisms likely contribute.

Role of Intraocular Pressure

Although IOP is within the statistically normal range, it remains the most important modifiable risk factor.

The Collaborative Normal-Tension Glaucoma Study demonstrated that approximately:

30% reduction from baseline IOP

reduces the risk of disease progression in many patients.

Thus:

NTG is not an IOP-independent disease.

Mechanical Mechanisms

Even “normal” levels of IOP may produce damage in a susceptible optic nerve.

Possible mechanisms include:

• Lamina cribrosa deformation
• Impaired axoplasmic transport
• Structural weakness of the optic nerve head
• Reduced tolerance to translaminar pressure gradients

Vascular Mechanisms

Vascular factors may contribute through inadequate optic nerve perfusion.

Potential mechanisms include:

• Low systemic blood pressure
• Excessive nocturnal hypotension
• Vascular dysregulation
• Vasospasm
• Migraine-related vascular instability
• Sleep apnea-associated nocturnal hypoxia

Ocular Perfusion Pressure

A simplified concept is:

Ocular perfusion pressure ≈ blood pressure − IOP

Therefore, optic nerve perfusion may fall because of:

• Increased IOP
• Reduced systemic blood pressure
• Both occurring simultaneously

This may be particularly relevant at night.

Associated Conditions

Common associations include:

• Migraine
• Raynaud phenomenon
• Obstructive sleep apnea
• Systemic hypotension
• Peripheral vascular dysregulation

These associations are not present in all patients.

Diagnosis

NTG should only be diagnosed after demonstrating:

• Typical glaucomatous structural damage
• Corresponding functional loss
• Open angles
• No convincing history of significantly elevated IOP
• No better explanation for the optic neuropathy

History

Important questions include:

• Visual symptoms
• Family history of glaucoma
• Previous IOP measurements
• Previous steroid use
• Ocular trauma
• Prior ocular surgery
• Migraine
• Raynaud symptoms
• Sleep apnea symptoms
• Snoring
• Daytime somnolence
• Systemic hypertension
• Antihypertensive medication timing
• Episodes of severe hypotension
• Blood loss or shock
• Neurologic symptoms

Visual Symptoms

Early disease is often asymptomatic.

Later symptoms may include:

• Difficulty with contrast
• Paracentral blur
• Reading difficulty
• Reduced peripheral vision

Central vision may remain good until advanced disease.

Pupillary Examination

A relative afferent pupillary defect may occur when damage is significantly asymmetric.

Slit-Lamp Examination

Look for signs suggesting secondary glaucoma, including:

• Pseudoexfoliation material
• Pigment dispersion
• Prior inflammation
• Iris transillumination defects
• Previous trauma or surgery
• Steroid-related changes

Gonioscopy

Gonioscopy is essential.

Typical NTG shows:

Open angles without a secondary cause of glaucoma

Gonioscopy also helps exclude:

• Angle closure
• Neovascularization
• Pigment dispersion
• Recession
• Inflammatory synechiae

Central Corneal Thickness

Pachymetry should be obtained.

A thin central cornea may:

• Cause Goldmann applanation IOP to underestimate true pressure
• Independently correlate with glaucoma risk

Therefore, an apparently low IOP should be interpreted in context.

Optic Nerve Findings

Typical glaucomatous findings include:

• Neuroretinal rim thinning
• Focal notching
• Vertical cup enlargement
• Cup-to-disc asymmetry
• RNFL defects
• Acquired optic nerve pits
• Disc hemorrhage
• Parapapillary atrophy

Disc Hemorrhage

Optic disc hemorrhage is particularly important in NTG.

It is associated with:

• Active disease
• Higher risk of progression
• Subsequent localized RNFL loss

A disc hemorrhage should prompt reassessment of:

• Target IOP
• Adherence
• Rate of progression

Neuroretinal Rim

Damage often preferentially involves the:

• Inferotemporal rim
• Superotemporal rim

This corresponds to characteristic arcuate visual field loss.

Optic Disc Pallor

Glaucoma causes cupping that is generally greater than pallor.

Marked pallor out of proportion to cupping should raise suspicion for another optic neuropathy.

Visual Field Findings

NTG may produce:

• Paracentral scotomas
• Nasal steps
• Arcuate defects
• Altitudinal-like defects
• Advanced generalized field constriction

Paracentral defects may occur relatively early and can threaten fixation.

Central Visual Field Testing

Because NTG can produce defects close to fixation, consider:

• Standard 24-2 or 24-2C testing
• 10-2 visual fields when central or paracentral damage is suspected

This can detect defects missed or underestimated by wider-spaced field strategies.

Optical Coherence Tomography

OCT is essential for documenting:

• RNFL thickness
• Macular ganglion cell complex
• Ganglion cell–inner plexiform layer
• Progressive structural loss

Macular OCT is particularly valuable when paracentral field loss is suspected.

Optic Disc Photography

Baseline and serial stereoscopic disc photography can document:

• Progressive rim thinning
• Disc hemorrhage
• RNFL changes

IOP Assessment

Single office IOP readings may miss clinically relevant pressure peaks.

Consider:

• Repeated measurements
• Measurements at different times of day
• Diurnal testing in selected patients

A patient classified as NTG may occasionally be found to have previously unrecognized pressure spikes.

Laboratory Testing

There is no routine laboratory test for NTG.

Testing should be guided by suspected associated or alternative disease.

For example:

• CBC if severe anemia is suspected
• ESR/CRP if arteritic ischemic optic neuropathy is a concern
• Other investigations based on systemic findings

When to Consider Neuroimaging

MRI of the brain and orbits should be considered when findings are atypical for glaucoma.

Red flags include:

• Young age
• Rapid progression
• Markedly asymmetric or unilateral disease
• Central visual acuity loss out of proportion to glaucoma
• Central scotoma
• Color vision loss disproportionate to field damage
• Neurologic symptoms
• Hemianopic field defect
• Optic disc pallor greater than cupping
• Unusual visual field pattern

Differential Diagnosis

Important mimics include:

• Compressive optic neuropathy
• Optic neuritis
• Non-arteritic anterior ischemic optic neuropathy
• Arteritic ischemic optic neuropathy
• Dominant optic atrophy
• Leber hereditary optic neuropathy
• Traumatic optic neuropathy
• Toxic or nutritional optic neuropathy
• Congenital optic nerve anomalies
• Optic nerve coloboma
• Optic disc pits
• Tilted disc syndrome

Previously Elevated IOP

Before labeling a patient as NTG, exclude previous periods of elevated IOP from:

• Steroid use
• Trauma
• Uveitis
• Ocular surgery
• Pigment dispersion
• Pseudoexfoliation
• Intermittent angle closure

Treatment Goals

The goal is to reduce IOP sufficiently to slow disease progression to a rate compatible with useful lifetime vision.

The target pressure is individualized according to:

• Baseline IOP
• Age
• Life expectancy
• Visual field status
• Rate of progression
• Central field involvement
• Fellow-eye status
• Treatment burden

Target IOP

A common initial target is approximately:

30% below untreated baseline IOP

based on the Collaborative Normal-Tension Glaucoma Study.

However, target IOP is dynamic and should be adjusted according to progression.

Observation

Not every untreated patient progresses rapidly.

Observation may be reasonable in selected patients with:

• Minimal damage
• No documented progression
• Advanced age
• Significant treatment burden

However, close structural and functional monitoring is essential.

Medical Therapy

Prostaglandin Analogs

Common first-line agents include:

• Latanoprost
• Travoprost
• Bimatoprost
• Tafluprost

Advantages include:

• Strong IOP lowering
• Once-daily dosing
• Minimal systemic cardiovascular effects

Rho Kinase Inhibitors

Agents such as:

• Netarsudil

may be useful as additional therapy, particularly when further IOP reduction is required.

Carbonic Anhydrase Inhibitors

Topical options include:

• Dorzolamide
• Brinzolamide

They may be used alone or in combination.

Alpha-2 Agonists

Brimonidine lowers IOP and is commonly used as adjunctive therapy.

Experimental neuroprotective effects have been proposed, but independent human neuroprotection remains unproven.

Beta-Blockers

Topical beta-blockers such as:

• Timolol

can effectively reduce IOP.

Use cautiously in patients with:

• Asthma
• Bradycardia
• Heart block
• Significant nocturnal hypotension

Because systemic blood pressure and optic nerve perfusion may be relevant in NTG, medication choice and timing should be individualized.

Miotics

Pilocarpine can lower IOP but is now used much less commonly because of:

• Frequent dosing
• Brow ache
• Induced myopia
• Reduced quality of life
• Retinal detachment considerations in susceptible eyes

Oral Carbonic Anhydrase Inhibitors

Agents such as:

• Acetazolamide
• Methazolamide

may be used temporarily in selected cases but are generally not suitable for long-term routine therapy because of systemic adverse effects.

Selective Laser Trabeculoplasty

SLT is an effective treatment option for NTG.

It may be used:

• As primary therapy
• As adjunctive therapy

Because baseline IOP is already relatively low, the absolute pressure reduction may be smaller than in high-pressure glaucoma.

Nevertheless, even modest additional IOP lowering can be clinically meaningful.

Filtering Surgery

When progression continues despite maximally tolerated medical or laser therapy, surgery may be required.

Options include:

• Trabeculectomy
• Glaucoma drainage device in selected circumstances

Trabeculectomy in NTG

Trabeculectomy can achieve very low IOP levels and is often the most effective surgical method when a very low target is required.

However, NTG patients have a relatively narrow therapeutic window between:

• Desired low IOP
• Excessive hypotony

Therefore, careful postoperative management is essential.

Hypotony Risk

Potential complications include:

• Hypotony
• Hypotony maculopathy
• Choroidal effusion
• Shallow anterior chamber

This is particularly relevant when very low postoperative IOP is sought.

Minimally Invasive Glaucoma Surgery

MIGS may provide useful IOP reduction in selected patients, especially when combined with cataract surgery.

However, angle-based MIGS is limited by:

Episcleral venous pressure

and may not achieve the very low target pressures required in advanced or rapidly progressive NTG.

Tube Shunts

Glaucoma drainage devices may be considered when:

• Trabeculectomy is unsuitable
• Previous filtration surgery has failed
• Conjunctival scarring is significant

They are not necessarily the first surgical choice when extremely low IOP is required.

Cyclodestructive Procedures

Cyclophotocoagulation is generally reserved for:

• Refractory glaucoma
• Eyes with limited visual potential
• Selected surgical circumstances

It is not usually first-line treatment for typical NTG.

Systemic Vascular Considerations

Management should also address potentially relevant systemic factors.

Consider evaluation for:

• Obstructive sleep apnea
• Significant nocturnal hypotension
• Severe anemia
• Cardiovascular disease

Nocturnal Hypotension

In patients who progress despite low IOP, ask whether antihypertensive medications are taken at bedtime.

Excessive overnight blood pressure reduction may potentially reduce optic nerve perfusion.

However:

Antihypertensive therapy should not be stopped or changed without coordination with the treating physician.

Sleep Apnea

Patients with:

• Loud snoring
• Witnessed apneas
• Daytime somnolence
• Morning headaches

should be considered for evaluation of obstructive sleep apnea.

Neuroprotection

Many neuroprotective strategies have been investigated.

At present:

No IOP-independent neuroprotective medication has been definitively proven to prevent glaucomatous progression in routine clinical practice.

Effective IOP lowering remains the main evidence-based treatment.

Referral

Consider neuro-ophthalmology referral when:

• The diagnosis is uncertain
• Visual loss is atypical
• Pallor exceeds cupping
• Visual fields suggest neurologic disease
• Progression is unusually rapid

Low-vision referral is appropriate when visual disability affects daily function.

Follow-Up

After starting or changing treatment, reassess:

• IOP response
• Medication tolerance
• Adherence

High-risk patients may require early review.

Once stable, follow-up is commonly every:

3–6 months

depending on disease severity and progression.

Monitoring

Long-term monitoring should include:

• IOP
• Optic nerve examination
• Disc photographs
• OCT RNFL
• Macular ganglion cell analysis
• Standard automated perimetry
• 10-2 fields when central damage is present

Rate of Progression

The most important long-term question is:

How fast is the disease progressing?

A young patient with slow progression may still accumulate major lifetime visual loss, whereas an older patient with stable mild damage may require less aggressive treatment.

Patient Education

Patients should understand that:

• Normal-range IOP does not exclude glaucoma.
• Treatment still focuses on lowering pressure.
• Medication adherence is essential.
• Regular visual fields and OCT are necessary.
• Progression can occur even without symptoms.

Prognosis

Prognosis depends primarily on:

• Age
• Baseline damage
• Rate of progression
• Presence of central field defects
• Ability to reduce IOP
• Adherence to follow-up

Many patients maintain useful vision throughout life when progression is recognized early and adequately slowed.

Fellow Eye

Patients with unilateral or asymmetric NTG remain at risk for development or progression of glaucomatous damage in the fellow eye.

Both eyes require continued surveillance.

Complications

Untreated or inadequately controlled disease can cause:

• Progressive visual field loss
• Paracentral scotoma
• Fixation-threatening defects
• Severe peripheral field loss
• Permanent visual impairment
• Blindness in advanced disease

Treatment-related complications include:

• Medication adverse effects
• Laser-related inflammation or pressure spikes
• Surgical hypotony
• Infection
• Filtration failure

Ophthalmology Pearls

• NTG = glaucomatous optic neuropathy with open angles and IOP that remains within the statistically normal range.
• NTG is a diagnosis of exclusion; do not assume every cupped optic nerve with normal IOP is glaucoma.
• A “normal” IOP may still be too high for an individual optic nerve.
• Disc hemorrhage is an important marker of progression, especially in NTG.
• NTG commonly produces paracentral visual field defects, so 10-2 testing can be valuable.
• Thin corneas may cause measured IOP to underestimate the true pressure.
• The Collaborative Normal-Tension Glaucoma Study supports an initial target of roughly 30% IOP reduction from baseline in patients requiring treatment.
• SLT remains useful, although the absolute IOP reduction may be smaller because baseline pressure is already low.
• Trabeculectomy may be required to reach very low target pressures, but hypotony is an important risk.
• Migraine, Raynaud phenomenon, sleep apnea, and excessive nocturnal hypotension may be relevant systemic associations.
• Marked optic disc pallor, rapid progression, central acuity loss, or neurologically patterned field defects should prompt neuroimaging rather than automatic labeling as NTG.
• IOP reduction remains the only established treatment proven to slow glaucomatous progression.