George L. Spaeth

BASICS

DESCRIPTION

Normal tension glaucoma (NTG) is used to describe glaucomatous damage with statistically normal intraocular pressure (IOP).

EPIDEMIOLOGY

Incidence

• There is wide discrepancy in the literature regarding prevalence and incidence of NTG due to inconsistent definitions of the disease, different methods of evaluation of the optic nerve and visual field, and different populations studied.

• NTG is likely an under-reported condition.

• The incidence of chronic open angle glaucoma (COAG) was estimated to be: 0.24% per year in Dalby, Sweden, 0.1% per year in Melbourne, Australia, and 0.55% per year in Barbados.

• NTG accounts for 40–75% of patients with newly diagnosed COAG (1).

Prevalence

• In the Baltimore Eye Survey, prevalence of COAG was: African–Americans 1.23% (40–49 years old) and 11.26% (80 years or older); Caucasians 0.92% and 2.16%, respectively.

– 16.7% of glaucomatous eyes never had a recorded IOP exceeding 21 mm Hg.

• In 7 regions throughout Japan, the prevalence of NTG in patients older than 40 years is ∼2.04%. (2)[B]

RISK FACTORS

• Migraine

• Female gender (3)[A]

Genetics

• The following genes have been reported as causative for NTG; however, the associations between these genes and NTG are either weak, or often not replicated in other populations (4)[B]:

– Apolipoprotein E

– Optic atrophy 1

– Optineurin (associated with a significant fraction of autosomal dominant, NTG cases)

• Other unknown genetic factors likely play a role in the development of NTG.

GENERAL PREVENTION

There is no known prevention of NTG prior to manifestations of the disease.

PATHOPHYSIOLOGY

Retinal ganglion cell damage and death leads to decrease in visual function in patients with glaucoma.

ETIOLOGY

• Intraocular pressure:

– Studies have shown a significant influence of IOP on the progression of visual field or neuroretinal rim damage.

– In the Collaborative Normal Tension Glaucoma Study, IOP reduction of at least 30% is significantly associated with protection of visual field and nerve status compared to untreated patients (∼20% of treated eyes and ∼40% of untreated eyes had glaucomatous progression within 3 years).

• Ocular vascular abnormalities:

– Drance described 2 forms of NTG – a nonprogressive form perhaps associated with transient vascular compromise, and a more common progressive form possibly due to chronic vascular insufficiency of the optic nerve head (1).

COMMONLY ASSOCIATED CONDITIONS

• Migraine headaches

• Raynaud’s phenomenon

• Sleep apnea (1)[B]

DIAGNOSIS

HISTORY

• Presence, onset, and nature of visual disturbance

• Systemic vascular risk factors such as diabetes and hypertension

• History of ocular trauma or surgeries

• Family history of glaucoma or other optic neuropathies

• Neurological disturbances such as headaches, dizziness, and weakness of extremities

PHYSICAL EXAM

• Check for an afferent pupillary defect.

• Slit-lamp examination of the anterior segment to check for signs of secondary glaucoma, such as iris transillumination defects, pseudoexfoliation material, anterior chamber cell or flare, and signs of prior surgery or trauma

• Gonioscopy

• Direct and indirect stereoscopic slit-lamp biomicroscopy to evaluate the optic nerve head, paying particular attention to:

– Narrowing of the neuroretinal rim width (relatively narrow rim inferotemporally or superotemporally can indicate glaucomatous damage)

– Asymmetry in disc cupping

– Optic disc hemorrhage

– Optic nerve color

– Beta zone parapapillary atrophy

– Loss of retinal nerve fiber layer

– Acquired pit of the optic nerve head

DIAGNOSTIC TESTS & INTERPRETATION

Lab

Initial lab tests

Lab tests as indicated for clinical suspicion of other or associated conditions.

Follow-up & special considerations

If an oral carbonic anhydrase inhibitor is prescribed for treatment, blood work including CBC and creatinine levels should be obtained.

Imaging

Initial approach

• Optic disc photographs should be obtained to evaluate and follow patients through their course of treatment.

• Other optic nerve head imaging devices are also currently being used for optic nerve head assessment:

– Scanning laser polarimetry

– Heidelberg retinal tomography

– Optical coherence topography

Follow-up & special considerations

Optic disc imaging should be obtained regularly (e.g., annually, or more often if a change in optic disc appearance is suspected) to document and assess the optic nerve head for change.

Diagnostic Procedures/Other

• Visual fields should be obtained at baseline and at regular intervals (e.g., annually, or more often if a change is suspected).

• Diurnal IOP testing may be useful if large IOP fluctuations or periods of IOP spike are suspected.

• Consider neuroimaging for:

– Loss of central visual acuity or central visual field

– Acute onset or rapidly progressive visual loss

– Markedly asymmetric or strictly unilateral optic neuropathy

– Hemianopic visual field loss

– Neuroretinal rim pallor (5)[B]

Pathological Findings

Atrophy of the retinal ganglion cell layer can be noted in advanced glaucoma.

DIFFERENTIAL DIAGNOSIS

• Optic nerve anomalies including coloboma, pits, oblique insertion

• Autosomal dominant optic atrophy

• Glaucoma associated with elevated (higher than normal) intraocular pressure – for example, past history of trauma or surgery that may have led to elevated IOP, wide diurnal fluctuation in IOP, and past history of steroid use

• Hemodynamic crisis

• Methyl alcohol poisoning

• Optic neuritis

• Arteritic ischemic optic neuropathy

• Nonarteritic ischemic optic neuropathy

• Compressive lesions of optic nerve and tract

• Traumatic optic neuropathy

TREATMENT

MEDICATION

First Line

• Prostaglandin agonists (e.g., latanoprost, bimatoprost, travoprost)

• Topical carbonic anhydrase inhibitors (e.g., dorzolamide, brinzolamide)

• Beta-blockers (e.g., timolol, levobunolol) –

• Selective alpha 2 receptor agonists

• Miotics (e.g., pilocarpine)

Second Line

Systemic carbonic anhydrase inhibitors (e.g., methazolamide, acetazolamide).

ADDITIONAL TREATMENT

General Measures

• The goal is to achieve and maintain an intraocular pressure level in which there will presumably be no further optic nerve damage.

• This intraocular pressure level is dynamic and should be reevaluated at each visit – this level is determined by history, examination, and risk factors for progression.

• If the intraocular pressure cannot be maintained low enough to prevent optic nerve damage with noninvasive therapy, glaucoma filtering surgery may be indicated.

• It is important to keep in mind the expense, inconvenience, and side effects of therapy.

Issues for Referral

• Neuro-ophthalmologist – if there is suspicion that the cause of the patient’s optic neuropathy or visual field defect is neurological.

• Low-vision specialist – for patients who are functionally inhibited by their quality of vision.

Additional Therapies

Neuroprotective agents are currently being studied for treatment of glaucoma patients.

SURGERY/OTHER PROCEDURES

• Laser procedures, for example, argon laser trabeculoplasty, selective laser trabeculoplasty:

– Laser procedures have been shown to be effective in lowering intraocular pressure.

– They should be used with caution in patients with highly pigmented trabecular meshwork.

– Laser procedures can be considered first line therapy in some patients.

• Filtering procedures:

– Trabeculectomy

– Tube shunt implant (e.g., Ahmed, Baerveldt, Molteno)

• Cyclodestructive procedures are not the first operation of choice because of their destructive nature and potential risk of damage to adjacent ocular structures:

– Endocyclophotocoagulation

– Laser transscleral cyclophotocoagulation

ONGOING CARE

FOLLOW-UP RECOMMENDATIONS

• Patients are reexamined after starting a new medication or after laser or surgical procedures to evaluate the efficacy.

• If the patient is at high risk for visual loss at their intraocular pressure level, it may be reasonable to recheck the patient within days to ensure that adequate intraocular pressure lowering is achieved with therapy.

• Once the patient has achieved an intraocular pressure level that is at low risk for causing additional glaucomatous damage, they can be reevaluated in 3–6 month intervals.

Patient Monitoring

See “Follow-Up Recommendations” and “Diagnostic Tests and Interpretations” sections.

PATIENT EDUCATION

It is important to counsel patients on the importance of adherence to medical therapy and with ophthalmological follow up for treatment of glaucoma.

PROGNOSIS

• Prognosis of glaucoma depends on the patient’s age and rate of progression.

• The prognosis is good if the patient’s rate of progression can be slowed to a rate such that they maintain good vision for their lifetime.

• Normal tension glaucoma patients with unilateral field loss are at high risk of developing field damage in the fellow eye (6)[A].

COMPLICATIONS

Uncontrolled glaucoma can lead to loss of vision.

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