Normal Tension Glaucoma
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Title: Normal Tension Glaucoma
Authors: William Clark, BA; Brian Stagg, MD
Date: 05/22/20
Keywords/Main Subjects: Glaucoma, Open-Angle Glaucoma, Normal Tension Glaucoma
- Definitions
-Glaucoma: array of clinical disorders characterized by progressive optic neuropathy and visual field loss1
-Normal intraocular pressure (IOP): Gaussian distribution with an established mean of 15 mmHg and upper limit of normal of 21 mmHg2,3,4
-Normal tension glaucoma (NTG): open chamber angles and glaucomatous optic neuropathy without a measured elevation (>21mmHG) in IOP 1,5
- Characteristic glaucomatous clinical findings
-Optic disc: Progressive inferior and superior temporal rim thinning; focal rim atrophy or “notching”; disc hemorrhages; increased inferior and superior cup-to-disc ratios6,7
-Retinal nerve fiber layer (RNFL): focal band-like defects and diffuse atrophy in the peripapillary region8
-Visual field: Nasal step-like defects, paracentral and arcuate scotomas obeying the horizontal midline9,10
- Differential Diagnosis
–Overlooking NTG: in the setting of normal IOP, diagnosis requires clinical suspicion and vigilant optic nerve examination; single center study found NTG was diagnosed via population screening 4 times more often than during routine clinical examination11
–Diagnosis of exclusion: imperative to investigate multiple etiologies for optic neuropathy and visual field loss; masquerading syndromes are often misdiagnosed as NTG and vice versa12,13,14
-Nonglaucomatous etiologies:
-Developmental: colobomas, megalopapilla, congenital pits, tilted optic nerve, physiologic cupping [visual field defects are non-progressive]12,1
-Acquired: compressive lesions of anterior visual pathway, anterior ischemic optic neuropathy (AION), optic neuritis, syphilis, toxic-metabolic optic neuropathy (methanol poisoning, heavy metal exposure, B12/folate deficiency) [predominately optic nerve pallor, but may cause cupping]12,14,15
-Glaucomatous etiologies:
-Primary open angle glaucoma: underestimated IOP [ruled out with pachymetry and diurnal curve of IOP measurement]12
-Secondary open angle glaucoma: pigment dispersion, exfoliation syndrome, neovascularization, inflammation [ruled out with anterior segment examination]12
-Angle-closure glaucoma [ruled out with gonioscopy]
-Previous insult: steroids, uveitis, trauma, systemic hypotension [visual deficits typically stable]12
- Key findings from NTG Trials
-Collaborative Normal Tension Glaucoma Study (CNTGS):
-IOP reduction by 30% lowers risk of visual field progression 16
-Majority (65%) of non-treated eyes showed no progression after 5 years and 12% of subjects progressed despite IOP reduction16
-Benefits of IOP reduction on visual field progression blunted by cataract formation17
-Natural history and risk factors should be weighed before initiating aggressive IOP reduction therapy17
-Low-Pressure Glaucoma Treatment Study (LGTS):
-Brimonidine tartrate 0.2% and timolol maleate 0.5% have similar efficacy in lowering IOP18
-Brimonidine is more likely to prevent progressive visual loss than timolol18
-9% of brimonidine patients had visual field progression as opposed to 39% of timolol patients over the 4 year study period18
-Brimonidine patients had a 28% drop-out rate due to ocular side effects, compared to a 11% drop-out rate in timolol patients18
-Proposed explanations: timolol may cause greater systemic hypotension and reduced ocular perfusion, brimonidine has demonstrated neuroprotective effects in animal models18
Clinical Epidemiology
- Prevalence
-Asian populations: NTG represents 47-92% of open angle glaucoma (OAG) cases19,20
-White populations: NTG represents 30-39% of OAG cases19,20
-African populations: NTG represents 21% of OAG cases in a Ghanaian population and 57% in a South African population21,20
- Risk Factors
-High-normal IOP and wide diurnal IOP fluctuation22
-Systemic hypotension, especially nocturnal hypotension23,24,25
-Vasospastic disease (especially migraine and Raynaud’s phenomenon)26,27
-Sleep apnea28
-Myopia19
-Large optic discs29
-Family history30
Pathophysiology
- Proposed mechanisms
-Vascular dysregulation: leads to unstable ocular perfusion, ischemia and optic nerve damage31,12
-Elevated translaminar pressure: may be due to structural variation in the lamina cribosa or low intracranial pressure 32,33
-Increased sensitivity to normal IOP: may be due to anatomic or genetic variation34
Evaluation
- History
-History of Present Illness: [assess for episodes of acute vision loss or color vision loss; visual impairment in everyday activities; episodes of eye irritation]35,12
-Pertinent Review of Systems:[assess for dizziness, headaches, weakness, syncope, fatigue, vasospasm]36
-Past Medical/Surgical History: [assess for anemia, cardiovascular disease, hypotensive crisis, hypercholesterolemia, diabetes, respiratory disease, renal disease, vasospasm, migraine headaches, sleep apnea, vitamin deficiency, heavy metal exposure]35,12
-Past Ocular History: [assess for myopia, glaucoma, inflammation, trauma, steroid use, elevated IOP, ocular surgery including laser in situ keratomileusis (LASIK)]12,35
-Family History: [assess for congenital ocular disease, blindness and glaucoma] 1
-Medication History: [assess with special attention to steroid and antihypertensive use]12
- Baseline Examination
-Best-corrected visual acuity (VA): VA disproportionate to optic nerve exam or visual field testing warrants further workup12
-Pupillary response: abnormal pupillary light reflexes may occur37
-Intraocular pressure: measurement <22 mmHg required for diagnosis12
-Slit lamp examination: exclusion of masquerading syndromes required
-Gonioscopy: exclusion of angle closure and angle recession required; narrow-angle NTG shown to be more progressive than wide-angle NTG38
-Dilated fundus examination: increased frequency of splinter hemorrhages near the optic nerve head and more localized RNFL defects near fixation compared to OAG39,40
-Visual fields: (automatic static perimetry) scotomas are deeper, more focal and closer to fixation compared to OAG; dense paracentral scotomas are common41,42,12
- Adjunctive procedures of value
-Pachymetry: central corneal thickness (CCT) affects IOP measurement, especially when measured via applanation tonometry (thinner CCT underestimates IOP)43,12,35
-Color vision testing: abnormalities raise suspicion for underlying neurological disorder12,13
-Neuroimaging: several patient factors have been linked with a higher probability of positive neuroradiological findings (age<50, lower visual acuity, and optic rim pallor>cupping)30
-Laboratory workup: toxic-metabolic [heavy metals and vitamin B12/folate], infectious [syphilis and lyme antibody testing], and inflammatory [erythrocyte sedimentation rate and C-reactive protein levels]12
-24-hour blood pressure monitoring: assesses ocular blood flow12
-Optical coherence tomography: provides quantitative imaging of optic nerve head and RNFL35
Management
- General Principles
-Target IOP: reduce IOP relative to the extent of glaucomatous damage and risk factors for progression, with 30% reduction as a general target1,16,35
-Forms of therapy: medication, laser trabeculoplasty and incisional surgery35
-Mitigate risk factors: coordinate with primary care physician regarding adequate cardiac/respiratory/renal functioning, sleep apnea, blood pressure control (avoid nocturnal anti-hypertensive dosing), and iron levels44,12,28
- Medical therapy
-IOP-lowering medications: prostaglandin analogs, selective alpha-2 adrenergic agonists and carbonic anhydrase inhibitors12
-Evidence: prostaglandins typically first-line due to efficacy and safety profile; brimonidine more efficacious than timolol in preventing visual field loss18,12,35
- Surgical therapy
-Selective laser trabeculoplasty (SLT): effective as initial therapy; proven to lower mean IOP and variability of IOP between visits; shown to lower IOP by 15% and topical medication use by 27% at 1 year post-procedure12,45,46,47
-Trabeculectomy: reduces IOP to sub-normal and single-digit levels; reduces IOP fluctuation; mitomycin-C augmented trabeculectomy shown to have a >60% success rate (>30% IOP reduction from baseline) at 4-year follow up; shown to slow further progression of visual field loss48,49,50
- Follow-up
-Evaluation: perform thorough interval history with special attention to medication adherence and side effects; measure IOP and VA; perform slit lamp exam; evaluate optic nerves; assess visual fields12,35
-Interval: Progressive optic nerve or visual field damage (1-2 months); unachieved target IOP( 3-6 months); achieved target IOP (6-12 months)12,35
References
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Faculty Approval by: Brian Stagg, MD
Identifier: Moran_CORE_27919
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