RNFL Thinning in CNS TB on Antitubercular Therapy
Retinal nerve fiber layer thinning that is most pronounced in the temporal sector in a patient receiving antitubercular therapy for CNS tuberculosis is suggestive of ethambutol-induced optic neuropathy (toxic optic neuropathy). 1
Ethambutol Ocular Toxicity: The Primary Concern
Ethambutol causes dose-related retrobulbar neuritis that characteristically produces temporal RNFL thinning, decreased visual acuity, central scotomas, and impaired red-green color discrimination. 1
Key Diagnostic Features
Temporal RNFL thinning is the hallmark pattern of ethambutol toxicity, distinguishing it from other causes of optic neuropathy where inferior and superior thinning predominate. 2, 3, 4
The risk of ethambutol ocular toxicity is 6% at standard dosing (25 mg/kg/day for 2 months, then 15 mg/kg/day), but increases to 18% at doses exceeding 30 mg/kg/day. 1
Ocular toxicity is more common in tuberculosis treatment than in other conditions due to the prolonged duration of therapy required. 1
Clinical Presentation Pattern
Visual symptoms often precede measurable changes on examination, making patient self-reporting critical. 1
Characteristic symptoms include blurred vision, decreased acuity, central scotomas, impaired red-green color discrimination, and peripheral visual field defects. 1
OCT may not detect RNFL defects in the early stage but becomes useful for follow-up monitoring. 2
Immediate Management Algorithm
Step 1: Discontinue Ethambutol Immediately
Ethambutol should be discontinued immediately and permanently if there are any signs of visual toxicity. 1
- Do not wait for ophthalmological confirmation before stopping the drug—visual symptoms alone warrant immediate cessation. 1
Step 2: Urgent Ophthalmological Assessment
Arrange immediate ophthalmological evaluation including visual acuity (Snellen chart), color discrimination (Ishihara tests), visual field testing, and funduscopy. 1
Obtain OCT imaging to document RNFL thickness, particularly in the temporal quadrant. 2, 3, 4
Step 3: Modify Antitubercular Regimen
Continue rifampicin, isoniazid, and pyrazinamide without ethambutol. 1
Extend treatment duration to 18 months if ethambutol must be omitted from the regimen. 1
Consider alternative agents if drug resistance is a concern, but never reintroduce ethambutol once toxicity has occurred. 1
Prognosis and Recovery
RNFL thickness may gradually normalize after ethambutol discontinuation, with progressive improvement in both RNFL and GCIPL measurements. 4
Visual acuity and visual fields typically improve after drug cessation, though recovery is variable. 2, 5
In two-thirds of patients with toxic optic neuropathy from antitubercular drugs, RNFL thinning becomes manifest over time (4-26.5 months), especially in the temporal segment. 2
Differential Considerations
While ethambutol toxicity is the primary concern, other causes of RNFL thinning in this context include:
Tuberculous optic neuritis (part of CNS TB itself) typically presents with optic disc swelling, enhancement on MRI, and responds to antitubercular therapy rather than worsening during treatment. 5, 6
Isoniazid can rarely cause optic neuropathy, though this is far less common than ethambutol toxicity. 3
Glaucomatous optic neuropathy follows the ISNT rule with inferior and superior thinning predominating, not temporal thinning. 1
Prevention and Monitoring
Baseline visual acuity and color discrimination testing should be performed before starting ethambutol. 1
Monthly visual acuity and color discrimination testing is recommended for patients receiving ethambutol doses >15-25 mg/kg, treatment >2 months, or those with renal insufficiency. 1
Serial OCT monitoring can detect subclinical RNFL thinning before symptoms develop, particularly measuring temporal quadrant thickness. 3, 4
Patients should be educated to report visual changes immediately and contact their physician if any vision changes occur. 1