Is the patient's loss of vision and blurry vision due to the HRZE (human immunodeficiency virus/tuberculosis treatment regimen) or pyridoxine (Vitamin B6)?

Ethambutol-Induced Optic Neuritis: Cause of Visual Symptoms

The patient's blurry vision and loss of vision are most likely due to ethambutol (part of the HRZE regimen) causing optic neuritis, which is a well-documented adverse effect of this medication. 1, 2

Mechanism and Clinical Presentation

Ethambutol causes retrobulbar neuritis, which is the most severe adverse effect of this medication. This is typically dose-related and presents with:

• Blurred vision (early symptom)
• Red/green color blindness (early symptom)
• Central scotomata (later symptom)
• Potential progression to visual acuity loss

The patient is experiencing both blurry vision and visual loss, which are classic symptoms of ethambutol toxicity. The tingling and pain in the skin may be a separate issue related to peripheral neuropathy, which could be from isoniazid or high-dose pyridoxine.

Ethambutol Toxicity Risk Factors

• Dosage >15 mg/kg/day increases risk (frequency increases to almost 3% at 25 mg/kg/day) 1
• Renal impairment (increases blood levels of ethambutol)
• Inability to report visual symptoms
• Pre-existing optic neuritis

Pyridoxine Considerations

The patient is receiving 100 mg of pyridoxine daily, which is appropriate for preventing isoniazid-induced neuropathy. However:

1. Standard prophylactic dose is typically 10-50 mg/day 1
2. Very high doses of pyridoxine (>200 mg/day) can actually cause sensory neuropathy 3, 4
3. The 100 mg dose is unlikely to cause visual symptoms but could contribute to the tingling sensations 4

Management Algorithm

1. Immediate action: Stop ethambutol immediately upon appearance of visual symptoms 1

   • Visual symptoms should never be ignored as they can progress to irreversible optic atrophy

2. Evaluation:

   • Perform visual acuity testing and color discrimination testing
   • Monitor for improvement after drug discontinuation
   • Consider ophthalmology consultation

3. Treatment modification:

   • Continue other anti-TB medications (isoniazid, rifampin, pyrazinamide)
   • Consider alternative regimen without ethambutol
   • Maintain pyridoxine at current dose to prevent isoniazid neuropathy

4. Follow-up:

   • Regular monitoring of visual acuity
   • Monitor for improvement of symptoms after ethambutol discontinuation

Important Considerations

• Optic neuritis from ethambutol is usually reversible if the drug is stopped promptly 1
• The peripheral tingling symptoms could be from:
  • Isoniazid-induced neuropathy (despite pyridoxine supplementation)
  • Less likely from pyridoxine at 100 mg/day (toxicity usually occurs at much higher doses)

Pitfalls to Avoid

1. Do not continue ethambutol despite visual symptoms - this can lead to irreparable ocular damage
2. Do not attribute visual symptoms to pyridoxine - pyridoxine toxicity typically causes sensory ataxic neuropathy, not visual disturbances 3, 4
3. Do not stop pyridoxine - this is necessary to prevent isoniazid-induced neuropathy
4. Do not delay action - prompt discontinuation of ethambutol is essential to prevent permanent visual impairment

In conclusion, the patient's visual symptoms are most consistent with ethambutol-induced optic neuritis, which requires immediate discontinuation of ethambutol to prevent permanent damage. The peripheral tingling symptoms are likely related to isoniazid-induced neuropathy despite pyridoxine supplementation.