Local studies on ETON are lacking. The actual incidence of ETON in Filipinos taking anti-TB drugs is unknown. At the most modest estimate of 1% in foreign literature (estimates as high as 15%), there should be at least 2500 cases of ETON in our country each year.

Regionally, neighbor nations not classified under "high TB burden countries" (Hong Kong, Singapore, Malaysia, and South Korea) have reported expectedly less incidence of ETON.29-33 The clinical profile of ETON in our country probably mirrors that of other countries with similar TB profiles and demographics (e.g. Indonesia).34

The following is a summary of published and unpublished local studies on ETON:

  • Inocencio and Castillo presented 34 patients diagnosed with ETON seen at the Neuro-ophthalmology Clinic of the Department of Ophthalmology-Philippine General Hospital.23 To date, it remains the largest local case series on ETON ever published. Even at the "standard dosage" of anti-TB drugs, the severity of ocular toxicity was pronounced, as 26 of 34 patients ended up with best-corrected visual acuity (BCVA) of 20/200 or worse at 1 year. Only 3 patients recovered vision back to 20/20 by 1 year, and only 4 patients by the 2nd year. From the clinical profiles, the authors concluded that discontinuation of EMB intake does not assure reversal of visual impairment and recommended careful monitoring of patients on EMB treatment for early detection and prevention of visual loss.
  • Tamesis et al. reported 14 patients aged 27-82 years who developed ETON while on standard dosage of anti-TB treatment.28 The time period from start of anti-TB intake to onset of visual symptoms stretched from 2 to 12 months, and visual acuity ranged anywhere from 20/60 to hand motions. This report was significant in being the first to highlight the local issues and controversies surrounding ETON. It contained the random survey mentioned previously (see above), and was the very first local paper that proposed recommendatory guidelines on the monitoring of patients under TB treatment.
  • Another local case series was presented back in 2005 during the Joint Meeting of the Philippine Academy of Ophthalmology and American Academy of Ophthalmology (PAO-AAO) in Manila.27 In 19 patients diagnosed with ETON at a private clinic over a period of 1 year, the time from intake of anti-TB drugs to onset of visual symptoms ranged from 4-12 months. Best-corrected visual acuity (BCVA) at the time of diagnosis showed 20/70 to 20/200 in 6 patients, and 20/400 or worse in 13 patients. Fifteen patients eventually developed mild pallor (usually temporal sector) of the optic nerves.
  • A case series from the Cebu City Medical Center reported during the Philippine Academy of Ophthalmology Annual Meeting in 2004 looked at 100 patients enrolled in the national TB program. They found 11% of patients having either visual acuity or color perception deficits35.
  • In the same meeting, another large case series seen at a private clinic was reported. With 39 diagnosed cases of ETON, the report looked at the patient demographic risk profile and found that a pre-existing co-morbidity in the eye predisposed patients to development of ETON36.
  • At the Joint Meeting of the Asian Neuro-ophthalmology Society and International Neuro-ophthalmology Society in Tokyo back in 2006, a paper was presented describing histopathologic changes in the retinal ganglion cell layer and optic nerve of adult albino rats fed with so called safe doses of Ethambutol37. No observable changes in the retinal ganglion cell layer nor the proximal 2 mm of the optic nerve were observed among the treatment groups. However, changes in the outer nuclear layer of the retina were noted among rats given ethambutol at doses as low as 15mg/kg/day.
  • Noche and others demonstrated early ultra-structural changes in the optic nerves of rabbits fed with ethambutol.38 Electron microscopy demonstrated axonal dilatation as early as two weeks after EMB administration, i.e., earlier than the clinical appearance of toxic optic neuropathy. In addition, they found decreased Vitamin B6 or pyridoxine levels in the retina of test subjects. To date, this is the only histological and bioch emical study on ETON published locally.
  • Currently, there are three ongoing studies on ETON being spearheaded by residents in various ophthalmology training institutions in Metro Manila. Results should be due in the next 1-2 years. These include:
    1. Determination of Color Vision Patterns Using Ishihara Plates in Newly-Diagnosed Tuberculosis Patients on Ethambutol Therapy
      Gapay FMA, Guevara GV
      Department of Ophthalmology-OSPITAL NG MAYNILA
    2. Clinical Profile of Ethambutol-related Toxic Optic Neuropathy
      Gonzales R
      Department of Ophthalmology and Visual Sciences- SENTRO Oftalmologico Jose Rizal-PILIPPINE GENERAL HOSPITAL
    3. Color Vision Changes Among Filipino Patients On Directly-Observed Treatment Short Course for Tuberculosis
      Cruz EM, Puentespina FG, Alejo KPL, Morabe, Nanagas MLR Department of Ophthalmology-JOSE R. REYES MEMORIAL MEDICAL CENTER