Toxic amblyopia, now more commonly termed toxic optic neuropathy, refers to the development of visual loss as a result of exposure to various toxic substances, such as tobacco, alcohol, and certain drugs. Patients firstly present with dyschromatopsia, followed by painless bilateral and progressive vision loss. The condition is frequently not timely recognized and many patients develop severe and often irreversible changes at the time of diagnosis. A detailed patient history with a physical examination is crucial for initial recognition, whereas advanced imaging and ocular tests are used for confirmation.
The term "toxic amblyopia" denotes a visual impairment that stems from the toxic effects of various substances    . The term "toxic optic neuropathy" is used to describe the damage to the papillomacular bundle by tobacco, methanol, numerous drugs (amiodarone, ethambutol, linezolid, phosphodiesterase 5 inhibitors, etc.), organic solvents (toluene, ethylene glycol), heavy metals (thallium, mercury, lead), or large amounts of alcohol   . Nutritional deficiencies (such as of thiamine, riboflavin, niacin, cobalamin, folic acid) have also been included in the pathogenesis of toxic amblyopia . The clinical presentation almost universally starts with dyschromatopsia, particularly affecting the brightness of red color   . Difficulties in distinguishing red from green is also a common complaint . Over the course of weeks to months, the progressive damage leads to the formation of scotomas, bilateral defects in visual acuity, and eventually vision loss    . A key feature of toxic amblyopia is the absence of pain during its development   . Peripheral vision is spared in most cases, but total vision loss has been reported . Manifestations in extraocular sites, such as polyneuropathy and Wernicke-Korsakoff syndrome, are typically seen in alcoholics .
An early diagnosis is imperative in the case of toxic amblyopia, as proper treatment may reverse the damage and save the patient's vision  . For this reason, physicians must perform a detailed clinical assessment when evaluating patients with painless and progressive vision loss   . Recent or chronic exposure to toxic substances must be checked, together with a proper dietary history, because a significant number of patients with toxic amblyopia will have accompanying nutritional disorders such as Wernicke's encephalopathy or pellagra  . A heteroanamnesis can sometimes be useful when alcohol or tobacco toxicity is suspected . Once important pieces of information are gathered, the physician should proceed to the next step of the workup, namely physical examination with an emphasis on the eyes   . Evaluation of visual acuity and visual fields, color vision testing, the light reflex (which is present except in severe methanol poisoning), and fundoscopy are sufficient to make an initial diagnosis   . Laboratory studies, including a complete blood count (CBC), basic biochemistry tests, and screening for toxic substances (or vitamin deficiencies) are recommended as well . Visual evoked potentials (VEP) and pattern electroretinography (PERG) can be employed to solidify the diagnosis, whereas magnetic resonance imaging (MRI) is the most appropriate imaging modality.