Toxic ampylopia:
It's optic nerve damage due to exogenous toxins.
Toxins which cause central scotoma as tobacco and ethyl alcohol
Toxins which cause contracted field as quinine and salicylates
Toxins which cause severe optic atrophy as methyl alcohol and arsenic.
A) Tobacco amblyopia:
It's due to degeneration of the ganglion cells of the papillo macular bundle by the decomposition of products of nicotine and cyanide decreasing vitamin B 12 as it normally neutralises the effect of cyanide.
The fundus is normal and the field shows Centro cecal scotoma for red and green.
B) quinine amblyopia:
Its due to idiosyncrasy to the drug.
The fundus has cherry red spot (arterial spasm) and the field is tubular ( night blindness)
Severe vaso spasm leads to closing the peripheral vessels of the retina causing ischemia and tubular field.
In severe cases there may be closed CRA leading to cherry red spot.
C) methyl alcohol amblyopia:
It's breakdown of methyl alcohol forming Formaldehyde+Formic acid causing acidosis and anoxia.
It causes coma and death , degeneration of ganglion cells leading to optic atrophy.
Treatment:
Stop the drug
Vasodilators
Vitamin B complex.
In methyl alcohol amblyopia do digastric lavage with ethyl alcohol which makes competitive inhibition in liver with methanol and give NaHco3 injection for acidosis.
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