A female presented with loss of vision in both eyes and on examination has normal pupillary responses and normal fundus, Her visually evoked response (VER) examination shows extinguished responses.The most likely diagnosis is –
Correct Answer: Cortical blindness
Description: Lesions upto optic tract (i.e. retina, optic nerve, optic chiasma and optic tract) affect pupillary (light) reflex. In the question light reflex is normal, thus optic neuritis and retinal detachment can be ruled out.
Now we are left with hysteria and cortical blindness. This can be differentiated by VER which is normal in hysteria and shows abnormality in cortical lesions. So, answer is cortical blindness.
It is worth noting that VER will also be abnormal in retinal detachment and optic neuritis. However, they have been excluded on the basis of normal pupillary response. Here I am giving brief idea about VER.
We know when light falls on retina, a series of nerve impulses are generated and passed on to the visual cortex via the visual pathway.
The visual evoked response measures the electrical potential resulting from a visual stimulus.
Because it is measured by scalp electrodes placed over the occipital cortex, the entire visual pathway from retina to cortex must be intact in order to produce a normal electrical waveform reading.
Visual evoked response is the only clinically objective technique available to assess the functional state of the visual system beyond the retinal ganglion cells.
Interruption of neuronal conduction by any lesion in this pathway will result in reduced amplitude of the VER.
In the question extinguished visual response suggests any abnormality some where, in the visual conducting system.
This abnormality in the VER can be easily attributed to cortical blindness because normal pupillary response and normal fundus rule out any chance of optic neuritis and retinal detachment.
Category:
Ophthalmology
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