In Horner’s syndrome, what is the effect of sympathomimetic? In ophthal, it is said that apraclonidine dilates pupil in Horner’s… in medicine, it is said that failure to dilate with cocaine confirms Horner’s…
Cocaine test is used for the diagnosis of Horners syndrome , absence of dilatation confirns Horner’s .
On the other hand , apraclonidine/amphetamine is used to localize the lesion , so when we administer amphetamine HORNER’S pupil may or may not dilate. So , the diagnosis is already made based on cocaine test … Cocaine acts centrally via blocking the cortical control centre . Amphetamine works via the peripheral nerve fibres . So , when the first and second order neuron gets block pupil still dilates due to amphetamine reaching the third order neuron via relay fibres. But when the final gateway i.e. third order neuron lesion occurs there is no way the signal reaches the dilator pupillae ; hence no dilation on amphetamine administration indicates 3rd order neuron lesion in Horner’s eye.
apraclonidine Causes mydriasis in horners because of upregulation of Aplha 1
apraclonidine has similar effects to amphetamine.
Apraclonidine has predominantly + effect on ALPHA 2 that downregulates the release of NE = MIOSIS.
But it has aganist effect on ALPHA 1 also.
IN HORNER… DUE TO ALREADY NE … upregulation of NE receptos in dilator ppl mscle… lots of NE RECEPTROS are now present… thats why the little effect that apraclonidine has on ALPHA 1… creates MYDRIASIS.