Phenylephrine 1% can be used in the diagnosis of Horner's syndrome. If postganglionic sympathetics are affected, will there be dilation or no dilation?

In the context of diagnosing Horner's syndrome, the application of phenylephrine 1% serves as a vital tool for differentiating the causes of the syndrome. When phenylephrine, an alpha-1 adrenergic agonist, is instilled in the eye, it causes dilation of the pupil by stimulating the dilator muscle of the iris.

In cases of postganglionic lesions affecting the sympathetic pathway (such as those seen in Horner's syndrome), the response to phenylephrine can help elucidate the extent and location of the lesion. When the postganglionic sympathetic fibers are damaged or impaired, the normal inhibitory signaling to the dilator muscle is lost. Thus, upon applying phenylephrine, one would expect significant pupil dilation because this pathway is unblocked by the agonist, revealing the underlying sympathetic function.

In contrast, if the lesion were preganglionic, the expected response to phenylephrine would differ accordingly. Therefore, the expectation of dilation following application of phenylephrine helps confirm that postganglionic sympathetic fibers are affected. The dilation observed in this scenario signifies that the dilator muscle is receiving adequate stimulation despite the prior impairment.

Thus, this dilatory response indicates that the postgang