What is the probable diagnosis in a patient with dilated pupil not responsive to 1% pilocarpine
**Core Concept**
The diagnosis of a dilated pupil not responsive to 1% pilocarpine is a clinical challenge that requires understanding of the autonomic nervous system, specifically the sympathetic and parasympathetic pathways that control pupil size. Pilocarpine is a muscarinic receptor agonist that stimulates the parasympathetic nervous system to constrict the pupil.
**Why the Correct Answer is Right**
A dilated pupil not responsive to 1% pilocarpine suggests an afferent pupillary defect (APD) or a third-order neuron lesion, which involves the afferent pathway of the pupillary light reflex. The afferent pathway involves the optic nerve, optic chiasm, optic tract, and the pretectal nuclei. A lesion in this pathway can cause a pupil that is dilated and non-reactive to light or to pilocarpine. In this case, pilocarpine is unable to stimulate the parasympathetic nervous system to constrict the pupil because the afferent signal is not being transmitted.
**Why Each Wrong Option is Incorrect**
**Option A:** A second-order neuron lesion would cause a pupil that is mid-dilated and reactive to light, not non-responsive to pilocarpine.
**Option B:** A third-order neuron lesion would cause a pupil that is dilated and non-reactive to light, but the pupil would still constrict with pilocarpine.
**Option C:** A Horner's syndrome would cause a constricted pupil, not a dilated one.
**Clinical Pearl / High-Yield Fact**
A patient with a dilated pupil not responsive to 1% pilocarpine may have an afferent pupillary defect (APD), which requires further evaluation with a swinging flashlight test to confirm the diagnosis.
**Correct Answer: D. Third-order neuron lesion**