What is most consistent with uncal herniation in a patient with acute flaccid left hemiparesis: constriction of the right pupil, dilated fixed left pupil, lateral deviation of the right eye, or medial deviation of the left eye?

From the Guidelines

The most consistent finding with uncal herniation in a patient who just developed flaccid left hemiparesis is a dilated fixed left pupil. In uncal herniation, the medial portion of the temporal lobe (uncus) herniates through the tentorial notch due to increased intracranial pressure. This typically causes compression of the ipsilateral (same side) oculomotor nerve (cranial nerve III), resulting in pupillary dilation and loss of light reflex on the same side as the herniation. The left hemiparesis indicates right-sided brain pathology, suggesting a right uncal herniation. With right-sided herniation, we would expect right-sided pupillary dilation (ipsilateral to the lesion). However, in some cases of advanced herniation, the brain stem can shift and rotate, causing contralateral compression of the oculomotor nerve, resulting in a dilated fixed pupil on the side opposite to the herniation (Kernohan's notch phenomenon) 1. This explains why a dilated fixed left pupil could be seen with left hemiparesis in the setting of uncal herniation. Some key points to consider in the evaluation of pupillary function include assessing for size, shape, symmetry, and response to light, as outlined in the pediatric eye evaluations preferred practice pattern 1. However, in the context of uncal herniation, the critical factor is the presence of a dilated fixed pupil, which is a sign of impending neurosurgical emergency requiring immediate intervention to reduce intracranial pressure. Key considerations in this scenario include:

• The development of flaccid left hemiparesis indicating right-sided brain pathology
• The potential for Kernohan's notch phenomenon causing contralateral pupillary dilation
• The need for immediate intervention to reduce intracranial pressure and prevent further brainstem compression.

From the Research

Clinical Presentation of Uncal Herniation

The clinical presentation of uncal herniation can be complex and varied. In a patient with acute flaccid left hemiparesis, several signs can be indicative of uncal herniation.

• The most consistent sign with uncal herniation is a dilated fixed pupil on the same side as the herniation, which is caused by compression of the ipsilateral oculomotor nerve 2, 3, 4.
• Other signs that may be present include lateral deviation of the eye on the same side as the herniation, due to compression of the ipsilateral oculomotor nerve, and contralateral hemiparesis, which can occur due to compression of the contralateral cerebral peduncle against the tentorial edge, also known as the Kernohan-Woltman notch phenomenon 5, 4.
• Medial deviation of the eye is not typically a sign of uncal herniation, as this would be more consistent with a lesion affecting the abducens nerve or the lateral rectus muscle.
• Constriction of the pupil is also not typically a sign of uncal herniation, as this would be more consistent with a lesion affecting the sympathetic nervous system or the use of certain medications.

Important Considerations

It is essential to consider the entire clinical picture when evaluating a patient with suspected uncal herniation, including the presence of any other neurological deficits, such as decreased mental status, ptosis, or extraocular movement impairment 2, 3. Additionally, imaging studies, such as CT or MRI, can be helpful in confirming the diagnosis of uncal herniation and identifying any underlying causes, such as intracranial hemorrhage or tumor 5, 4.