Which pupil dilates in a patient with a subdural (subdural hematoma) hematoma?

Pupillary Changes in Subdural Hematoma

In subdural hematoma, the pupil typically dilates on the same side (ipsilateral) as the hematoma due to compression of the third cranial nerve against the tentorium. 1

Pathophysiology of Pupillary Dilation in Subdural Hematoma

The mechanism behind pupillary changes in subdural hematoma involves a specific sequence of events:

1. Mass Effect and Herniation: As the subdural hematoma expands, it creates a mass effect that pushes brain tissue.

2. Third Nerve Compression: The expanding hematoma causes uncal herniation, where the medial temporal lobe (uncus) is pushed against the tentorium cerebelli, compressing the ipsilateral third cranial nerve (oculomotor nerve) 1

3. Pupillary Fiber Damage: The pupillary fibers run peripherally in the third nerve, making them particularly susceptible to external compression 2

4. Clinical Manifestation: This compression results in ipsilateral pupillary dilation (mydriasis) and impaired pupillary light response on the same side as the hematoma

Clinical Recognition and Monitoring

Pupillary changes are critical warning signs that require immediate attention:

• Early Warning Sign: Ipsilateral pupillary dilation is often one of the earliest signs of deterioration in subdural hematoma 1

• Progression Pattern: Deterioration typically presents in one of two ways:

  1. Gradual rostrocaudal deterioration with development of midposition pupils
  2. Sudden presentation of a unilaterally dilated pupil progressing to bilateral pupillary dilation 1

• Associated Signs: Along with pupillary changes, look for:

  • Decreasing level of consciousness
  • Worsening limb power
  • Extensor posturing
  • Contralateral Babinski sign
  • Abnormal respiratory patterns (late sign) 1

Monitoring Recommendations

The American Heart Association/American Stroke Association recommends:

• Frequent monitoring of level of arousal and ipsilateral pupillary dilation in patients at high risk for deterioration (Class I; Level of Evidence C) 1

• Particular vigilance in patients with acute subdural hematoma >10 mm thick or midline shift >5 mm on CT scan 3

Clinical Implications

The presence of pupillary abnormalities significantly impacts prognosis and management decisions:

• Surgical Decision-making: A comatose patient with asymmetric or fixed and dilated pupils should undergo surgical evacuation of the subdural hematoma 3

• Prognostic Value: Pupillary abnormalities are associated with worse outcomes, though some studies report good recovery in a subset of patients with pupillary abnormalities 1

• Timing of Intervention: While pupillary changes indicate the need for urgent intervention, the underlying brain injury and ability to control intracranial pressure may be more critical to outcome than absolute timing of hematoma removal 4

Common Pitfalls and Caveats

1. False Localizing Signs: Rarely, a subdural hematoma can cause contralateral pupillary dilation as a false localizing sign 2

2. Bilateral Dilation: Late-stage bilateral pupillary dilation indicates severe brainstem compression and is associated with poor outcomes

3. Confounding Factors: Pre-existing pupillary abnormalities, direct ocular trauma, or medications can confound pupillary assessment

4. Incomplete Assessment: Always assess both pupils for size, shape, and reactivity to light, as comparing both sides is essential for accurate interpretation

Remember that while pupillary dilation is a critical sign in subdural hematoma management, it must be interpreted within the context of the patient's overall clinical condition, neurological examination, and neuroimaging findings.