What percentage of posterior reversible encephalopathy syndrome (PRES) cases present with strictly unilateral (right‑side) involvement in a 40‑year‑old hypertensive man with blood pressure around 140/100 mm Hg?

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Strictly Unilateral PRES Occurs in Approximately 2.6% of Cases

Strictly unilateral PRES, as seen in this right-sided case, is rare and occurs in approximately 2.6% of confirmed PRES cases 1. This represents an uncommon variant of a syndrome that typically presents with bilateral, symmetric involvement.

Distribution Patterns in PRES

The classic teaching of symmetric parietooccipital involvement is actually incomplete. In a large series of 76 confirmed PRES cases 1:

  • Parietooccipital regions: 98.7% (nearly universal)
  • Posterior frontal lobes: 78.9%
  • Temporal lobes: 68.4%
  • Thalamus: 30.3%
  • Cerebellum: 34.2%
  • Brainstem: 18.4%
  • Basal ganglia: 11.8%
  • Unilateral variant: 2.6%

Mechanism of Unilateral Presentation

A recent 2026 case report 2 provides mechanistic insight into why unilateral PRES can occur. In a 38-year-old hypertensive man (similar to your patient), unilateral right-sided PRES developed in the setting of contralateral (left) middle cerebral artery occlusion. The proposed mechanism: vascular occlusion on one side triggers compensatory increased blood flow to the opposite hemisphere, making that side vulnerable to hypertensive breakthrough and vasogenic edema.

This suggests that unilateral PRES may occur when:

  • Pre-existing vascular asymmetry exists
  • Compensatory hemodynamic changes favor one hemisphere
  • Acute blood pressure fluctuations overwhelm autoregulation asymmetrically

Blood Pressure Management in This Case

Your patient's BP of 140/100 mmHg is not in the hypertensive emergency range (which requires >180/120 mmHg with acute target organ damage) 3. However, PRES itself constitutes acute neurologic target organ damage.

For PRES with BP 140/100 mmHg:

  • This represents a controlled state after initial presentation
  • Gradual BP reduction is critical to avoid hypoperfusion 4
  • Target systolic BP ≤130 mmHg over days, not hours 2
  • Avoid rapid drops that could worsen cerebral perfusion in areas with impaired autoregulation

Clinical Pitfalls

The unilateral presentation creates diagnostic challenges:

  • May be misdiagnosed as acute stroke, particularly given the asymmetry
  • Look for vasogenic (not cytotoxic) edema on diffusion-weighted imaging
  • Consider underlying vascular asymmetry or occlusion on the contralateral side 2
  • The atypical presentation does NOT correlate with severity or prognosis 1

Prognosis Considerations

Despite the atypical unilateral presentation, prognosis follows general PRES patterns 5:

  • Mortality: 10-19%
  • Persistent neurological deficits: 40-44% of cases
  • Most show clinical and radiologic recovery within weeks with appropriate management
  • Early recognition and BP control remain the most critical prognostic factors

The 2.6% incidence of strictly unilateral PRES means this patient has an uncommon but well-documented variant that requires the same management principles as typical bilateral cases, with particular attention to gradual BP normalization and investigation for underlying vascular asymmetry.

References

4
Research

Posterior Reversible Leukoencephalopathy Syndrome During Hypertensive Crisis in Obstructive Sleep Apnea Syndrome: Searching for a Link.

High blood pressure & cardiovascular prevention : the official journal of the Italian Society of Hypertension, 2025

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