Absent Left P1 Segment and PCA Territory: Neurovascular Anatomy and Clinical Implications

Imaging showing absent left P1 segment (proximal posterior cerebral artery) and absent left PCA territory perfusion indicates a major posterior circulation variant or pathology.

Anatomic Clarification

• P1 segment = PCA segment from basilar bifurcation to posterior communicating artery (PCom) origin.
• If P1 is absent, the PCA may be fetal-type (supplied entirely by ICA via large PCom) → normal variant (10–30%).
• If PCom also absent → true PCA hypoplasia/occlusion → ischemia risk.

Pathologic Causes of PCA Territory Loss

• Atherosclerotic occlusion (basilar or P1)
• Embolism (cardiac, arterial)
• Vasculitis (PACNS, giant cell arteritis)
• Dissection (vertebral/basilar)
• Trauma
• Posterior reversible encephalopathy syndrome (PRES) – transient

Clinical Syndromes

• PCA infarct: Homonymous hemianopia, alexia, memory deficits (hippocampus), thalamic pain syndrome.
• Top-of-basilar: Bilateral visual, consciousness, motor deficits.

Management

• Acute: IV tPA/thrombectomy if large vessel occlusion.
• Secondary prevention: Antiplatelet/anticoagulant, risk factor control.
• Rehab: Vision therapy, cognitive strategies.