Right PCOM Infarct: Signs and Symptoms
A right posterior communicating artery (PCOM) infarct most commonly presents with ipsilateral third nerve palsy featuring pupillary involvement, along with contralateral motor and sensory deficits affecting the left side of the body.
Primary Clinical Manifestations
Cranial Nerve Findings (Ipsilateral - Right Side)
The right PCOM courses adjacent to the third cranial nerve in the subarachnoid space, making third nerve palsy the hallmark presentation 1, 2:
- Pupillary involvement (dilated right pupil) - critical distinguishing feature suggesting compression from aneurysm or ischemia
- Complete ptosis of the right eyelid
- Ophthalmoplegia with inability to adduct, elevate, or depress the right eye
- Eye positioned "down and out" at rest
- Diplopia when ptosis is lifted
Critical distinction: Pupil-involving third nerve palsy in the context of PCOM pathology raises urgent concern for posterior communicating artery aneurysm and requires immediate neuroimaging with MRA or CTA 1, 2.
Contralateral Motor and Sensory Deficits (Left Side)
PCOM perforators supply critical structures including the thalamus, hypothalamus, and portions of the midbrain 3, 4:
- Left-sided weakness (hemiparesis) - particularly if cerebral peduncle involvement
- Left-sided sensory loss or paresthesias - from thalamic involvement
- Left hemibody numbness/tingling 5, 6
Visual Field Deficits
When PCOM infarction extends to involve PCA territory 6, 7:
- Left homonymous hemianopia (loss of left visual field in both eyes)
- Patients typically describe this as a negative visual phenomenon (darkness, blackness) rather than positive symptoms
- 66% of patients can correctly lateralize the deficit to a hemifield 6
Associated Neurological Features
Thalamic Involvement
The PCOM gives rise to thalamogeniculate and premammillary perforators 3, 4:
- Sensory deficits (most prominent feature with lateral thalamic infarction) 5, 7
- Slight motor weakness (typically reversible) 7
- Neuropsychological deficits: attention impairment, memory dysfunction 4
- Altered level of consciousness if bilateral thalamic involvement 3
Midbrain Involvement
If infarction affects the midbrain (precommunal segment involvement):
- Ipsilateral cerebellar ataxia (superior cerebellar peduncle) 1, 2
- Ipsilateral flapping hand tremor (red nucleus) 1, 2
- Contralateral hemiplegia (cerebral peduncle) 1, 2
- Decreased consciousness - indicates severe proximal involvement 7
Presenting Symptoms
Acute Onset Features
- Unilateral headache (ipsilateral to the lesion) - common presenting symptom 7
- Dizziness (47% in posterior circulation strokes) 5
- Nausea/vomiting (27-41% in posterior circulation ischemia) 8, 5
- Sudden onset in 90% of cases without preceding warning 6
Warning Symptoms (10% of cases)
May have preceding transient symptoms before complete infarction 6
Anatomical Correlation
The right PCOM connects the right internal carotid artery to the right posterior cerebral artery, supplying:
- Perforators to the ipsilateral thalamus (thalamogeniculate artery)
- Premammillary artery territory
- Hypothalamus
- Portions of the midbrain
- Runs immediately adjacent to the third cranial nerve 1, 2
Clinical Pitfalls
Pupil-sparing vs. pupil-involving: Pupil-sparing third nerve palsy suggests microvascular ischemia (diabetes, hypertension), while pupil involvement with PCOM pathology mandates urgent aneurysm evaluation 1, 2
Bilateral infarctions possible: Despite unilateral PCOM occlusion, bilateral thalamic infarctions can occur via compromised circuminfundibular plexus 3
Fetal-type PCOM anatomy: When the PCA is predominantly supplied by a fetal-type PCOM (hypoplastic P1 segment), PCOM occlusion causes more extensive ischemia including lateral thalamus, medial temporal lobe, and occipital cortex 4
Delayed recognition: Visual field deficits may not be immediately apparent if complete ptosis is present, as patients cannot see the deficit until the lid is elevated 1
Diagnostic Urgency
Immediate neuroimaging with MRI/MRA or CT/CTA is mandatory to:
- Rule out posterior communicating artery aneurysm (especially with pupil involvement) 1, 2
- Identify extent of infarction
- Assess for hemorrhagic transformation
- Evaluate Circle of Willis anatomy and collateral flow 9
The combination of ipsilateral third nerve palsy with pupillary involvement plus contralateral motor/sensory deficits should immediately trigger consideration of right PCOM pathology requiring emergent neurovascular imaging.