What is the appropriate management for hypoattenuation (decreased attenuation), particularly in critical areas such as the brain?

Management of Hypoattenuation on Brain CT

Hypoattenuation on brain CT in acute ischemic stroke represents ischemic brain injury and requires urgent assessment to determine eligibility for thrombolytic therapy, with the critical decision point being whether hypoattenuation involves more than one-third of the middle cerebral artery (MCA) territory.

Immediate Diagnostic Interpretation

Extent Assessment is Critical

• Hypoattenuation involving ≤33% of MCA territory: IV alteplase (rtPA) is recommended, as these patients benefit from thrombolytic therapy despite the presence of early ischemic changes 1.

• Hypoattenuation involving >33% of MCA territory: IV alteplase should NOT be administered, as these patients have extensive irreversible injury with poor prognosis and significantly increased risk of symptomatic hemorrhage (8-fold increase) 1.

• "Frank hypodensity" or "obvious hypodensity": This represents irreversible injury and is a contraindication to thrombolytic therapy 1.

Early CT Signs to Recognize

The following findings indicate acute ischemia and should be systematically assessed 1:

• Loss of gray-white matter differentiation (particularly at insular cortex and lentiform nucleus)
• Sulcal effacement or compression of CSF spaces
• Hyperdense MCA sign (thrombus in proximal MCA)
• Focal brain swelling or mass effect

Important caveat: Physician ability to reliably detect these changes is variable (70-80% accuracy), so when uncertain about extent, err on the side of caution 1.

Time-Critical Management Protocol

For Patients Within 3-4.5 Hour Window

1. Complete CT within 25 minutes of ED arrival with interpretation within additional 20 minutes (total door-to-interpretation: 45 minutes) 1.

2. If hypoattenuation is mild-to-moderate (<33% MCA territory): Proceed with IV alteplase if other eligibility criteria are met 1.

3. If extensive hypoattenuation (>33% MCA territory) or frank hypodensity: Do NOT administer alteplase due to high hemorrhage risk and lack of benefit 1.

Pathophysiological Context

What Hypoattenuation Represents

• Early hypoattenuation indicates extended critical hypoperfusion: Patients with visible hypoattenuation have significantly larger volumes of critically hypoperfused tissue (mean 116 cm³) compared to those with normal CT (mean 14 cm³) 2.

• Not all hypoattenuation is irreversible: Some tissue showing early hypoattenuation may still be salvageable, particularly at the periphery of the affected region 2.

• Brain swelling without hypoattenuation: This CT sign (loss of gray-white distinction with preserved attenuation) may represent penumbral tissue rather than core infarct and does not preclude treatment 3.

Risk Stratification Based on CT Findings

High-Risk Features Predicting Poor Outcome

The following CT findings predict clinical deterioration from brain swelling 1:

• Hypoattenuation involving ≥50% of MCA territory within 12 hours
• Hyperdense MCA sign
• Mass effect with compression of frontal horn
• Shift of septum pellucidum or pineal gland
• Additional vascular territory involvement

Hemorrhagic Transformation Risk

Patients with widespread early infarct signs have significantly increased risk of symptomatic hemorrhage after thrombolysis 1:

• Early edema or mass effect: 8-fold increased hemorrhage risk
• Involvement of >33% MCA territory: contraindication to late-window thrombolysis (3-6 hours)

Management of Brain Swelling from Hypoattenuation

If Patient Deteriorates After Initial Assessment

Supportive measures 1:

• Elevate head of bed 20-30 degrees to facilitate venous drainage
• Maintain normothermia (36-37°C)
• Avoid hypotonic fluids (use 0.9% saline only)
• Correct hypoxemia (PaO₂ ≥13 kPa), hypercarbia (PaCO₂ 4.5-5.0 kPa), and hyperthermia
• Avoid antihypertensive agents that cause cerebral vasodilation

Osmotic therapy for impending herniation 1:

• Mannitol 0.5 g/kg IV, OR
• Hypertonic saline 3% at 2 mL/kg IV

Surgical decompression: Consider for malignant MCA infarction with life-threatening swelling 1.

Common Pitfalls to Avoid

• Do not delay imaging interpretation: Time is critical, and every minute counts for treatment decisions 1.

• Do not dismiss subtle findings: Even mild hypoattenuation indicates significant underlying hypoperfusion 2.

• Do not administer rtPA if uncertain about extent: When hypoattenuation extent is unclear and appears extensive, the risk of hemorrhage outweighs potential benefit 1.

• Do not use contrast-enhanced CT routinely: It provides no additional information for acute stroke assessment unless evaluating for tumor or infection 1.

• Do not assume all hypoattenuation is irreversible: Some penumbral tissue may be salvageable, particularly in early presentations 2, 3.