Subarachnoid Hemorrhage on CT: Key Imaging Features and Clinical Implications

Subarachnoid hemorrhage (SAH) is a critical neurological emergency that demands immediate medical attention. It occurs when blood leaks into the subarachnoid space—the area between the brain and the thin tissues that cover it. This condition can stem from various causes, including ruptured aneurysms, arteriovenous malformations, cerebral amyloid angiopathy, or traumatic head injuries. Among these, aneurysmal rupture remains the most common non-traumatic cause, often linked to underlying vascular weaknesses such as those exacerbated by hypertension or atherosclerosis.

Understanding CT Imaging in Acute SAH

Computed Tomography (CT) scans are the gold standard for diagnosing acute subarachnoid hemorrhage. In a healthy brain, cerebrospinal fluid (CSF) appears as a low-density area within sulci, fissures, and cisterns on non-contrast CT images. However, when bleeding occurs, the presence of fresh blood—being denser than CSF—displaces this normal appearance, showing up as hyperdense (bright white) regions along the brain's surface, particularly in the interhemispheric fissure, Sylvian fissure, basal cisterns, and around the brainstem.

Why CT Is the First-Line Diagnostic Tool

In emergency settings, whether dealing with trauma-related injury or spontaneous intracranial bleeding, unenhanced CT is universally preferred due to its speed, availability, and high sensitivity—especially within the first 6 to 24 hours after symptom onset. The contrast between the dark CSF spaces and the bright hemorrhagic areas makes early detection highly reliable. Studies show that CT can detect SAH with over 95% accuracy during the acute phase, making it indispensable in ruling in or out life-threatening bleeds quickly.

Limitations of CT Over Time

However, the visibility of hemorrhage on CT diminishes as time progresses. After several days—typically beyond 72 hours to one week—the density of the blood begins to fade as red blood cells break down and hemoglobin undergoes biochemical changes. As a result, subacute or chronic SAH may not be clearly visible on follow-up CT scans, potentially leading to false-negative results if imaging is delayed.

What Happens When CT Is Negative but SAH Is Suspected?

If clinical suspicion remains high despite a normal CT scan—such as in patients presenting with sudden "thunderclap" headaches, neck stiffness, or altered mental status—a lumbar puncture is typically performed next. Cerebrospinal fluid analysis can reveal xanthochromia (yellowish discoloration due to bilirubin), which confirms the presence of prior bleeding. Additionally, advanced imaging techniques like CT angiography (CTA) or magnetic resonance imaging (MRI) may be used to identify aneurysms or subtle vascular abnormalities that could have caused the bleed.

Best Practices for Early Detection

To maximize diagnostic accuracy, clinicians strongly recommend performing a non-contrast head CT as soon as possible after symptom onset. Early imaging—within the first few hours—is crucial for identifying hyperdense blood in the sulci, fissures, and basal cisterns. Rapid diagnosis not only improves patient outcomes but also enables timely interventions such as securing a ruptured aneurysm via endovascular coiling or surgical clipping.

In summary, recognizing the characteristic CT findings of subarachnoid hemorrhage is essential for prompt and effective treatment. While CT excels in detecting acute bleeds, awareness of its limitations over time ensures that alternative diagnostic pathways are pursued when necessary. For both medical professionals and patients, understanding the timeline and imaging features of SAH can make a significant difference in survival and recovery.