The first investigation of choice in a patient with suspected subarachnoid haemorrhage should be –
**Core Concept**
In patients with suspected subarachnoid hemorrhage (SAH), prompt and accurate diagnosis is crucial to initiate timely management. Non-contrast computed tomography (CT) is the preferred initial imaging modality due to its high sensitivity and speed in detecting acute hemorrhage. This is because CT scans can quickly identify blood in the subarachnoid space, which is a hallmark of SAH.
**Why the Correct Answer is Right**
Non-contrast CT is the first-line investigation for SAH because it can detect acute blood in the subarachnoid space with high sensitivity. This is attributed to the high density of fresh blood on CT scans, which is visible even in the absence of contrast. The sensitivity of non-contrast CT for SAH is particularly high in the first 24 hours after the event. The blood appears as a hyperdense area in the subarachnoid space on the CT scan, which is a characteristic feature of SAH.
**Why Each Wrong Option is Incorrect**
**Option B:** CSF examination may be performed to confirm the diagnosis of SAH, but it is not the first investigation of choice. CSF analysis may show blood in the CSF, but it may take several hours for blood to appear in the CSF, and it may not be detectable until 12-24 hours after the hemorrhage.
**Option C:** MRI is a sensitive imaging modality for SAH, but it is not the first-line investigation. MRI takes longer to perform than CT and may not be immediately available in emergency situations. Additionally, MRI may not be as sensitive as CT in detecting acute blood in the subarachnoid space.
**Option D:** Contrast-enhanced CT is not the first investigation of choice for SAH. The use of contrast may obscure the detection of acute blood in the subarachnoid space, and it is not necessary for the initial evaluation of SAH.
**Clinical Pearl / High-Yield Fact**
In the setting of SAH, it is essential to remember that a negative non-contrast CT scan does not rule out SAH, particularly if the scan is performed more than 24 hours after the event. In such cases, further investigation with MRI or lumbar puncture may be necessary to confirm the diagnosis.
**✓ Correct Answer: A. Non-contrast computed tomography**