What is the likely cause of thunderclap headaches in a 42-year-old patient with no evidence of aneurysm on computed tomography (CT) scan, whose headaches consistently occur during physical exertion or at the point of sexual climax?

Reversible Cerebral Vasoconstriction Syndrome (RCVS)

The most likely diagnosis in this 42-year-old patient with recurrent thunderclap headaches triggered by sexual climax and exertion, with negative CT angiography for aneurysm, is reversible cerebral vasoconstriction syndrome (RCVS). 1, 2

Why RCVS is the Leading Diagnosis

RCVS has emerged as the most frequent cause of thunderclap headache in patients without aneurysmal subarachnoid hemorrhage and is the most common cause of recurrent thunderclap headaches. 2 The clinical presentation described is pathognomonic for RCVS:

• Exertional and sexual activity triggers: The typical thunderclap headaches of RCVS are brought on by exertion, sexual activities, emotion, Valsalva maneuvers, or bathing—all triggers that induce sympathetic activation. 2
• Recurrent pattern: RCVS characteristically presents with multiple thunderclap headaches recurring over days to weeks, which distinguishes it from single-episode causes like aneurysmal SAH. 2
• Age appropriateness: Patients with RCVS tend to be older (mean age ~51 years) compared to other primary headache disorders. 3

Diagnostic Workup Required

Despite the negative CT scan, you must complete a comprehensive vascular imaging evaluation because standard CT without angiography has limited sensitivity for detecting the segmental vasoconstriction characteristic of RCVS:

• CT angiography (CTA) or MR angiography (MRA) is essential to identify the multifocal narrowing and dilatation of cerebral arteries that defines RCVS. 1, 4
• Brain MRI with susceptibility-weighted imaging (SWI) should be obtained to evaluate for complications including intracerebral hemorrhage, posterior reversible encephalopathy syndrome (PRES), or ischemic stroke. 1
• Lumbar puncture remains mandatory if not yet performed, as up to 50% of RCVS patients may have evidence of subarachnoid hemorrhage on CSF analysis despite negative initial CT. 4
• Serial transcranial Doppler sonography can track vasoconstriction over time. 5
• Follow-up angiography at 3 months is required to confirm reversibility of vasoconstriction, which is the defining feature of RCVS. 4

Critical Differential Diagnoses to Exclude

While RCVS is most likely, you must systematically exclude life-threatening alternatives:

Subarachnoid Hemorrhage

• Even with negative CT, SAH accounts for 10-25% of thunderclap headaches with 27-44% mortality. 1
• CT sensitivity drops significantly after 6 hours, making lumbar puncture mandatory when clinical suspicion remains high. 1
• Xanthochromia testing (performed >6 hours after onset) has 100% sensitivity and 95.2% specificity. 6

Cervical Artery Dissection

• Presents with thunderclap headache in up to 20% of cases, particularly during exertion. 1
• Look for associated Horner syndrome, neck pain, or focal neurological deficits. 1

Cerebral Venous Sinus Thrombosis

• Can mimic SAH with thunderclap presentation. 1
• MR venography or CT venography is required if not already performed. 7

Primary Exertional or Sexual Headache

• These are diagnoses of exclusion only after complete vascular imaging rules out secondary causes. 8
• The recurrent pattern over days to weeks makes primary headache disorders less likely. 2

Incidence and Clinical Context

RCVS accounts for approximately 8.8% to 45% of thunderclap headaches without aneurysmal SAH in emergency department populations, depending on the rigor of diagnostic evaluation. 3, 5 The wide range reflects under-recognition of this syndrome when comprehensive vascular imaging is not performed. 5

Management Implications

Early diagnosis of RCVS is critical because:

• Stroke risk: A minority of RCVS cases progress to ischemic or hemorrhagic stroke, and rarely death. 2
• Trigger removal: Identifying and eliminating precipitants (vasoactive substances, sympathomimetics) is essential. 2
• Avoiding harmful interventions: Misdiagnosis can lead to unnecessary procedures or inappropriate treatments. 4

Red Flags Present in This Case

This patient meets multiple criteria from the Ottawa SAH Rule that mandate aggressive workup: 1, 6

• Age ≥40 years
• Onset during exertion
• Thunderclap headache quality

Never assume exertional headache is benign without proper vascular imaging, as missing reversible cerebral vasoconstriction syndrome or subarachnoid hemorrhage carries catastrophic consequences. 6

Probable RCVS Diagnosis

According to the 2013 International Classification of Headache Disorders revision, you can diagnose "probable RCVS" when the headache pattern is typical (recurrent thunderclap headaches with exertional/sexual triggers), even before angiographic confirmation, if other causes are excluded. 2 However, definitive diagnosis requires demonstrating reversibility on follow-up imaging at 3 months. 4