Overview

The PHASES score is a validated clinical prediction model that estimates the 5-year risk of aneurysmal subarachnoid hemorrhage (aSAH) from an unruptured intracranial saccular aneurysm. Each letter in the acronym represents an independent prognostic domain: Population, Hypertension, Age, Size (maximum diameter), Earlier subarachnoid hemorrhage from a different aneurysm, and Site (vascular location). Points are assigned to each domain and summed; higher totals correspond to higher modeled rupture risk over five years.

The score was developed by Greving and colleagues through a pooled individual-patient analysis of six prospective cohort studies comprising thousands of patients with unruptured intracranial aneurysms followed with serial imaging. PHASES is widely used in neurosurgery, neuroradiology, and stroke neurology to support shared decision-making between observation with surveillance imaging and preventive treatment (microsurgical clipping or endovascular coiling).

This calculator is for educational use. It does not replace specialist consultation, institutional pathways, or individualized assessment of procedural risk, comorbidity, and patient values.

Clinical background: unruptured intracranial aneurysms

Intracranial aneurysms are focal dilations of cerebral arteries, most commonly saccular (berry) aneurysms at arterial bifurcations. They are detected incidentally on brain imaging performed for unrelated symptoms, in screening contexts, or during evaluation of unrelated neurologic complaints. Population-based studies suggest a prevalence of several percent in adults, though most aneurysms never rupture.

When an aneurysm ruptures, blood enters the subarachnoid space, producing aneurysmal subarachnoid hemorrhage, a neurologic emergency with substantial morbidity and mortality even with modern critical care and endovascular techniques. The central clinical question for patients with an unruptured aneurysm is whether the expected benefit of preventive repair outweighs the risks of intervention and the baseline risk of rupture during observation.

Rupture risk is not uniform. Small internal carotid artery aneurysms in normotensive middle-aged North American or European patients carry lower modeled risk than large posterior circulation aneurysms in hypertensive elderly Finnish patients. PHASES quantifies this heterogeneity using readily available clinical and angiographic variables.

Why the PHASES score was developed

Before PHASES, clinicians relied on size thresholds (often 7 mm), location rules of thumb, and population-specific registries with differing follow-up protocols. Prior scores and guidelines emphasized maximum diameter but underweighted population-specific baseline incidence, prior hemorrhage from a separate aneurysm, and geographic ancestry effects on rupture rates.

PHASES was designed to produce a single integer score that could be mapped to 5-year rupture probabilities stratified by population group. By pooling prospective cohort data rather than retrospective case series alone, the developers aimed to reduce selection bias and provide externally applicable risk estimates for counseling patients about surveillance versus treatment.

Who should have PHASES calculated

PHASES applies to patients with a confirmed unruptured intracranial saccular aneurysm who are being considered for management planning. Typical scenarios include:

• Incidentally discovered aneurysm on CT angiography, MR angiography, or catheter angiography.
• Multiple aneurysms, where PHASES is calculated for each lesion individually.
• Serial imaging surveillance, where recalculation may be warranted if size category changes or new clinical risk factors emerge.

PHASES is not intended for:

• Already ruptured or acutely symptomatic aneurysms requiring emergency evaluation.
• Non-saccular aneurysms (fusiform, dissecting, infectious/mycotic) where natural history differs.
• Intracranial pseudoaneurysms or vascular malformations without a saccular aneurysm morphology.

Scoring domains in detail

P: Population

Geographic ancestry group modifies baseline rupture incidence in the derivation cohorts:

• North American or European (other than Finnish): 0 points (reference population).
• Japanese: +3 points, reflecting higher observed rupture rates in Japanese cohorts compared with North American and European cohorts at comparable aneurysm characteristics.
• Finnish: +5 points, reflecting the highest population-related weighting, consistent with elevated incidence in Finnish unruptured aneurysm registries.

Population assignment should follow the patient''s epidemiologic background as used in the original studies, recognizing that individual ancestry may be mixed and that local validation data may differ from derivation cohorts.

H: Hypertension

• No: 0 points
• Yes: +1 point for treated or untreated systemic arterial hypertension

Chronic hypertension contributes to hemodynamic stress on the aneurysm wall and is a modifiable risk factor. Blood pressure control remains a cornerstone of conservative management regardless of treatment strategy.

A: Age

• < 70 years: 0 points
• ≥ 70 years: +1 point

Advanced age increases modeled rupture risk in the pooled analysis. Age also interacts with treatment planning: older patients may face higher periprocedural risk, while younger patients have longer exposure time during observation, which favors preventive repair in higher-risk lesions.

S: Size (maximum diameter)

Maximum aneurysm diameter on imaging is the strongest single contributor to the score:

• < 7.0 mm: 0 points
• 7.0–9.9 mm: +3 points
• 10.0–19.9 mm: +6 points
• ≥ 20.0 mm: +10 points (giant aneurysm category)

Measurement should use the largest dimension on high-quality CTA, MRA, or DSA, with consistent technique across follow-up studies. Oblique measurements, partial thrombosis, and aneurysm neck morphology can complicate sizing; specialist review is valuable when growth is borderline between categories.

E: Earlier SAH from a different aneurysm

• No: 0 points
• Yes: +1 point if the patient had prior aneurysmal SAH attributed to rupture of a separate aneurysm

A history of prior aSAH identifies patients with demonstrated vulnerability to aneurysm rupture, independent of the index lesion''s current size. This variable does not apply to the same aneurysm after prior treatment; it specifically captures hemorrhage from a different aneurysm.

S: Site (vascular location)

• Internal carotid artery (ICA): 0 points (reference site)
• Middle cerebral artery (MCA): +2 points
• Anterior cerebral artery (ACA), posterior communicating artery (PCOM), or posterior circulation (basilar, vertebral, and related branches): +4 points

Posterior circulation and PCOM aneurysms have been associated with higher rupture rates in observational data. MCA aneurysms carry intermediate site-related risk compared with ICA lesions.

Complete scoring table

Domain	Finding	Points
Population	North American or European (other than Finnish)	0
	Japanese	3
	Finnish	5
Hypertension	No	0
	Yes	1
Age	< 70 years	0
	≥ 70 years	1
Size	< 7.0 mm	0
	7.0–9.9 mm	3
	10.0–19.9 mm	6
	≥ 20.0 mm	10
Earlier SAH (different aneurysm)	No	0
	Yes	1
Site	ICA	0
	MCA	2
	ACA, PCOM, or posterior circulation	4
Total score range		0–22

Interpreting the total score

The original PHASES publication provides score-specific 5-year rupture risk estimates that vary by population stratum. This calculator groups totals into three relative risk bands commonly used for counseling, while emphasizing that rupture can still occur at lower scores and that absolute percentages should be taken from the primary risk tables when precision is required.

Lower relative risk (score 0–3)

A total of 0–3 points is frequently cited as a lower-risk band in reviews and secondary analyses. Some studies use a threshold of 3 or less to denote comparatively lower modeled 5-year rupture risk. Typical profiles in this band include small ICA aneurysms in younger North American or European patients without hypertension or prior SAH.

Management often favors periodic vascular imaging surveillance with interval dictated by specialist guidelines, patient age, and any interval growth. Conservative management remains appropriate only when procedural risks, comorbidity, and life expectancy are weighed explicitly. Patients should receive education about thunderclap headache and acute focal neurologic deficits warranting emergency evaluation.

Intermediate relative risk (score 4–10)

Scores of 4–10 occupy a middle zone between lower-risk benchmarks and the highest-weighted combinations of factors. Examples include 7–9.9 mm aneurysms, MCA location, Japanese population weighting, or modest accumulation of several +1 and +2 factors.

Shared decision-making is central in this range. Clinicians balance modeled rupture risk against risks of open microsurgery or endovascular treatment, including periprocedural stroke, hemorrhage, cranial nerve injury, radiation exposure (for some endovascular approaches), and need for antiplatelet therapy after stent-assisted coiling. Neurosurgical or neurointerventional consultation is often appropriate. Serial imaging frequency may be increased when growth is detected or when morphology is unfavorable.

Higher relative risk (score 11–22)

Totals of 11–22 reflect accumulation of major drivers, such as large or giant size (≥10 mm categories, especially ≥20 mm), Finnish or Japanese population weighting, posterior circulation or PCOM site, advanced age, hypertension, and prior SAH from another aneurysm. These profiles associate with materially higher modeled 5-year rupture risk in the derivation analysis.

Patients in this band generally merit prompt specialist evaluation. Treatment versus continued surveillance is decided through multidisciplinary assessment, informed consent, and local expertise. Acute symptoms (sudden severe headache, cranial neuropathy, seizure, or focal deficit) require emergency work-up regardless of PHASES score.

Clinical application and management context

PHASES informs but does not dictate management. Professional guidelines and institutional pathways integrate rupture risk with:

• Patient age and life expectancy, including competing mortality risks.
• Aneurysm morphology not captured by PHASES (irregular dome, daughter sac, high aspect ratio, wide neck, branch vessel incorporation).
• Interval growth on serial imaging, a dynamic marker that may outweigh a single static score.
• Symptoms such as mass effect, cranial nerve palsy, or thromboembolic events from partial thrombosis.
• Family history of subarachnoid hemorrhage or connective tissue disorders (for example polycystic kidney disease, Ehlers-Danlos spectrum), which may lower the threshold for intervention in some settings.
• Procedural candidacy and center volume for clipping versus coiling.
• Antithrombotic requirements if the patient requires antiplatelet or anticoagulant therapy for other conditions.

All patients benefit from modifiable risk factor reduction: blood pressure control, smoking cessation, avoidance of stimulant drugs associated with hypertensive surges when clinically relevant, and clear return precautions for sudden headache or neurologic change.

Relationship to other risk tools

PHASES is among the most widely cited scores for unruptured aneurysms because of its prospective pooled derivation and population-specific risk tables. Other approaches include simpler size-location rules, the UIATS (Unruptured Intracranial Aneurysm Treatment Score) for multidisciplinary treatment consensus, and morphology-focused assessments. PHASES excels at quantitative 5-year risk framing; UIATS and team-based review excel at integrating factors not in the acronym (comorbidity, patient preference, technical feasibility).

Using PHASES alongside specialist judgment and serial imaging is standard practice rather than relying on the integer score alone.

Special considerations

Multiple aneurysms: Calculate PHASES separately for each lesion. Management prioritizes the highest-risk aneurysm or symptomatic lesion while monitoring others.

Post-treatment remnants: Residual neck or regrowth may require reassessment; PHASES was developed for untreated unruptured aneurysms.

Pediatric aneurysms: PHASES was derived in predominantly adult cohorts; pediatric cases require specialist pathways.

Ethnicity and migration: Population points reflect cohort epidemiology; patients with mixed heritage or long-term residence outside ancestral regions may need contextual interpretation.

Imaging modality differences: CTA, MRA, and DSA may yield slightly different size measurements; consistency across follow-up is essential.

Limitations

• PHASES estimates population-averaged risk; individual rupture probability remains uncertain for any single patient.
• Exact 5-year percentages depend on population stratum and should be read from primary risk tables when counseling requires numeric precision.
• Morphologic instability features are not fully encoded in the six domains.
• External validation across contemporary treatment eras, diverse health systems, and ethnic groups may show calibration differences.
• The score does not incorporate patient-reported outcomes, occupational factors, or geographic access to specialized care.
• Rupture despite low PHASES scores reinforces that no score eliminates risk entirely.

How to use this calculator

On the Calculator tab, select one option for each PHASES domain: Population, Hypertension, Age, Size, Earlier SAH from another aneurysm, and Site. The tool sums points (range 0–22), displays a criterion-by-criterion breakdown, and assigns a relative risk band (lower, intermediate, or higher). Use the Formula Explained tab for the full scoring table and clinical cautions.

Recompute the score when maximum diameter crosses a size threshold on follow-up imaging or when major clinical inputs change. Document the score, the counseling discussion, and the agreed surveillance or treatment plan in the medical record.

Disclaimer: This content is for educational purposes only and is not medical advice. Management of intracranial aneurysms requires evaluation by qualified neurosurgical or neurointerventional specialists familiar with the individual patient and local resources.