Neurointervention · IR Playbook

Balloon Test Occlusion of the Internal Carotid Artery (BTO)

Balloon test occlusion is a temporary endovascular trial of internal carotid artery sacrifice performed to predict whether a patient will tolerate permanent occlusion. A compliant balloon arrests antegrade flow so that the hemisphere is perfused only by circle of Willis and leptomeningeal collaterals; adequacy of that collateral supply is then judged clinically, angiographically and haemodynamically, which is why every failure mode of the test is a failure of collateral reserve or of embolus control. Abrupt carotid sacrifice without testing has been reported to carry a cerebral infarction risk of approximately 26 percent. The test itself is a catheter angiographic procedure and its principal hazard is thromboembolic stroke.

Learning objectives

• Select patients in whom carotid sacrifice is being contemplated and who can cooperate with neurological testing
• Position and inflate the occlusion balloon to arrest flow without causing dissection or carotid sinus reflex
• Interpret clinical, venous phase and stump pressure endpoints to classify the patient as pass or fail
• Recognise and manage thromboembolic and access complications arising during the test

Indications

• Planned therapeutic internal carotid artery sacrifice for giant, fusiform, blister or dissecting aneurysm not amenable to reconstruction
• Planned en bloc resection of skull base or head and neck malignancy encasing the internal carotid artery
• Carotid blowout syndrome in the head and neck where deconstructive management is contemplated
• Traumatic or iatrogenic carotid injury or carotid cavernous fistula where parent vessel occlusion is a treatment option

Absolute contraindications

• Inability to perform a neurological examination during occlusion
• Uncontrolled active haemorrhage requiring immediate sacrifice without time for testing

Relative contraindications

• Severe uncorrected coagulopathy or contraindication to systemic heparinisation
• Severe contrast allergy or renal impairment
• Heavy atherosclerotic or ulcerated plaque in the segment to be occluded

Equipment

• 6 Fr femoral sheath and 5 Fr contralateral sheath
• 6 Fr guide catheter, 90-100 cm
• 5 Fr diagnostic catheter for contralateral control injection
• Compliant occlusion balloon catheter, 4-7 mm diameter
• 0.014 inch or 0.016 inch steerable guidewire
• Diluted iodinated contrast for balloon inflation to allow fluoroscopic monitoring
• Heparinised saline flush and unfractionated heparin
• Activated clotting time analyser
• Pressure transducer for stump pressure measurement
• Non-invasive blood pressure monitoring, and sodium nitroprusside or labetalol if a hypotensive challenge is planned

Procedure steps

1. Record speech, orientation, cranial nerves, and power in all four limbs with the patient awake on the table, and agree a repeatable motor task such as counting aloud while squeezing a squeaky toy in the contralateral hand. This baseline is the reference against which every subsequent examination is compared, so an incomplete baseline makes an equivocal deficit uninterpretable.
2. Place a 6 Fr sheath in the right common femoral artery and a contralateral sheath if a control injection will be needed. Perform selective injections of both internal carotid arteries and at least one vertebral artery to map the circle of Willis, define the anterior and posterior communicating arteries, identify a fetal posterior cerebral artery, and record baseline cortical venous phase timing.
3. Give unfractionated heparin 70-100 units per kilogram intravenously and confirm an activated clotting time above 250 seconds before the balloon is inflated. Stasis of blood in the column distal to an inflated balloon is intensely thrombogenic and full anticoagulation is what separates a diagnostic test from an iatrogenic stroke.
4. Advance the guide catheter into the cervical internal carotid artery and track the compliant balloon over a soft wire to sit at or just below the intended level of permanent occlusion, keeping it clear of the carotid bulb. Size the balloon to the measured vessel diameter, typically 4-7 mm, so that flow arrest is achieved at low inflation volume.Pearl: Placing the balloon above the bulb avoids carotid sinus reflex bradycardia, which otherwise drops systemic pressure and produces a false positive test.
5. Inflate slowly with diluted contrast while watching balloon profile, and confirm complete arrest by a gentle hand injection through the guide catheter showing a static contrast column with no antegrade flow. Stop inflating at the first sign of vessel straightening or waisting resolution; oversizing risks dissection.
6. Transduce the distal balloon lumen or guide catheter tip distal to the occlusion and record mean stump pressure against simultaneous systemic mean arterial pressure. Express the result as a ratio; a stump pressure ratio above 60 percent supports adequate collateral reserve.
7. Repeat the neurological examination every 1-2 minutes for the first 5 minutes and then every 5 minutes, watching for dysphasia, hemiparesis, neglect, reduced conscious level or failure of the agreed motor task. Any reproducible new deficit ends the test immediately — deflate at once rather than completing the planned duration.Pearl: Most deficits that will occur appear within the first few minutes; a patient who is neurologically intact at 5 minutes rarely fails later, but the full period is still needed because delayed failures occur.
8. With the balloon still inflated, inject the contralateral internal carotid artery, and the vertebral artery if posterior collateral supply is relevant, and compare the time to cortical venous opacification between the tested and control hemispheres. A delay under 2 seconds supports tolerance; a delay above 4 seconds is regarded as a contraindication to permanent sacrifice even when the patient is clinically intact.Pearl: Venous phase asymmetry detects a hemisphere that is perfused but with exhausted reserve — the group that fails after permanent occlusion despite passing the clinical test.
9. Standard textbook technique: in a patient who is clinically intact but has borderline venous delay or stump pressure, reduce mean arterial pressure by approximately one third with a short acting agent while maintaining occlusion, and re-examine. This unmasks marginal collateral reserve; any deficit during the challenge is a failed test.
10. Deflate fully under fluoroscopy, wait briefly, then perform a control angiogram of the tested vessel to exclude dissection, thrombus and distal embolus before withdrawing the balloon. Reverse or allow heparin to wear off according to whether the sacrifice will proceed in the same sitting.

Complications

• Thromboembolic stroke
• Haemodynamic ischaemia during occlusion
• Arterial dissection at the balloon site
• Groin access site haematoma or pseudoaneurysm
• Contrast induced nephropathy

Exam pearls

• During balloon test occlusion of the internal carotid artery, a cortical venous phase delay of less than 2 seconds relative to the control hemisphere supports safe permanent occlusion, whereas delay greater than 4 seconds is treated as a contraindication.
• A stump pressure ratio above 60 percent of systemic mean arterial pressure is the accepted haemodynamic threshold for tolerance of permanent internal carotid artery occlusion.
• A new deficit that resolves within minutes of balloon deflation indicates haemodynamic collateral failure; one that persists indicates thromboembolic stroke and demands immediate angiography.
• Inflating the occlusion balloon at or within the carotid bulb triggers carotid sinus reflex bradycardia and hypotension, which can produce a false positive balloon test occlusion.
• Abrupt internal carotid artery sacrifice without prior balloon test occlusion has been reported to carry a cerebral infarction risk of approximately 26 percent.

Viva questions

Your patient is neurologically intact throughout a 30 minute occlusion but the venous phase on the tested side lags the control hemisphere by 3.5 seconds. Do you sacrifice the carotid?

No, not on that result alone. Clinical tolerance at rest only shows perfusion is above the threshold for symptoms, whereas venous delay reflects exhausted collateral reserve, and delay in this range predicts delayed infarction after permanent occlusion. Add a hypotensive challenge or perfusion imaging, and plan bypass or a reconstructive strategy if the result remains borderline.

Why does balloon position relative to the carotid bulb matter, and what happens if you get it wrong?

Inflation within or immediately above the bulb stimulates carotid sinus baroreceptors, causing bradycardia and systemic hypotension. That simultaneously drops collateral perfusion pressure and confounds the neurological assessment, producing a false positive test in a patient who would in fact have tolerated occlusion.

The patient develops dense hemiparesis at 12 minutes. You deflate and it does not resolve after 10 minutes. What is your reasoning and what do you do?

Haemodynamic ischaemia reverses within minutes of restoring flow, so a persistent deficit after deflation indicates thromboembolism rather than collateral failure. Check the activated clotting time, give further heparin, and perform immediate angiography with a view to mechanical thrombectomy of a large vessel occlusion.

How does a fetal posterior cerebral artery on the side being tested change your interpretation?

It means the occipital lobe is supplied by the tested internal carotid artery rather than the basilar, so occlusion removes both anterior and posterior circulation supply to that hemisphere and there is no posterior communicating collateral to recruit. The territory at risk is larger and a pass should be regarded with more caution.

Why must the patient not be sedated, and what do you do if they cannot cooperate?

Serial neurological examination is the primary endpoint of the test and sedation abolishes it, leaving only surrogate angiographic and pressure measures. In an uncooperative patient, either abandon clinical BTO and rely on quantitative perfusion imaging during occlusion, or reconsider a reconstructive rather than deconstructive strategy.

What does a stump pressure ratio of 45 percent tell you in a patient with no deficit and symmetric venous phase?

It indicates collateral perfusion pressure below the accepted 60 percent threshold, so the patient has functional but low-pressure collateral supply that may fail under any future hypotensive stress. This is the classic indication to proceed to a hypotensive challenge or perfusion imaging rather than accepting the test as passed.

References