General Description

The circle of Willis provides vital, if theoretical, redundancy for collateral cerebral blood supply. Balloon test occlusion (BTO) verifies the adequacy of collateral supply should the need arise to sacrifice or potentially compromise one of the major afferent vessels. It is a way of testing whether a permanent clinically significant neurologic deficit will result based on temporary occlusion of a particular vessel.

Indications

BTO plays an important role in treatment planning for a variety of disease processes. Head and neck cancers that invade, encase, or compromise the carotid arteries can complicate resection. Foreknowledge that the vessel can be sacrificed, should the need arise, can simplify treatment strategies or enable backup plans. Large and complex aneurysms, particularly those near the skull base, are increasingly treated with flow diversion strategies, but vessel sacrifice remains a viable alternative if the interventionist can assure adequate intracranial flow through BTO.

Neuroendovascular Anatomy

The cervical internal carotid arteries (ICAs) are the most common target for BTO. To isolate flow to the intracranial compartment, the BTO is undertaken just distal to the carotid bifurcation. The external carotid artery can supply collateral or retrograde flow that can confuse results if this is not isolated during the test.

Specific Technique and Key Steps

Carotid artery puncture is best used for anterograde access to ipsilateral lesions. Carotid pulses are palpated to identify the target vessel starting approximately 2 cm above the clavicle and traced cephalad to approximately the level of the hyoid cartilage. Whereas access to distal sites can be undertaken with the patient under conscious sedation, carotid access is preferably performed under general anesthesia ( Fig. 5.1 and Video 5.1 ).

1. 
   

   Distal access with a 6 or 7 French (F) sheath is obtained.

   
   
2. 
   

   Using a diagnostic catheter of choice, a complete cerebral angiogram is performed to assure understanding of the underlying anatomy ( Fig. 9.1, 9.2 and Video 9.1, 9,2).

   
   
3. 
   

   The target vessel, usually a cervical ICA, is selected with the diagnostic catheter, at which point an exchange-length 0.035-inch guidewire is advanced into the target vessel with good distal purchase and the diagnostic catheter is exchanged out.

   
   
4. 
   

   Depending on the nondiseased size of the vessel, a 6 or 7F balloon guide catheter is advanced into the target vessel and positioned distal to the bifurcation, thereby isolating the blood supply of interest.

   
   
5. 
   

   The patient is given heparin, and therapeutic activated clotting time (ACT) is documented.

   
   
6. 
   

   A baseline neurological examination is performed prior to balloon inflation and under conditions of normotension.

   
   
7. 
   

   The balloon is inflated with a two-hand technique, with the practitioner puffing contrast material through the lumen of the catheter while gently inflating the balloon. The balloon is inflated only sufficiently to induce contrast stasis within the ICA or vertebral artery. Aggressive inflation can cause carotid dissection and should be avoided. The static contrast material will serve as an indicator of occlusion over the course of the test.

   
   
8. 
   

   Every 3 minutes for a duration of 15 minutes, the practitioner should (1) perform fluoroscopy to evaluate for contrast status, keeping in mind that the contrast material is expected to slowly wash out with the forward flow of heparinized saline through the flush; and (2) perform a neurological examination. If changes are observed in the neurological examination, the patient has demonstrated that the occluded circulation is essential and the test should be aborted ( Video 9.1, 9,2).

   
   
9. 
   

   If no neurological changes are observed under normotensive conditions over the course of 15 minutes, the practitioner then starts an intravenous infusion of an antihypertensive agent (e.g., sodium nitroprusside), with the goal of dropping the mean arterial pressure (MAP) to 75% of the patient’s baseline.

   
   
10. 
    

    Once the goal MAP is obtained, the practitioner repeats frequent neurological examinations as per step 8.

    
    
11. 
    

    If the patient is able to maintain good neurological function throughout the period of hypotension, the patient is deemed to have passed the test. The balloon is deflated, and the catheter is withdrawn into the common carotid artery ( Video 9.1, 9.2 ).

    
    
12. 
    

    Follow-up angiographic evaluations of both the cervical and intracranial arterial trees should be performed to ensure that no dissection and no thrombosis have occurred.

Device Selection

1. 
   

   6–7F arterial sheath insertion requires the following:

   
   
   
   
   1. 
      

      Microaccess kit (microneedle, microwire).

      
      
   2. 
      

      6–7F sheath.

   
   
2. 
   

   Diagnostic arteriography requires the following:

   
   
   
   
   1. 
      

      Simmons 2 diagnostic catheter (Cordis).

      
      
   2. 
      

      0.035-inch Glidewire (Terumo).

   
   
3. 
   

   BTO requires the following:

   
   
   
   
   1. 
      

      6 or 7F balloon guide catheter.

      
      
   2. 
      

      0.035-inch exchange-length Glidewire (Terumo).

      
      
   3. 
      

      3-mL syringe with hemostatic valve filled with 50% contrast material.

      
      
   4. 
      

      Infusion of an antihypertensive agent (e.g., sodium nitroprusside).

Pearls

• 
  

  Use the most compliant balloon available.

  
  
• 
  

  Avoid overinflation and repeated inflation of balloons and do not manipulate a balloon while it is inflated.

  
  
• 
  

  Always inflate the balloon under fluoroscopic visualization ( Video 9.1, 9.2 ).

  
  
• 
  

  Anticoagulate with heparin and maintain ACT above 300 seconds.

  
  
• 
  

  It is important to keep tight control of blood pressures throughout the duration of the examination because the natural tendency of the patient’s blood pressure is to increase to counteract decreased blood flow. Maintaining hypotensive pressure goals can be particularly difficult in younger patients with good cardiovascular function ( Video 9.1, 9.2 ).

  
  
• 
  

  If a leak is detected around the balloon, the balloon should be reinflated and the schedule of neurological examinations should be restarted to avoid a false-negative test.

  
  
• 
  

  Patients with severe atherosclerotic disease may not be good candidates for BTO. As an alternative, a small compliant balloon microcatheter may be advanced into the petrous segment of the carotid artery if the burden of disease is less in that location. There is a greater risk of dissection within the petrous carotid, so care must be taken to avoid overinflation of the balloon.

  
  
• 
  

  Even though a patient may not demonstrate any deficit on an appropriately performed BTO, this does not guarantee that the same patient will not develop a deficit once the parent artery is definitively sacrificed. A false-negative rate up to 20% has been reported.