Why are vagus nerve stimulators (VNS) typically placed on the left side instead of the right side?

Why Vagus Nerve Stimulators Are Typically Placed on the Left Side

Vagus nerve stimulators (VNS) are implanted on the left side rather than the right side primarily to reduce the risk of cardiac side effects, as the right vagus nerve has more cardiac innervation and stimulation could potentially cause bradyarrhythmias or other cardiac complications. 1

Anatomical and Physiological Rationale

The placement of VNS on the left side is based on several important considerations:

1. Cardiac Innervation Differences:

   • The right vagus nerve provides more significant innervation to the sinoatrial node (the heart's natural pacemaker)
   • Left vagus nerve stimulation carries lower risk of cardiac complications like bradycardia and asystole
   • Stimulation of the right vagus nerve can more readily cause heart rate changes and arrhythmias

2. Recurrent Laryngeal Nerve Anatomy:

   • Both vagus nerves contain fibers that contribute to the recurrent laryngeal nerve
   • The left recurrent laryngeal nerve follows a longer path, looping around the aortic arch
   • This anatomical arrangement makes the left side more predictable for surgical approach 2

Clinical Evidence Supporting Left-Sided Placement

The standard practice of left-sided VNS placement is supported by multiple guidelines and research:

• Clinical trials for VNS in epilepsy have predominantly used left-sided implantation 3
• The ANTHEM-HF trial showed that left-sided VNS could be performed with higher stimulation amplitudes (2.2 ± 0.5 mA) compared to right-sided VNS (1.7 ± 0.5 mA) 4
• Surgical techniques for VNS implantation are standardized for the left side to minimize complications 1

Right-Sided VNS: Exceptions and Considerations

While left-sided placement is standard, right-sided VNS may be considered in specific circumstances:

• When left vagus nerve is damaged or inaccessible due to previous surgery
• In cases of left-sided anatomical constraints or infection at the left implant site
• When VNS lead deficiency occurs on the left side 5

However, right-sided VNS carries important caveats:

• Requires thorough preoperative cardiac assessment (84% of right-sided VNS patients had this in a multicenter study)
• Three patients in a 38-patient study experienced postoperative cardiac side effects with right-sided placement
• May have comparable effectiveness (55% responder rate) but potentially lower tolerability 5

Surgical Approach and Complications

The surgical technique for VNS implantation involves:

• Blunt dissection technique to identify and isolate the vagus nerve
• Placement of a helical electrode with three contacts around the nerve
• Connection to a pulse generator typically placed in the subclavian region 1

Common complications include:

• Laryngopharyngeal dysfunction (hoarseness, dyspnea, coughing) in approximately 66% of patients
• Vocal cord paralysis (rare, 1-2.7%)
• Infection (3-8%)
• Potential cardiac complications, particularly with right-sided placement 1, 6

Clinical Implications

When considering VNS implantation:

• Left-sided placement should be the default approach for most patients
• Right-sided placement should only be considered when left-sided implantation is impossible
• Careful cardiac monitoring is essential, especially with right-sided implantation
• Stimulation parameters should be adjusted based on patient tolerance and side effects 3

In conclusion, the preference for left-sided VNS placement is based on solid anatomical and physiological principles that prioritize patient safety while maintaining therapeutic efficacy. The right-sided approach remains a viable but less preferred alternative when left-sided implantation is not feasible.