Primer on Nerve Anatomy to Avoid Complications: An Interview With Matthew Callstrom, MD, PhD

Dr Callstrom, Professor of Radiology at Mayo Clinic College of Medicine in Rochester, Minnesota, spoke with IO Learning at the 2019 Symposium for CIO and shared his thoughts about the complications associated with ablation in various areas of the body due to nerve anatomy, and the importance of learning such anatomy to avoid these complications. 

Your presentation dealt with avoiding complications during image-guided ablation procedures. Can you explain?

At a very high level, there are challenges to doing ablation anywhere in the body. One of the primary risks is causing a nerve injury, which can be very significant, such as a motor injury. For example, injury of the sciatic nerve can cause permanent disability, such as loss of movement and feeling in the affected leg. Some nerves, like the femoral nerve, innervate the muscles of the anterior thigh. It can be difficult to see the nerve, so we must ask ourselves the following: Where is the nerve? How do I avoid it? Do I understand the risks involved if the nerve is damaged? The point is that the operator must have an awareness of nerve anatomy, recognize the potential risk, and plan for the risk and/or monitor the situation to try and avoid the nerve. In many of the procedures that we do, we may not see the nerves because they are such small structures, so we must have a sound knowledge of nerve anatomy.

Are there any imaging modalities that do allow this visibility?

No. If the proceduralist looks hard enough, he or she can see anything. With magnetic resonance imaging, it is a little easier to see the nerve if one knows exactly where to look. Usually, the operator can find it. Often, when we do procedures, the imaging is not quite as good as it might be in the diagnostic realm, so the ability to see nerves is more challenging. 

Is there a knowledge gap in training on nerve anatomy?

Yes, it’s very interesting. The liver is where the world of ablation started. There’s no real risk of nerve damage in the liver, but as we started to branch into doing ablation outside the liver, we needed to learn neural anatomy. Renal ablation and lung ablation, for example, are challenging, and we had to learn about the different treatment areas as far as what to look for during treatment, as well as what happens if nerve damage occurs in these areas. We have the knowledge of anatomy and can understand issues that might arise if nerve damage occurs.

Do you have a suggestion as to a first step for readers interested in brushing up on their nerve anatomy knowledge? 

Dr Nick Kurup from our group published a paper on nerve anatomy that is very helpful as a reference for ablation proceduralists.¹

You mentioned injury to the sciatic nerve and spinal cord. Can you elaborate?

Clearly, the proceduralist must avoid the spinal cord and major motor nerves, so one must learn where the motor nerves in any region of the body are located because that’s the primary risk. For example, when treating an upper extremity, where are the nerves that feed the hand? Where are the nerves in the anterior and posterior leg? Where is it safe to do the procedure and where must extra care be taken? The nerves obviously course throughout the body, so when treating a lung tumor, the brachial plexus, or upper chest, one must worry about the phrenic nerve. When treating the abdominal wall, the nerves in the flank must be considered. In the lower extremity, one must worry about the sciatic and femoral nerves. 

What would you say is the most challenging area?

We’ve had more femoral nerve injuries than anything, because the course is so complicated. It’s in a location that’s a little bit variable as it courses through the pelvis, so we focus on that area, as it is high risk.

Are there any additional steps that you can recommend?

There are a few. With CT imaging, there are a few landmark areas one can use to find the femoral nerve, or one knows where it “should” be, so must understand where it exits from the spinal cord and traverses to the groin. Basically, proceduralists must be familiar with the neural anatomy, and proceed accordingly.

Is this similar to a mapping study for the femoral nerve?

Yes, in some ways. The nerve follows a specific course, and it follows a course relative to other anatomy that one can identify. For example, the muscle that allows a person to lift the leg is called the iliopsoas muscle. The nerve sits on the lateral margin of that muscle, so we avoid that region. We can’t always see it, but we know it’s there. There are variants in the human body in many different ways, but the variance is still relatively narrow. The variance is enough that one must be more respectful of that area, because it might be 1 centimeter away from its expected location. As the proceduralist gains experience, they discover that it’s very predictable. The proceduralist just has to know where to look.

Should these considerations be part of the thought process behind patient selection and informed consent? 

Yes, but probably more from a philosophical perspective. When patients come to us for treatment, they don’t know the relative risk of what we’re doing, but we do. When we discuss informed consent with the patient and state that “we’re treating a tumor that’s near your sciatic nerve,” we are saying that we think we could hurt it, but the patient hears that we will avoid the sciatic nerve and not hurt them. Proceduralists must clearly communicate the risk to the patient. The proceduralist must be as careful as possible and understand that knowledge of nerve anatomy is important, and this boundary is based on one’s experience. If one is highly knowledgeable about nerve anatomy, one can more safely treat patients.

How do you explain this particular aspect of risk to patients?

Usually, I explain that the treatment will be close to an important nerve. I’m going to do my best to avoid that nerve. I’m going to be very careful and use nerve monitoring or imaging, and I’m going to do everything I can to avoid nerve damage.

Any final thoughts?

As proceduralists expand the types of ablation procedures they perform beyond the liver, knowledge of neural anatomy is important to avoid complications.

Reference

1. Kurup NA, Morris JM, Schmit GD, et al. Neuroanatomic considerations in percutaneous tumor ablation. Radiographics. 2013;33:1195-1215.

Disclosures: None.

Address for correspondence: Matthew Callstrom, MD, PhD, Department of Radiology, Mayo Clinic, 200 First St SW, Rochester, MN 55905. Email: callstrom.matthew@mayo.edu