Fig. 19.1
The postsynaptic dorsal column pathway. Importantly, these fibers ascend ipsilaterally within the spinal cord
Divergently, the somatic nerves that supply the abdominal wall have primary afferents within the dorsal horn and ascend contralateral in the spinothalamic tract, before synapsing with thalamus and then unto the somatosensory cortex. The nerves that are within the transversus abdominis plane (the plane between the transversus abdominal muscle and the internal oblique) provide sensation to the anterior abdominal wall bilaterally. Branches of spinal nerves from T7–L1 are located within this plane.
The transversus abdominis plane (TAP) block procedure was first described in 2001 by Rafi [1] as a blind landmark technique targeting the lumbar triangle of Petit (Fig. 19.2).
Fig. 19.2
The transversus abdominal plane is between the internal oblique and the transversus abdominis muscles
More recently, an ultrasound-guided subcostal approach has been utilized to place the stimulating leads in the plane between the internal oblique and the transversus abdominis muscles. A third method of lead placement has been described in this area that involves a percutaneous nerve stimulator to guide placement of a small novel lead. This device, StimRouter (Bioness; Valencia, CA) is now in U. S. Food and Drug Administration (FDA) studies.
19.2.1 The Percutaneous Method of Epidural Stimulation
In some settings physicians have chosen to treat this complex problem with spinal cord stimulation (SCS). Once the epidural needle is in place, the lead is targeted to the nerves that are involved in the generation of pain. The traditional approach involves placing the lead at the level of T4–7. This is often done in parallel or a staggered array. If these lead arrays are successful in controlling abdominal pain, no further adaptations are needed. In many settings, the pain best treated by this method is visceral in nature. Kapural et al. [2] have shown the most common lead placement for this abnormality is at T5 or T6. This pattern often leads to total coverage of the entire abdomen. In selected patients the paresthesia is unpleasant or does not adequately cover the patient’s pain. In these cases the epidural approach is not the ideal treatment option.
19.2.2 The Method of Transversus Abdominis Plane Stimulation
The anatomical boundaries of the lumbar triangle of Petit are external oblique muscle anteriorly, latissimus dorsi posteriorly, and iliac crest inferiorly. This may act as the landmark in order to facilitate the placement of neurostimulator leads in the neurovascular plane.
Ultrasound-guided access to the transversus abdominis neurovascular plane may be performed using an oblique subcostal approach or a horizontal approach at the midaxillary line between the postal margin and the iliac crest. Prior to attempting this method the physician should be well versed in ultrasound imaging and the anatomy of the musculature of this region.
Hydrodissection of the transverse abdominis plane with normal saline may be employed to facilitate placement of neurostimulator leads. However, utilizing this approach may limit the ability to stimulate the surrounding tissues until there has been absorption of the fluid.
Using a needle with a plastic stylet, the needle should be bent to mimic the convexity of the anterior abdominal wall in order to place the lead electrode in the transversus abdominal plane along this convexity. This may be more easily facilitated by advancing the lead from a lateral to a medial direction. Furthermore during permanent placement of leads, careful dissection to the anterior abdominal wall should be performed before needle entry and placement in order to ensure that all contacts of the lead placed within the transversus abdominis plane.
Another option for lead placement is the use of an introducer sheath to separate tissue planes and allow adequate room for lead placement and program. Novel sheaths exist for this method.
19.3 Risk Assessment
1.
The patient may be at high risk with epidural lead placement or major neurosurgical interventions may be a candidate for transversus abdominis plane lead placement. The risks are limited, but careful preoperative planning, as well as extensive knowledge of the regional anatomy, is necessary.
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