Lesson 1 of 3
Dorsal columns, dorsal roots and the usage ratio
Place a thoracic trial lead, then read the therapeutic window of its program.
The key idea
Conventional SCS aims to put paraesthesia over the painful area. It comes from recruiting large dorsal column fibres. The dorsal roots beside the lead compete: recruiting them gives a segmental band, such as tightness around the chest (Struijk 1993a, p. 101), and at higher amplitude, discomfort.
Two thresholds frame every program: the perception threshold (PT) and the discomfort threshold (DT). Their ratio is the usage ratio. When it is 1.4, dorsal roots are generally activated first and coverage is limited; when it is higher, the largest dorsal column fibres fire first and paraesthesia widens before it becomes uncomfortable (Holsheimer 1997, p. 496).
The simulator sets DT at 1.4 × the lowest dorsal root threshold (Manola 2006, p. 40). A usage ratio of exactly 1.4 therefore means a root fibre sets the perception threshold. This is a modelling convention, not a ratio measured in patients.
The dorsal CSF layer under the lead drives much of this. Perception thresholds differ between spinal levels mainly because its thickness differs (He 1994, p. 55). In Holsheimer’s model the bipole’s range falls from 2.50 to 1.40 as dorsal CSF thickens from 2.4 to 4.8 mm (Holsheimer 1997, p. 495, Table 1).
For a thoracic trial the Tuohy enters low and the lead travels up. The needle goes into a high lumbar interlaminar space, usually T12–L1 or L1–L2 (Polsunas 2023, p. 237), at a shallow paramedian angle so the lead exits into the dorsal epidural space (Kim 2018, p. 325). On AP, the lead is then advanced cephalad, riding the midline (Kim 2018, p. 330), to its target: often T7–T8 for axial pain and T8–T10 for appendicular pain (Kim 2018, p. 325), for example the T8–T9 interspace (Polsunas 2023, p. 240).
So the lead is anchored at its caudal entry and its distalmost contact, numbered 1 as on Boston Scientific leads (and in the console), is the most cephalad. Confirm the final position on AP plus a lateral or contralateral oblique view (Kim 2018, p. 332).
Benefit and risk
Benefit
- Who benefits
SCS is generally used when other treatments have failed, most often after spine surgery. Outcomes are best for neuropathic, appendicular pain rather than axial pain. Other indications include complex regional pain syndrome, painful peripheral neuropathy and phantom limb pain (Kim 2018, p. 325).
- What programming adds
In modelling, a program that recruits dorsal columns first widens paraesthesia coverage before discomfort (Holsheimer 1997, p. 496).
Risk
- Root recruitment
A segmental band or discomfort at amplitudes the patient needs for coverage (Struijk 1993a, p. 101).
- Off-midline or ventral lead
A lead in the lateral gutter or too ventral can cause discomfort or abdominal stimulation, and is hard to bring back to midline (Kim 2018, pp. 333–334).
- Access
Entry at or below L2–L3, below the conus, may be safer for the cord but leaves a longer path to steer the lead (Kim 2018, p. 326).
- Posture
In 42 patients with percutaneous leads, 95 % had a higher perception threshold sitting or standing than supine, mostly 11–25 % higher (Olin 1998, p. 171).
Try it on the simulator
In the console, place the Tuohy at T12–L1 to loss of resistance, thread the lead cephalad to T8 on the midline, then choose Program the lead. Compare the default guarded tripole with a bipole.