Target: The Muscle as a Pain Generator and Key Element to Protect during MIS Surgery

5 Target: The Muscle as a Pain Generator and Key Element to Protect during MIS Surgery

Norman Marcus and Roger Härtl

Summary

Dr. Normam Marcus is a skilled clinician who has dedicated part of his career to the study of muscle physiopathology and its role as “back pain” generator. His approach has proven to be an effective tool in those patients where pain diagnosis/treatment is challenging. In this chapter, Dr. Marcus guides the reader through the rationale of his approach and how to implement this approach into a multidisciplinary practice.

Keywords: Ehlers Danlos ED muscle pain back pain low back pain discogenic pain mechanical back pain radicular pain myogenic pain

5.1 The Role of Muscles in Back and Neck Pain—before, during, and after Minimally Invasive Surgery

Soft tissue includes ligaments, fascia, and muscle/tendon. For simplicity, muscles will be used in this chapter to represent soft tissue, acknowledging that the other soft tissue representatives are important pain generators as well. Appreciation of the pathophysiologic and functional causes of muscle pain may improve spine surgery outcomes.

5.2 Introduction

Muscles constitute approximately 40% of the body by weight and are thought to be an important cause of nonspecific/idiopathic low back pain in as many as 85% of patients presenting with nonradiating back pain.1 The overriding concept of this chapter is that muscle is an important primary source of back pain. It is then reasonable to assume that persistent low back pain, often encountered as a common symptom in failed back surgery syndrome (FBSS), can be a function of muscle generated pain, and may be present before as well as after spine surgery. Therefore, careful preoperative evaluation for muscle generated pain is crucial in avoiding FBSS. This requires a thorough musculoskeletal evaluation and emphasis on muscle strengthening and flexibility training in order to avoid mistakenly operating on a patient who suffers from muscle related pain.

With the increasing rates of spine surgery, there is an increased incidence of FBSS.² Newer soft tissue sparing procedures (minimally invasive surgery [MIS]) hold the promise of diminished trauma and subsequent incidence of FBSS, but even though postoperative pain is less in MIS compared to open procedures for the first 6 months, a significant number of patients still experience mild-to-moderate pain and disability 5 years after surgery.³ This raises the concern that, regardless of the surgery, some of this pain may have been present before as “muscle generated pain” (MGP). Muscle-related back or neck pain may exist even after a perfectly performed MIS procedure. The surgical team that anticipates this possibility will have a greater degree of success in appropriately selecting patients for surgery and managing postoperative pain.

5.3 Background

Since the seminal report of Mixter and Barr in 1934⁴ of a disc herniation causing sciatic pain and a second paper in 1935,⁵ suggesting that nonradiating back pain was disc related, the field of spine surgery has looked to the neuraxis as the source of the back pain and surgery as often inevitable. Faculty at the Weill Cornell Center for Comprehensive Spine Care, believing that ignored MGP may be a contributing etiology for FBSS in technically successful spine surgery, have begun to examine the role of muscles in clinical back pain presentations. Fishbain et al⁶ pointed out that it is difficult to distinguish neuropathic pain from soft tissue generated pain: Without knowledge of the pathophysiologic mechanisms causing muscle pain, in a patient presenting with regional pain, the putative diagnosis will usually be neuraxial. For example, a patient complaining of pain in the low back, buttock, and thigh with magnetic resonance imaging (MRI) evidence of an L4/L5 disc will most often be assessed as having pain that reflects root compression and not be assessed for muscles in the low back and buttock as a possible source of the pain. A 2011 study⁷ described variables contributing to the incidence of FBSS: (1) preoperatively, (2) perioperatively, and (3) postoperatively. MGP as a putative source of back and/or leg pain was not mentioned.

5.4 Clinical Presentation of Muscle Pain

Muscle pain is typically described as a dull aching sensation, made worse with aggressive activity and prolonged positioning. A muscle that is a source of pain is found to have inhibition of electrical activity with associated problems in coordination and flexibility related to increased activity in antagonist groups.⁸ This phenomenon is known as the “pain-adaptation model.”

Sensitized muscles and muscles that are painful due to referred pain patterns (see below) may present with increased muscle tone.⁹ Nerves traveling through or adjacent to stiffened muscles may become compressed, suggesting a neuropathic problem of which the piriformis syndrome is only one example.¹⁰

5.5 Neuroanatomy and Neurophysiology of Muscle Pain¹¹: Myogenic (MGP) versus Neurogenic Inflammation

The nociceptive signal is transmitted to the central nervous system where it may be ultimately experienced in the cerebral cortex as pain. The nociceptor may be stimulated by compression of the afferent neuron close to the spinal cord (see Fig. 5.1, for example, from a herniated lumbar or cervical disc, severe lateral recess stenosis, or spondylolisthesis causing compression of an exiting nerve root in the foramen). In this case, the neural compression elicits an action potential that can travel back to the spinal cord producing neuropathic pain. At the same time, however, an action potential can also travel peripherally to the muscle tissue with release of neuropeptides resulting in neurogenic inflammation. Clinically, the patient with afferent nerve compression could then have neurogenic induced muscle inflammation complicating the neuropathic presentation.

Fig. 5.1 Schematic of group IV muscle nociceptor. Neurogenic and myogenic pain.
Patho-anatomical changes in the neuraxis may produce compression of the afferent group IV fibers close to the spine producing neurogenic inflammation consisting of (a) a retrograde impulse distally to the muscle causing release of neuroexcitatory and vasoactive substances from varicosities on the nerve, arteriolar endothelium, mast cells, and platelets and an anterograde stimulation to synapse with a dorsal horn neuron (DHN).
Myogenic excitation results from noxious stimuli to the muscle tissue strong enough to depolarize high threshold mechano/chemo receptors causing an action potential and release of glutamate into the synapse with the DHN and peripheral release of the same neuroexcitatory substances above.

A microsurgical discectomy or decompression and fusion to treat the nerve compression may eliminate or diminish the anterograde stimulation travelling to the spinal cord and retrograde stimulation peripherally toward the muscle. Although the neuropathic pain component will decrease, immediately or over time, the significant muscle changes induced by the neurogenic inflammation may persist for a variable period and can result in significant persistent postoperative MGP, which, if not considered, will be difficult to treat. The patient may then be pessimistically diagnosed as suffering from FBSS.

If noxious stimuli are persistent or recur repeatedly, the peripheral nerve may become sensitized with accompanying persistent tenderness and local spread of hypersensitivity as a reflection of retrograde stimulation to uninvolved branches of the free nerve ending, and concomitant sensitization of dorsal horn neurons (DHNs), which may result in the opening of ineffective channels to DHN at different levels in the spinal cord, producing referred pain patterns to muscles that are not the direct source of pain. For example, a sensitized serratus anterior may refer pain to cervical paraspinal muscles causing pain in the lateral neck that may mistakenly be attributed to degenerative changes in the cervical spine.

5.6 Clinical Examination

The possible sources of pain may be identified with a soft tissue assessment for muscle dysfunction. The examination includes the following steps:

a)Observation of asymmetric muscle stiffness and/or spasm. Asymmetry of:

1.Hip height with associated equal leg length (from anterior superior iliac spine [ASIS] to medial malleolus).

2.Asymmetry of resting ankle rotation while lying supine with legs extended.

b)Kraus-Weber test for trunk muscle strength and flexibility (Table 5.1).

c)Palpation of muscles and fascia in lower body most often involved as pain generators, demonstrating increased tenderness to pressure relative to a muscle in an uninvolved nonpainful region. (A pressure algometer may provide greater accuracy in determining how much pressure is perceived as uncomfortable.)¹²

Table 5.1 Kraus-Weber test for trunk muscle strength and flexibility: exercises and measurements

Test	Measurement
Floor touch—knees extended, feet together	Flexibility of low back muscles, hamstrings, and calves
Leg lift supine	Strength of abdominals and hip flexors
Sit up with legs extended	Strength of abdominals and hip flexors
Sit up with knees bent	Eliminating hip flexors, testing abdominal muscle strength
Lifting lower body and then upper body from the mat while prone	Back muscle extensor strength

The back or neck examination is done in prone position with pillows under hips and ankles (chest and head), which is a stressless position. The supine examination is done lying flat.

In order to apply consistent pressure, the examiners should have an idea of how much pressure they are applying with their fingers (thumb or fingers two to four). Normal muscle in an otherwise healthy adult is generally not tender with pressures below approximately 3.5 kg/cm².

The pressure should be applied to all putative pain generating muscles in the region of the pain complaint including putative antagonists. Muscles identified as tender (causing tenderness at lower pressures) should also prompt an examination of the contralateral muscle for comparison.

5.7 Muscle Generated Pain and Spinal Surgery

A pilot study in the Orthopedic Spine Surgery Division of Kaiser Permanente in San Diego to identify and treat specific muscles causing back and leg pain in patients scheduled for spine surgery resulted in four of seven patients’ surgeries being cancelled with three still having reported relief 77 weeks after treatment.13

If all patients being considered for spine surgery were screened for the presence of MGP, and if identified muscles were treated, it is suggested that successful surgical outcome rates would be higher because prior recognition and treatment of soft tissue pain will decrease unnecessary surgeries.

In the same paper, 97 patients presenting with persistent pain, who had failed typical pain medicine interventions such as epidural steroid injections, facet blocks, radiofrequency ablation, spinal cord stimulation, and spine surgery, were also assessed with the same protocol to identify and treat sensitized muscles. At 77 weeks post treatment, successful reduction in pain and improvement in activities of daily living (ADLs) were sustained, suggesting that soft tissue evaluation should be a standard element of care in FBSS.

5.8 Operative Considerations

Surgeons performing MIS have always understood that muscles are important in postoperative recovery. Indeed, surgical approaches often emphasize muscle sparing in an attempt to improve both the speed of postoperative recovery and functional outcome.

The traditional subperiosteal midline approach sacrifices large portions of the posterior stabilizing elements, including ligaments, tendons, and bony structures. It may also lead to denervation and devascularization of the adjacent muscles. The use of self-retaining retractors may also result in muscle atrophy,¹⁴^,¹⁵^,¹⁶ which in turn can lead to decreased force production capacity.¹⁷ Mayer et al¹⁸ evaluated trunk muscle strength in patients who had undergone lumbar surgery and found diminished strength in subjects who had undergone open fusion procedures compared to those who had undergone discectomy. Muscle denervation due to extensive exposure, especially over the facet joint and pars interarticularis, is another source of muscle atrophy. These factors conceptually impact patient recovery and long-term effects of surgery, both at the index level and adjacent segments.¹⁹^,²⁰ This topic has been recently reviewed by Kim et al.²¹^,²²

Stevens et al²⁰ assessed the appearance of the multifidus muscle via MRI in patients treated by open versus MISS lumbar interbody fusion techniques, and found marked intermuscular edema on MRI at 6 months postsurgery in the open surgery group. In contrast, normal muscle appearance was observed on MRI in patients in the MISS group (Fig. 5.2).