7.3 Complex Regional Pain Syndrome, Fibromyalgia & Myofascial Pain

Complex Regional Pain Syndrome, Fibromyalgia & Myofascial Pain

Chronic pain syndromes encompass a heterogeneous group of conditions characterized by altered peripheral and central pain processing. Mastering the diagnostic criteria, physiological mechanisms, and targeted therapies for Complex Regional Pain Syndrome (CRPS), Fibromyalgia, and Myofascial Pain Syndrome (MPS) is essential for physical medicine and rehabilitation specialists.

Complex Regional Pain Syndrome (CRPS)

Complex Regional Pain Syndrome is a debilitating neuropathic pain condition that typically affects a distal limb following tissue trauma, fracture, or surgery. The pain is characteristically disproportionate in magnitude and duration to the inciting event and is accompanied by autonomic, motor, sensory, and trophic disturbances.

CRPS Type I vs CRPS Type II

• CRPS Type I (formerly known as Reflex Sympathetic Dystrophy [RSD]): Represents approximately 90% of cases. Occurs without evidence of a specific, major peripheral nerve lesion. Common precipitating events include distal radius fractures, ankle sprains, crush injuries, stroke, or prolonged limb immobilization.
• CRPS Type II (formerly known as Causalgia): Defined by the presence of a confirmed, major peripheral nerve injury (e.g., median or sciatic nerve transection/laceration).

Pathophysiology

CRPS involves a complex interplay of neurogenic inflammation (exaggerated release of substance P and calcitonin gene-related peptide [CGRP]), autonomic dysregulation (sympathetic sprouting and catecholamine hypersensitivity, leading to sympathetic-maintained pain), and central sensitization (spinal cord dorsal horn hyperexcitability and cortical reorganization).

Budapest Clinical Diagnostic Criteria

Diagnosis is made clinically using the validated Budapest Diagnostic Criteria. To meet clinical diagnostic criteria, the patient must have continuing pain disproportionate to any inciting event AND:

1. Must report at least one symptom in three of the four following categories:
   • Sensory: Hyperalgesia (increased pain response to noxious stimuli) or allodynia (pain due to normally non-noxious stimuli such as light touch or clothing).
   • Vasomotor: Temperature asymmetry ($>1.0^\circ\text{C}$) or skin color changes/asymmetry.
   • Sudomotor / Edema: Edema, sweating changes, or sweating asymmetry.
   • Motor / Trophic: Decreased range of motion, motor dysfunction (weakness, tremor, dystonia), or trophic changes (altered hair growth, nail changes, skin atrophy).
2. Must display at least one sign at the time of examination in two or more of those same four categories.
3. No other diagnosis better explains the signs and symptoms.

Diagnostic Testing

• Triple-Phase Bone Scan: Phase 1 (flow) and Phase 2 (blood pool) show increased vascular perfusion; Phase 3 (delayed 3-hour tissue phase) characteristically shows diffuse, asymmetric pericapsular hypermetabolism (increased radiotracer uptake) in the periarticular joints of the affected extremity. This finding is highly specific during the early stages of CRPS.
• Radiographs: May reveal patchy, subperiosteal osteoporosis (Sudeck atrophy) in chronic stages.
• Diagnostic Sympathetic Blocks: Stellate ganglion block (for upper extremity) or lumbar sympathetic block (for lower extremity) using local anesthetic confirms sympathetic-maintained pain if significant temporary temperature elevation and pain relief occur.

Interdisciplinary Treatment

• Physical & Occupational Therapy: The primary cornerstone of treatment. Emphasizes active desensitization, gradual weight-bearing, edema control, and functional range of motion. Passive immobilization worsens outcome.
• Pharmacotherapy: Short-course oral steroids (prednisone taper for acute inflammation), bisphosphonates (to inhibit osteoclast activity and bone turnover), gabapentinoids (gabapentin, pregabalin), and SNRIs.
• Interventional Procedures: Sympathetic nerve blocks, spinal cord stimulation (SCS) for severe, refractory cases.

Fibromyalgia

Fibromyalgia is a non-articular chronic pain syndrome characterized by widespread musculoskeletal pain, fatigue, sleep disturbance, and cognitive dysfunction ("fibro-fog").

Pathophysiology: Central Sensitization

Fibromyalgia is fundamentally a disorder of central pain augmentation (central sensitization). Functional neuroimaging and CSF studies demonstrate elevated levels of excitatory neurotransmitters (substance P, glutamate) in the central nervous system alongside diminished activity in descending inhibitory monoaminergic (serotonergic and noradrenergic) pain pathways. This leads to generalized hyperalgesia and allodynia without peripheral tissue inflammation or tissue damage.

American College of Rheumatology (ACR) Diagnostic Criteria

The diagnostic framework evolved from the 1990 criteria (which required manual palpation of at least 11 of 18 specific "tender points") to the updated 2010/2016 ACR Criteria, which eliminate manual tender point counts in favor of quantitative self-reported indices:

1. Widespread Pain Index (WPI): Assesses pain presence across 19 specified body regions (score 0-19).
2. Symptom Severity Scale (SSS): Rates severity of fatigue, waking unrefreshed, and cognitive symptoms (score 0-12).
3. Diagnostic Threshold: WPI $\ge 7$ and SSS $\ge 5$, OR WPI 4-6 and SSS $\ge 9$.
4. Symptoms must be present at a similar level for at least 3 months.
5. Pain must be widespread, defined as pain in at least 4 of 5 regions (left upper, right upper, left lower, right lower, axial).

Pharmacological Management (FDA-Approved)

Three medications are currently FDA-approved for fibromyalgia:

1. Duloxetine (SNRI): Inhibits reuptake of serotonin and norepinephrine, enhancing descending inhibitory pain pathways. Highly effective for pain, fatigue, and comorbid depression.
2. Milnacipran (SNRI): Dual serotonin-norepinephrine reuptake inhibitor with greater selectivity for norepinephrine reuptake inhibition. Improves pain and fatigue.
3. Pregabalin (Alpha-2-delta ligand): Binds the $\alpha_2\delta$ auxiliary subunit of presynaptic voltage-gated calcium channels in the CNS, reducing calcium influx and inhibiting release of excitatory neurotransmitters (glutamate, substance P).

Contraindication: Opioids are ineffective and contraindicated in fibromyalgia; they do not target central sensitization and can exacerbate hyperalgesia (opioid-induced hyperalgesia).

Myofascial Pain Syndrome (MPS)

Myofascial Pain Syndrome is a regional musculoskeletal pain disorder characterized by painful, localized trigger points in skeletal muscle and surrounding fascia.

Trigger Points vs Fibromyalgia Tender Points

Distinguishing trigger points from tender points is a classic board examination distinction:

• Myofascial Trigger Point:
  • Located within a taut band of skeletal muscle.
  • Extremely focal tenderness; palpation produces a characteristic referred pain pattern distant from the site of pressure.
  • Mechanical stimulation (snapping palpation or needle insertion) elicits a local twitch response (LTR)—an involuntary contraction of muscle fibers within the taut band.
• Fibromyalgia Tender Point:
  • Widespread tenderness without a palpable muscle taut band.
  • Palpation produces local tenderness only, without referred pain patterns or local twitch responses.

Treatment Strategies

1. Trigger Point Injections (TPI): Infiltration of local anesthetic (e.g., 0.5% procaine or 0.25% bupivacaine) into the trigger point. The therapeutic mechanism is primarily mechanical disruption of the dysfunctional motor endplates combined with anesthetic pain relief.
2. Dry Needling: Insertion of an acupuncture needle into the trigger point to elicit an LTR without injectate. Equivalent in long-term efficacy to TPI with local anesthetic.
3. Spray-and-Stretch: Application of a vapocoolant spray (e.g., ethyl chloride) over the muscle to block cutaneous nociceptive input, followed immediately by passive stretching of the muscle to restore resting length.