Pain Maps

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Pain is a complex sensory and emotional experience, and its accurate description and classification are fundamental to diagnosis and management. A critical concept in pain medicine is referred pain, formally defined by the International Association for the Study of Pain (IASP) as “pain perceived at a location other than its origin”. This phenomenon, where noxious stimulation of a structure is felt elsewhere in the body, is distinct from local pain, which is experienced directly at the site of tissue damage or pathology.

Types of Pain

Understanding referred pain necessitates differentiating it from other types of pain that may present with similar distributions, particularly radicular pain and neuropathic pain. Radicular pain arises specifically from irritation or compression of a spinal nerve root or its dorsal root ganglion. It is typically characterized as sharp, shooting, or lancinating and often radiates in a band (sometimes but not always in a dermatome)

In contrast, referred pain is nociceptive in origin, stemming from the stimulation of pain receptors (nociceptors) within somatic (musculoskeletal) or visceral tissues, not directly from nerve root compromise. Consequently, referred pain is often described qualitatively differently – as deep, dull, aching, gnawing, or an expanding pressure – and its distribution, while often segmental, does not typically follow precise dermatomal boundaries. The IASP explicitly recommends that pain radiating into a limb be specifically described as either referred or radicular, highlighting the clinical importance of this distinction.

Neuropathic pain, distinct from both referred and radicular pain, is defined as pain caused by a primary lesion or disease affecting the somatosensory nervous system itself, either peripheral or central. Referred pain, conversely, originates from noxious stimulation of non-neural tissues.

Referred pain can be broadly categorized based on its tissue of origin:

1. Somatic Referred Pain: Originating from deep somatic structures such as muscles, fascia, tendons, ligaments, joint capsules, periosteum, and bone. This includes pain referred from spinal structures, peripheral joints, and myofascial trigger points.
2. Visceral Referred Pain: Originating from internal organs within the thoracic, abdominal, or pelvic cavities.

While the underlying neurophysiological mechanisms share common principles, particularly neural convergence, the specific pathways and clinical characteristics can differ between somatic and visceral referred pain.

Pain Maps

Pain maps (also referred to as pain charts or body maps) serve as essential tools in both clinical practice and research for assessing and documenting the spatial characteristics of pain. They involve a transformational representation of a patient's subjective experience of pain into a graphical, numerical, or descriptive format. Typically, this requires the patient to indicate the affected body regions on a diagram, but maps can also incorporate additional dimensions of the pain experience.

The primary purpose of pain mapping is to investigate and visualize the location, extent, and distribution of pain or other symptoms. Clinically, they aid in diagnosis by correlating pain patterns with known anatomical structures or pathological processes, monitoring treatment progress, and facilitating communication between clinicians and patients. In research, pain maps allow for the systematic collection and analysis of spatial pain data, enabling the identification of patterns associated with specific conditions or responses to interventions.  

Pain maps can capture various types of information. The core elements are pain location (specific points or segments) and pain distribution/extent (the pattern or spread, such as radiating or referred patterns related to dermatomes, myotomes, or sclerotomes). Beyond spatial data, maps may integrate information on pain intensity, often using scales like the Visual Analogue Scale (VAS) , and pain quality, using descriptors such as dull, aching, burning, throbbing, or tingling.  

It is important to distinguish between the tool ("pain map") and the phenomenon it represents ("referred pain pattern"). A pain map is the graphical or descriptive output generated through a specific assessment method , whereas a referred pain pattern is the actual neurophysiological phenomenon of pain being perceived distally from its origin.

Methodologies

The referred pain patterns documented in the literature are derived from a variety of methodologies, broadly classifiable as clinical observation/correlation and experimental stimulation studies.

A crucial consideration when interpreting pain maps is the methodology used in their generation. Maps derived from patient self-report following diagnostic blocks, such as the Cooper cervical facet maps , reflect the clinical reality of pain relief in individuals already experiencing symptoms. In contrast, maps generated through experimental stimulation in volunteers, like those by Dwyer or Dreyfuss , depict pain provocation, often in individuals who are asymptomatic at baseline. These methodological differences likely contribute to variations observed between different pain map studies for the same structure. Experimental stimuli may not fully replicate the complex neurobiological changes, including central sensitization, present in chronic pain states. Furthermore, the territory where pain is abolished by a block may not perfectly mirror the territory where pain can be provoked by stimulation.

Clinical Observations

Pain Drawings/Diagrams: A common method involves patients marking the location and extent of their pain on standardized body charts. Various techniques are used, such as shading areas, marking specific points, or checking predefined regions. These can be implemented using traditional paper-and-pen formats or increasingly via digital interfaces. This approach was utilized in Cooper et al.'s study of cervical facet pain based on patient relief and is frequently employed in studies of myofascial trigger points.

Clinical Correlation (Diagnostic Blocks): This method involves confirming the source of pain by observing pain relief following targeted injections of local anesthetic into specific structures (e.g., facet joint medial branch nerves, sacroiliac joint, nerve roots). Pain maps are then constructed based on the anatomical distribution of pain that is relieved by the block. Controlled diagnostic blocks (often using comparative agents with different durations) are considered the reference standard for diagnosing pain originating from structures like the facet joints and sacroiliac joints , despite inherent limitations such as the potential for false-positive responses.

Clinical History and Examination: Clinicians often infer pain sources based on the patient's description of pain characteristics and location, combined with physical examination findings such as localized tenderness or pain provocation with specific movements or maneuvers (e.g., SIJ provocation tests ). However, numerous studies have shown that clinical features alone are often unreliable for accurately identifying the specific anatomical structure responsible for spinal pain, particularly differentiating between facet, disc, and SIJ sources.

Experimental Stimulation in Volunteers

Noxious Stimulation of Structures: This involves intentionally provoking pain by stimulating specific tissues in volunteers or patients and mapping the resulting pain distribution. Common techniques include:

• Chemical Stimulation: Injecting substances known to activate nociceptors, such as hypertonic saline solution, into muscles, ligaments, or joints. This method was pioneered by Kellgren in his studies of deep somatic pain.  
• Mechanical Distension: Injecting contrast media or saline into joint capsules (e.g., facet joints, SIJ, GHJ) to cause distension and provoke pain. Dwyer et al. and Dreyfuss et al. used this for mapping spinal joint pain patterns in volunteers.

Electrical Stimulation: Applying electrical currents to stimulate specific nerves (e.g., medial branches innervating facet joints ) or tissues directly to map evoked pain sensations.

Mechanical Stimulation: Applying pressure, such as through manual palpation of myofascial trigger points or using pressure algometers to determine pain thresholds.

Thermal Stimulation: Using controlled application of heat or cold to evoke pain responses.

Indirect Mapping

Pressure Pain Threshold (PPT) Mapping: This technique involves systematically measuring the minimum pressure required to elicit pain at multiple points over a defined body region using an algometer. The resulting map illustrates areas of mechanical hyperalgesia (decreased PPT), which can be localized or widespread, reflecting peripheral or central sensitization processes associated with various pain conditions.

Neuroimaging (fMRI, SPECT-CT): Functional neuroimaging techniques like functional magnetic resonance imaging (fMRI) can identify patterns of brain activation associated with pain perception or processing. Single-photon emission computed tomography combined with CT (SPECT-CT) has been investigated for identifying inflammation or increased metabolic activity in potentially painful spinal structures like facet joints, although its correlation with confirmed facet joint pain remains inconsistent. These methods do not directly map perceived pain location but offer insights into the central processing or peripheral pathophysiology related to pain.