Impulse Pain

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Impulse pain is a sharp, often intense, and typically transient exacerbation of background pain, or the de novo appearance of such pain, that is directly and immediately precipitated by specific actions.

These actions commonly include physiological reflexes such as coughing or sneezing, or activities that involve straining, such as during defecation, lifting heavy objects (Valsalva-like maneuvers), or even forceful laughing. Additionally, sudden or unguarded body movements, such as quick turns, unexpected jolts, or rapid reaching, are characteristic triggers.

The term "impulse pain" is particularly used when these triggers elicit pain in the context of an underlying inflammatory process, where tissues are peripherally sensitised. For example in appendicitis, adhesive capsulitis, and radicular pain.

Distinction from General Movement-Related Pain

While many inflammatory processes are associated with pain on movement (kinesalgia), impulse pain is distinguished by its specific provocation. It is not pain with any movement, but rather pain elicited by actions that are sudden, forceful, or jarring. These actions often involve a rapid increase in intra-cavitary pressure (e.g., intra-thoracic, intra-abdominal, intraspinal) or a swift deformation, stretch, or compression of sensitised tissues.

This contrasts with the more common scenario where pain might gradually increase with the extent or duration of movement, or be present with sustained postures. The "impulse" nature of the trigger implies a rapid rate of tissue loading or a significant, swift change in pressure acting upon structures whose mechanical tolerance has been compromised by inflammation (peripheral sensitisation).

Pathophysiology

Inflammation creates a state of heightened neuronal excitability, which then makes the affected tissues abnormally sensitive to mechanical stimuli. The inflammatory response, whether triggered by infection, injury, or autoimmune processes, leads to profound changes in the local tissue environment, which alters the function of sensory nerves.

As part of the inflammatory process, a variety of cells, including immune cells (macrophages, mast cells, neutrophils, glial cells), endothelial cells, and damaged parenchymal cells, release a plethora of chemical substances known as inflammatory mediators. This "inflammatory soup" comprises molecules such as prostaglandins (especially PGE2), bradykinin, serotonin, histamine, adenosine triphosphate (ATP), protons (acidic pH), cytokines (e.g., tumor necrosis factor-alpha (TNF-α), interleukin-1beta (IL-1 beta), interleukin-6 (IL-6)), chemokines, and various growth factors, most notably nerve growth factor (NGF).

This biochemical onslaught effectively lowers the threshold for nociceptor activation and amplifies their response to subsequent stimuli, a phenomenon known as peripheral sensitisation. This is characterized by a reduction in the activation threshold of nociceptors and an increase in their responsiveness to both noxious and innocuous stimuli. Both the thinly myelinated A delta fibres and the unmyelinated C-fibers undergo sensitization. A stimulus that would normally be subthreshold can now trigger pain, and stimuli that would normally be only mildly painful evoke a much more intense and prolonged pain response (hyperalgesia).

Peripheral sensitisation directly leads to the clinical phenomena of mechanical allodynia and hyperalgesia, which are central to the experience of impulse pain.

Some important terms:

Mechanical allodynia is defined as pain resulting from a mechanical stimulus that does not normally provoke pain, such as light touch or gentle movement. In the context of impulse pain, the forces generated by a cough, sneeze, or sudden jolt, which might be perceived as innocuous or merely as movement in healthy tissues, become painful when applied to inflamed and sensitized structures.

Mechanical hyperalgesia is an exaggerated and often prolonged pain response to a mechanical stimulus that is normally considered painful. For example, a rapid movement or a minor bump might cause some discomfort even in healthy individuals, but in an inflamed joint or muscle, the same stimulus can provoke a disproportionately severe and lasting pain.

Biomechanics

The "impulse" in impulse pain refers to the nature of the mechanical event that triggers the pain. These actions generate forces or pressure changes that are often rapid and significant.

Actions such as coughing, sneezing, and the Valsalva maneuver (forceful expiration against a closed glottis, as occurs during straining) lead to substantial and rapid increases in intra-thoracic and intra-abdominal pressures. For instance, intratracheal pressure during a closed-mouth sneeze can surge dramatically, from a baseline of around 0.43 kPa to as high as 42 kPa. These pressure waves are not confined; they can be transmitted to adjacent body cavities and structures.

Increased intra-abdominal pressure can directly affect inflamed intra-abdominal organs (like the appendix, leading to peritoneal irritation) or structures within the abdominal wall or pelvis. Similarly, increased intra-thoracic pressure can stress inflamed costal structures or the pleura. These pressures can also be transmitted to the spinal canal, leading to an increase in intraspinal (intrathecal) pressure (studied in the early to mid 20th century with myelograms). This can, in turn, increase pressure within intervertebral discs (intradiscal pressure) or cause movement or tension on the dura mater and nerve roots, particularly if these structures are already inflamed or compromised by a lesion such as a disc herniation.

Sudden body movements, such as a quick twist of the torso, an unguarded reach, or an unexpected jolt (e.g., missing a step), impose direct mechanical stresses (tension, compression, shear, or torsion) on musculoskeletal tissues. If these tissues (e.g., joint capsules, ligaments, tendons, muscles, fascia) are inflamed, their structural integrity and mechanical tolerance are often reduced.

In adhesive capsulitis, for example, the joint capsule is inflamed and fibrotic; a sudden arm movement can abruptly stretch this non-compliant and sensitized tissue, triggering pain. In the case of an intercostal muscle strain, a forceful cough not only involves increased intra-thoracic pressure but also causes a rapid, forceful contraction and stretch of the already injured and inflamed muscle fibers between the ribs. Even minor deformations or stresses, if applied rapidly to tissues where mechanoreceptors and nociceptors are sensitized by an inflammatory milieu, can be sufficient to trigger a volley of action potentials, perceived as sharp pain.

Clinical Features

One should evaluate for both impulse symptoms and impulse signs.

History

The qualities of impulse pain are often distinct:

Onset: The pain appears abruptly, with a clear temporal link to the triggering action. There is typically no delay between the cough, sneeze, or sudden movement and the onset of the sharp pain.

Intensity: The pain is usually described as sharp, lancinating, stabbing, or shock-like. Its intensity is often perceived by the patient as being significantly greater than their baseline inflammatory ache or discomfort.

Duration: The acute, sharp component of impulse pain is generally brief, lasting only for the duration of the mechanical stressor or a few moments thereafter. However, it may be followed by a period of residual aching or heightened sensitivity in the affected area.

Location: The pain can be well-localized to the site of the underlying inflammation. In some instances, particularly involving spinal structures or nerve root irritation, the pain may radiate or refer to a broader anatomical area, following a dermatomal or sclerotomal pattern.

Examination

There have not been any studies on the diagnostic utility of impulse signs to the authors knowledge.

Heel drop test - involves a patient suddenly dropping from standing on their toes to their heels. Typically used in the acute abdomen setting, but may be positive in the setting of inflammatory processes within the spine (anecdotal)

Percussion tenderness - also used in acute abdomen, but again anecdotally may be positive in low back pain in the setting of inflammation.

Hopping test - also used in acute abdomen.

Bed shake test - also used in acute abdomen