International Association for Study of Pain definitions demonstrate that lactation-related pain and aversive sensations are not chronic neuropathic pain, nociplastic pain, or central sensitisation but may constitute peripheral sensitisation to inflammation
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Pain sensitisation
Central sensitisation
Increased responsiveness of nociceptive neurons in the central nervous system to their normal or subthreshold afferent input.
Note: See note for sensitization and nociceptive neuron above. This may include increased responsiveness due to dysfunction of endogenous pain control systems. Peripheral neurons are functioning normally; changes in function occur in central neurons only.
Peripheral sensitisation
Increased responsiveness and reduced threshold of nociceptive neurons in the periphery to the stimulation of their receptive fields.
Allodynia
Allodynia is defined as "pain due to a stimulus that does not normally provoke pain ... It was considered to be preferable to define allodynia in terms of the response to clinical stimuli and to point out that the normal response to the stimulus could almost always be tested elsewhere in the body, usually in a corresponding part. ... Allodynia is taken to apply to conditions which may give rise to sensitization of the skin, e.g., sunburn, inflammation, or trauma."
In the case of nipple pain, the term allodynia could be legitimately used to describe the unpleasant sensation of pain that results when a stimulus that does not normally provoke pain (for example, light touch of the bedsheets, or touch of the breastpad, or water in the bath or shower) results from inflammation of the nipple epidermis. The nipple inflammation, however, is due to ongoing repetitive mechanical microtrauma, an acute, constantly refreshed injury which requires urgent attention. When the repetitive mechanical microtrauma ceases, the tissue heals and the pain resolves.
Hyperalgesia
Hyperalgesia is a term "used for cases with an increased response at a normal threshold, or at an increased threshold, e.g. in patients with neuropathy. ... Current evidence suggests that hyperalgesia is a consequence of perturbation of the nociceptive system with peripheral or central sensitization, or both."
On the current ISAP definition, this term doesn't appear to have a role in describing lactation-related nipple pain, since the patient's sensation of pain correlates with acute tissue injury. When the repetitive mechanical microtrauma ceases, the tissue heals and the pain resolves.
Acute pain
Acute pain is generally defined as pain lasting up to 30 days from the initiating injury or event. In lactation-related nipple pain and inflammation, the mechanical injury is applied repeatedly, multiple times each day. When application of mechanical microtrauma ceases, the nipple pain heals.
The International Association for the Study of Pain (IASP), in its list of terminologies, states that pain is an experience of sensations and emotions and is always subjective. No pain, even acute pain in response to obvious tissue trauma, can be conceptualised as purely nociceptive. An individual’s perception of pain always emerges out of interactions between multiple factors.
Nociception
The neural process of encoding noxious stimuli. Note: Consequences of encoding may be autonomic (e.g. elevated blood pressure) or behavioral (motor withdrawal reflex or more complex nocifensive behavior). Pain sensation is not necessarily implied.
Nociceptive neuron
A nociceptive neuron is a "central or peripheral neuron of the somatosensory nervous system that is capable of encoding noxious stimuli."
Nociceptive pain
Nociceptive pain "arises from actual or threatened damage to non-neural tissue and is due to the activation of nociceptors. Note: This term is designed to contrast with neuropathic pain. The term is used to describe pain occurring with a normally functiong somatosensory nervous system to contrast with the abnormal function seen in neuropathic pain."
In lactation-related nipple pain, epidermal damage is detected by the dense number of nociceptive nerve endings in the dermis.
Neuralgia
Neuralgia describes pain which is in the distribution of a nerve or nerves. It is possible that the deep stabbing or radiating pain that patients describe when they are experiencing nipple pain, damage, and inflammation is a form of neuralgia.
Some people use the terms neuropathy and neuralgia interchangeably, but they are different: "Neuropathy refers to nerve damage while neuralgia is a type of nerve pain. Neuralgia can be a symptom of neuropathy. Often described as a severe shooting, burning, or stabbing pain, neuralgia can result from damage, irritation, or dysfunction of a nerve."
Chronic pain
Neuropathic pain
A discussion of acute neuropathic pain and the lactating nipple is here.
Neuropathic pain is "caused by a lesion or disease of the somatosensory nervous system. Neuropathic pain is a clinical description (and not a diagnosis) which requires a demonstrable lesion or a disease that satisfies established neurological diagnostic criteria. ... Somatosensory refers to information about the body ... rather than information about the external world (e.g. vision) ... The presence of symptoms or signs (e.g. touch-evoked pain) alone does not justify the use of the term neuropathic."
Note: Finnerup et all observe that the new IASP definition of neuropathic pain, above, means that a condition like chronic regional pain syndrome is not considered a neuropathic pain syndrome because in chronic regional pain syndrome the somatosensory system is intact. Neuropathic pain is a lesion of the somatosensory system. Neuropathic pain lacks robust classification or scoring systems, but is divided into three classes: possible, probable, and definite neuropathic pain.
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Examples of peripheral neuropathic pain are postamputation pain, trigeminal neuralgia, painful radiculopathy, painful polyneuropathy, postherpetic neuralgia, peripheral nerve injury pain.
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Examples of central neuropathic pain are central post-stroke pain, central pain in multiple sclerosis, spinal cord injury neuropathic pain.
The IASP Neuropathic Pain Special Interest Group (NeuPSIG) note that “the temporal relationship between the lesion or disease and the [onset of neuropathic] pain may vary” from immediately after an injury to years after the initiation of disease.
Neuropathy
Neuropathy is a disturbance of function or pathological change in a nerve: in one nerve, mononeuropathy; in several nerves, mononeuropathy multiplex; if diffuse and bilateral, polyneuropathy.
Nociplastic pain
Nociplastic pain arises from altered nociception despite no clear evidence of actual or threatened tissue damage causing the activation of peripheral nociceptors or evidence for disease or lesion of the somatosensory system causing the pain.
Recommended resources
Selected references
Finnerup NB, Kuner R, Jensen TS. Neuropathic pain: from mechanisms to treatment. Physiological Reviews. 2021;101:259-301.