Spatial Summation of Pain
Encyclopedia of Pain definition
This is the pain phenomenon that occurs when there are “progressively larger numbers of primary afferent (presynaptic) neurons are activated simultaneously, until sufficient neurotransmitter is released to activate an action potential in the spinal cord (postsynaptic) neuron.”
This is the pain pattern of an increased response to a stimulus resulting from stimulation of larger areas of the body such that there is an increase in pain intensity as a function of the area stimulated.
Spatial summation in fibromyalgia
Staud et al., in discussing spatial summation of pain (SSP), noted that the mechanisms “include pain coding dependent on impulse frequency and the number of recruited central neurons.” (Spatial summation of heat pain within and across dermatomes in fibromyalgia patients and pain-free subjects. Staud, Rolanda,*; Vierck, Charles J.b; Robinson, Michael E.c; Price, Donald D. PAIN: October 2004 - Volume 111 - Issue 3 - p 342–350, https://journals.lww.com/pain/Abstract/2004/10000/Spatial_summation_of_heat_pain_within_and_across.16.aspx. They also stated that spatial summation of pain might be “influenced by pain inhibitory mechanisms, such as diffuse noxious inhibitory controls.” They speculated that it is possible that there are abnormal interactions “between pain inhibitory mechanisms and SSP” and these might be “relevant for chronic pain conditions such as fibromyalgia” and might help one understand “why widespread pain is characteristic for this chronic pain syndrome.”
They conducted a study “to determine the difference of thermal SSP in the upper extremities between FM and normal control (NC) subjects “within and across dermatomes of the hand.” Each finger tip was immersed and there was also stepwise immersion of “the fingers, hands, and forearms in a hot water bath (46 °C) for 5s and 20s.”
They also compared “immersion of several fingertips across dermatome C7–C8 …progressive immersion of the index finger (dermatome C7).” Result demonstrated the presence of “significant spatial summation of heat-induced pain in both FM and NC subjects.” SSP was most extensive within the fingers, and became negligible as the stimulus area increased above the hand. Furthermore, SSP was more pronounced within one dermatome such as that of the index finger than across several dermatomes of the hand. These results were similar for both FM and NC subjects. Thus, mechanisms of SSP, including possible inhibitory factors that limit this relevant pain mechanism, appear to be similar for both FM and NC subjects.”
Staud et al. (2007) stated that both impulse frequency and “the number of recruited central neurons are relevant for pain encoding and temporal as well as spatial summation of pain (SSP).”(Spatial summation of mechanically evoked muscle pain and painful aftersensations in normal subjects and fibromyalgia patients, Roland Staud, Euna Koo, Michael E Robinson, Donald D Price, Pain . 2007 Jul;130(1-2):177-87, DOI: 10.1016/j.pain.2007.03.015, available in full online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2041939/.) They note that spatial summation of pain has been well characterized for heat induced pain.
They studied mechanical SSP (MSSP) in normal controls and fibromyalgia patients using 5s supra-threshold pressure-pain stimulations of forearm muscles. This was conducted using three identical probes (separated by 4 or 8cm). Stimulated areas ranged between 0.79 and 2.37 cm2.
Subjects rated the pain intensity of mechanical stimuli as well as pain aftersensations.
Results showed that MSSP increased monotonically in NC and FM subjects.
They also found that “pressure pain and pressure pain aftersensations were greater in FM subjects and highly associated with clinical pain intensity (r2=.44-.64)…”
They interpreted their results as suggesting that spatial and temporal summation factors might “contribute to overall clinical pain.” They noted that “despite higher experimental pain ratings, the magnitude of MSSP was not statistically different between NC and FM subjects. Furthermore, muscle stimuli elicited more MSSP when separated by 8 cm than 4 cm and this finding was not different between NC and FM subjects. Thus, mechanisms of MSSP were similar for both FM and NC subjects.”
Spatial Summation in Encyclopedia of Pain, 2007 Edition, DOI: https://doi.org/10.1007/978-3-540-29805-2_4092