Pain is a universal response to harmful stimuli, and is a fundamental aspect of the evolutionary process in living organisms ^[1][2][1,2]. Pain is a complex, multidimensional experience essential for survival and adaptation, serving as a vital communication system between the body and the brain [3]. It alerts an organism to potential threats and prompts it to take appropriate action [4]. Investigations of cellular mechanisms and behavioral responses related to nociceptor activation, tissue injury, inflammation, and the environmental context of these responses are starting to reveal the evolution of mechanisms and behaviors important for pain [2]. Consequently, pain has emerged as a universal response across diverse living beings, facilitating their ability to thrive and propagate their genetic material across generations ^[5][6][5,6].

2. Pain in DOC

Pain is a subjective experience, and its definition may vary from person to person. The International Association for the Study of Pain (IASP) approved a definition of pain in 1979, encompassing both the sensory and emotional dimensions of the experience and the association between tissue injury and pain ^[7][76]. The IASP modified its basic pain terminology in 2007, introducing new terms to describe the various aspects of pain ^[8][77]. However, the subjective nature of pain remains a fundamental aspect of the experience, and reporting on it becomes crucial, with the narrative approach being recommended to assess pain in subjects who can communicate (Figure 1, Table 2). Considering the current definition of pain, assessing it in non-communicative patients remains challenging ^[9][78]. Disorders of Consciousness (DOC) encompass a range of pathologies that affect a patient’s ability to interact with their surroundings, resulting from both traumatic and non-traumatic causes. Consciousness is generally defined as the brain’s capacity to perceive oneself and the environment, which requires adequate arousal (wakefulness) and awareness of content (sensory, cognitive, and affective experiences) ^[10][11][79,80]. These components are referred to as the level and content of consciousness. Following an acquired brain injury, two possible conditions may arise: Vegetative State/Unresponsive Wakefulness Syndrome (VS/UWS) or Minimally Conscious State (MCS). VS/UWS is characterized by spontaneous eye-opening and an absence of consciousness, with only residual reflexive responses to external stimuli ^[12][81]. On the other hand, MCS presents minimal yet discernible non-reflex behaviors in response to various stimuli, although these responses may be inconsistent ^[12][81]. Clinical assessments of these conditions rely on consensus and behavioral scales, such as the Coma Recovery Scale–Revised (CRS-R), to determine the severity and extent of the disorder ^[12][13][81,82]. Pain can be present during both the acute phase and the subsequent intensive rehabilitation period in patients with brain injuries ^[14][83]. This may result from various factors such as skin lesions, surgical wounds, neuropathic pain, and injuries of different types. Additionally, pain may arise from nursing maneuvers and devices used during hospitalization. During the rehabilitation and chronic phases, pain can be caused by peripheral nerve lesions, central pain, spasticity, joint limitations, bedsores, paraosteoarthropathy, constipation, and post-traumatic headaches ^[15][16][84,85]. Central nervous system damage may also lead to chronic pain, such as thalamic pain ^[17][18][19][86,87,88]. These conditions can lead to changes in pain processing in the central nervous system and to Complex Regional Pain Syndrome (CPRS), a neuropathic pain disorder characterized by various clinical features ^[20][89]. The underlying mechanism of CPRS is multifactorial, involving abnormal neuronal transmission, autonomic dysregulation, and central sensitization. The pro-inflammatory and immunological response further contributes to peripheral sensitization and alteration of the sympathetic nervous system ^[20][89]. Painful symptoms may interfere with rehabilitation processes, limiting or delaying their effectiveness ^[14][83]. Thus, it is crucial to implement appropriate early interventions to prevent secondary damage and pain-related functional limitations, such as bedsores or muscle–tendon retraction.

3. Pain Treatment in DOC

There is no consensus on the appropriate pharmacological treatment of pain in patients with DOC ^[21][90]. Medication should typically be given when there are clear behavioral indications of pain. Precise dosing of pharmacotherapy is crucial to prevent interference with the evaluation and therapy strategy for recovering consciousness. Moreover, if the strategy’s efficiency is still debatable, and yet to be proven through large-scale studies, the World Health Organization (WHO) proposes the WHO analgesic ladder, a pain management strategy developed in 1986 to provide adequate pain relief for cancer patients ^[22][91]. The ladder consists of three steps, with each step providing increasing levels of pain management options. The first step is for mild pain and involves the use of non-opioid analgesics such as NSAIDs or acetaminophen with or without adjuvants. The second step is for moderate pain and involves the use of weak opioids such as hydrocodone, codeine, or tramadol, with or without non-opioid analgesics and with or without adjuvants. The third step is for severe and persistent pain and involves the use of potent opioids such as morphine, methadone, fentanyl, oxycodone, buprenorphine, tapentadol, hydromorphone, or oxymorphone, with or without non-opioid analgesics and with or without adjuvants. It is essential to consider that inadequate pain control may impair intentional behavioral responses, whereas excessive treatment, using high doses of opioids to decrease pain, could negatively interfere with arousal ^[23][92] and may hinder cognitive recovery and attention ^[21][24][90,93]. The optimal drug dosage could preserve the patient’s arousal and consciousness, reducing the risk of misdiagnosis ^[25][26][94,95]. Different approaches are suggested in the presence of suspected symptomatic, mild, moderate, or neuropathic pain. In the case of managing pain with symptoms, the principles of proportionality and gradualness are considered, given their interactions with current therapies. In this case, treatment approaches typically involve the use of aspirin, paracetamol, nonsteroidal anti-inflammatory drugs, opioids, and γ-aminobutyric acid (GABA)-ergic agents ^[21][27][90,96]. In cases of suspected mild pain, administering aspirin, paracetamol, or nonsteroidal anti-inflammatory drugs is suggested ^[28][97]. For moderate or neuropathic pain, it is recommended to use high-dose aspirin or paracetamol, oral NSAIDs, and GABAergic agents ^{[14][21][29][30]}[83,90,98,99]. Finally, for suspected severe pain the use of mixed agonists/antagonists, partial agonist opioids, parenteral opioids, antidepressants, anticonvulsants, and atypical agents is usually suggested ^{[14][21][31][32]}[83,90,100,101]. Since around 89% of DOC patients are characterized by spasticity ^[33][102], which is associated with pain and other symptoms (i.e., increased hypertonia, altered sensorimotor control, and muscle spasms) ^[34][103], in cases of focal spasticity, or to treat severe or worsening cases, infiltration of botulinum ^[35][36][104,105] is suggested. For dystonia and diffuse spasticity, improvements were instead observed by administering intrathecal baclofen ^[37][106].

4. Pain and Consciousness in DOC

Pain treatment is a relevant aspect of the management of DOC patients. However, pain characteristics related to the presence/absence of behavioral responses, and the modifications observed in biomarkers during noxious stimuli, can provide information on the covert content of consciousness (Table 23). The Nociception Coma Scale (NCS) has been developed to assess pain in DOC patients ^[38][107]. The pressure of a fingernail on four limbs provides nociceptive stimulus, applied generally using an algometer, to quantify the necessary pressure to observe the behavioral response to the stimulus. The NCS consists of four subscales assessing motor, verbal, visual responses, and facial expression, allowing distinctions between reflexes (e.g., groaning or oral reflex movements) and higher-level behaviors (e.g., pain localization and crying or intelligible verbalization). Since the visual subscale does not show significant changes between a noxious and a non-noxious condition, NCS was recently substituted by its revised version (NCS-R) ^[39][108]. The absence of the visual subscale does not alter its sensitivity, maintaining the same clinometric property of the NCS ^[40][109], with higher total scores in MCS than in VS/UWS patients. In a study on 64 patients, Chatelle and colleagues ^[39][108] observed that the total scores and subscores (motor, verbal, and facial) of the NCS were higher in noxious than non-noxious stimulation conditions. They identified an NCS cut-off value of four that distinguished the patients who received a noxious stimulation from those who received a non-noxious stimulation. A successive neuroimaging study in DOC with fluorodeoxyglucose (FDG)-PET showed positive correlations between brain activity from the ACC and NCS-R scores, indicating a correlation with pain processing ^[41][110]. Considering the NCS-R, Chatelle and colleagues proposed a cut-off value of two to differentiate nociception from pain ^[42][111]. However, in a retrospective study on the neural basis for pain experience based on the preservation of brain metabolism as assessed by FDG-PET, Bonin and colleagues, suggested a conservative NCS-R cut-off score of less than five to identify pain in these patients ^[43][112]. Concerning the modality with which to administer the nociceptive stimulus, Formisano et al. ^[44][113] proposed the use of NCS(-R) with personalized stimulations (for example, opening the hand, abducting the upper limbs, and mobilizing the head), which may cause different reactions compared with simpler pressure applied to the fingernail bed. Again, a multicentric study involving 40 healthy volunteers and 60 DOC patients found that VS/UWS and MCS patients had lower pressure pain thresholds than healthy participants, suggesting further research on possible pain hypersensitivity in patients with severe brain injuries and multiple co-morbidities is needed ^[45][114]. In a recent study involving 70 VS/UWS patients, Cortese and colleagues ^[46][115] provided evidence to show that an accurate assessment of pain could predict changes in consciousness level with an accuracy of 84% when using the NCS, and 72% when adopting the NCS-R. The results indicated that a change in behavioral response, following a nociceptive stimulation, with a total score for the NCS of ≥5 and ≥3 for the NCS-R, can predict positive outcomes with regard to the condition of MCS. However, it is crucial to consider the patient’s clinical condition in the pain assessment. DOC patients could have developed severe spasticity or they may have been intubated, making pain assessment more complex ^[33][47][102,116]. In patients with tracheostomy, it is necessary to consider lower cut-off values to distinguish nociception from pain because of lower verbal sub-scares in these patients ^[48][117]. Interestingly, Cortese and colleagues ^[49][118] showed the possibility of observing trace conditioning in VS/UWS patients without any behavioral responses to nociceptive stimuli. The study measured the Galvanic Skin Response (GRS) and Heart Rate Variability (HRV) to assess responses to nociceptive stimuli in 13 healthy subjects and 37 VS/UWS patients. Eight VS/UWS, which all showed trace conditioning to the noxious stimulus, were diagnosed as MCS within one month. In the context of autonomic responses to pain, Heart Rate Variability (HRV) emerges as a valuable marker for assessing nociceptive responses in cases of experimentally induced pain. Enhanced parasympathetic activity has been associated with better self-regulation capacities and increased pain inhibition capacity ^[50][119]. In a study conducted by Tobalbini and colleagues, involving 24 patients diagnosed with disorders of consciousness (DOC), it was observed that nociceptive stimuli could lead to changes in autonomic function. This change was characterized by increased sympathetic activity and reduced vagal modulation ^[51][120]. In two separate studies—one involving 21 DOC patients, including 11 with UWS/VS ^[52][121], and the other involving 24 DOC patients, including 12 with UWS/VS ^[51][120]—a reduction in cardiac complexity (i.e., HRV entropy) was noted in UWS/VS patients during exposure to noxious stimuli. Additionally, in an EEG-based study involving 21 DOC patients, pain stimulation was linked with a higher parietal response in the delta frequency band, lower activation in the left frontal region, and increased Galvanic Skin Response (GSR) and Heart Rate ^[53][122].