Central Pain

Central Post-stroke Pain

Information about the frequency of pain after stroke is limited. Studies have shown that pain occurs in anywhere between 1 and 50% of patients after a stroke, both ischemic and hemorrhagic, but not all pain is related to stroke ( ; ; ), and it is also clear that different criteria may account for some of the variations. The most common forms of chronic pain after stroke are musculoskeletal pain, shoulder pain, central post-stroke pain (CPSP), tension-type headache, and spasticity ( Table 69-2 ). Estimates of the proportions of the different types of pain are 40% for musculoskeletal pain, 20% for shoulder pain, 10% for headache, 10% for CPSP, 7% for spasticity, and 13% for other types of pain ( ). No criteria for CPSP have been universally accepted thus far, but the following seem to be the minimal criteria, and they correspond to those used in recent publications ( ):

• A history suggestive of a stroke followed by pain

• Confirmation of a stroke-like relevant lesion on imaging

• Body part with pain corresponding to a specific CNS structure affected by the stroke lesion

• Negative (hyposensitivity) or positive (hypersensitivity) sensory signs in a body area corresponding to a CNS site damaged by the stroke

• Other causes of pain excluded or considered highly unlikely

A prospective study found that CPSP developed in 8% of stroke patients within the first year after stroke ( ). found an 11% frequency of CPSP in a group of elderly patients. In a recent population-based study of 964 stroke patients ( ) that used a grading system and in which patients were classified as having either definite or probable neuropathic pain ( ), 7.3% were found to have either probable or definite CPSP. CPSP can occur after lesions at different levels of the somatosensory pathways, but interestingly, the prevalence of CPSP is dependent on the anatomical location of the lesion ( , , ). A higher frequency of pain has been observed with brain stem infarctions. Thus, found that CPSP developed in 16 (25%) of 63 patients with lateral medullary infarctions. found two-thirds of patients with lateral medullary infarctions to have CPSP. , looking at thalamic lesions, found the prevalence of CPSP to be higher in persons with lesions affecting the inferolateral part of the thalamus (16.7%) than in those with thalamic infarcts in general (7.5%). One study found that CPSP developed in 4 of 13 patients after intracerebral hemorrhage (≈30%). This high prevalence may be the result of a predilection for hemorrhage in the basal ganglia and thalamic structures.

Multiple Sclerosis

The reported prevalence of pain in patients with MS varies, with figures ranging from 29–86% ( ). The large span in reported frequencies probably reflects the use of different methodologies and only few of the studies being population based. In addition, the difficulty distinguishing spasticity and nociceptive types of pain from central pain may contribute to the variation in reported frequency of pain. In a postal survey of 627 patients with definite MS who were compared with a sex- and age-matched control group, 79% of the MS patients and 75% of the control group had some form of pain in the preceding month, but patients with MS had higher pain intensity and higher consumption of analgesics than did their respective controls ( ). Pain may be an initial symptom of MS in about 10–25% of patients, but it is otherwise seen as part of a chronic condition during the course of MS in approximately 50% of patients. Entities considered to represent central pain conditions include central dysesthetic pain, trigeminal neuralgia (TN), Lhermitte’s sign, and tonic spasms ( ). In an Italian study involving 1627 patients, the following prevalence of central pain was found: central dysesthetic pain, 18%; TN, 2%; Lhermitte’s sign, 9%; and tonic spasms, 11% ( ). In the aforementioned postal survey of 627 patients with definite MS and a similarly sized age- and gender-matched group, found that 38% in the MS group and 12% in the control group had touch-evoked pain. In another study by in which all types of pain in the past month were considered, 79% in the MS group and 75% in the control group had pain complaints. In another study of 364 MS patients, 27.5% were considered to have central neuropathic pain (defined as a pain distribution consistent with a CNS lesion and with simultaneous exclusion of a peripheral neuropathic or nociceptive cause) ( ).

Syringobulbia and Syringomyelia

The prevalence of syringomyelia and syringobulbia was previously noted to be between 3 and 8/100,000 ( , ), but mainly because of improved neuroimaging methods, it is now reported to be higher. , in a study from New Zealand, reported a prevalence of 8.2/100,000 and higher in certain ethnic groups. Pain is considered the most important symptom in syringomyelia ( ), but no population-based studies have provided reliable data on this. In a consecutive study of 46 patients with syringomyelia with and without pain, 67% had spontaneous pain and 64% had pain evoked by one or several stimulus modalities ( ).

Ongoing Pain

Ongoing pain is spontaneous and may be continuous or paroxysmal. The character differs, but it can be shooting, shock-like, aching, cramping, crushing, smarting, and burning, among other descriptions. Episodic, paroxysmal types of pain are short-lasting shooting, electric, shock-like, or stabbing in character.

Evoked Pain

Stimulus-evoked pain is classified according to the type of stimulus that provokes it, such as mechanical, thermal, or chemical stimuli. In some patients all these symptoms may be present; in others only one type of hypersensitivity is present. So a series of stimuli need to be applied to document or exclude abnormality. Evoked pain is usually brief and lasts only for the duration of stimulation, but it may sometimes persist even after cessation of stimulation because of after-sensations, which can last for minutes, hours, or even days. In such cases, distinction between evoked and spontaneous types of pain can be difficult. Allodynia and hyperalgesia may be seen with central pain disorders; however, it is clinical experience that in CPSP, dysesthetic MS, and perhaps also other central pain conditions, the sensory quality of the abnormal evoked sensation more often has the character of dysesthesia rather than true pain (i.e., allodynia). Hyperpathia (i.e., explosive pain evoked in areas with an increased sensory threshold when the stimulus exceeds the threshold) is also a feature of central pain, as in peripheral neuropathic pain. It has been claimed that a specific set of descriptors is associated with the various types of central pain, just as it has been suggested for peripheral neuropathic types of pain ( ).

Central Post-stroke Pain

The lesion responsible for pain in CPSP was initially claimed to be located in the contralateral thalamus, and the syndrome was for years called “thalamic syndrome” ( ; ; ). It is now well established that lesions causing central pain after stroke can be located anywhere along the somatosensory projection system, such as in the lateral medulla, in other parts of the brain stem, in the thalamus, or beyond the thalamus, including the cortex and the operculum ( ; ; ; ; ; ; ). For this reason the term “thalamic pain” has been abandoned and substituted by the broader and more appropriate term CPSP ( ). The pain in CPSP can be both superficial and deep, and in the latter case it may be difficult to distinguish from musculoskeletal types of pain, which often accompany CPSP. No set of specific pain descriptors is associated with CPSP, but burning, pricking, lancinating, icy, tearing, cutting, and squeezing types of pain are often present in CPSP. Using the McGill Pain Questionnaire, the median number of words chosen was eight with a median pain rating index of 21.5% and a median pain intensity of 2 on a scale of 0–10 ( ). The onset of pain in CPSP varies and can occur anytime from immediately after the stroke to several months or, in some cases, years after the stroke ( , , ). In a prospective study it was found that two-thirds of patients had an onset of CPSP during the first month, 19% within the first 6 months, and the remaining after 6 months ( ). found that pain developed about 10 months after the stroke in their population. The pain of CPSP can be spontaneous or evoked; in particular, spontaneous dysesthesia seems to be common and is reported by more than 75% of CPSP patients. Some studies have found higher pain intensity with lesions located in the brain stem and thalamus ( , ) rather than outside these areas, but this does not seem to be a consistent finding ( ). The intensity of spontaneous pain often fluctuates and can be increased by emotional as well as physical distress and reduced by rest and distraction ( , , ), similar to other types of neuropathic pain in which internal and external factors can modulate the pain experience. Pain often represents a great burden to the patient, even when the intensity is low. There has been interest in the distribution of pain, which can involve anything from small areas to the entire half of the body. In patients with medullary infarctions, crossed pain distributions are characteristic and have specific significance. In patients with pain located unilaterally in the orofacial area and in the ipsilateral finger digits, the pain is likely to originate in the contralateral thalamus. The sensory loss may involve all sensory abnormalities, but loss of spinothalamic functions (cold, warmth, pinprick) appears to be essential and is found in more than 90% of patients with CPSP, whereas sensory loss with respect to other modalities is less common ( , , ). The area of pain is within the territory of the sensory abnormalities and typically occupies only a fraction of the sensory deficit. Another characteristic of CPSP is the paradoxical presence of hyper-phenomena in the painful area. Hyper-phenomena may be manifested either as hypersensitivity with allodynia or merely as dysesthesia to one or several sensory modalities. In a study by , cold allodynia or dysesthesia was present in 94% of patients with CPSP but in only 3% of stroke patients with sensory abnormalities but without pain. Similarly, 75% had touch-evoked allodynia or dysesthesia, but none in the group without pain had these abnormalities. Abnormal temperature and pain sensibility are the most consistent abnormalities in CPSP. In two studies it was found that 81% had reduced sensibility to temperature ( , ). The presence of hypersensitivity within the same territory of the sensory deficit can, for obvious reasons, occasionally obscure the detection of sensory loss or the extent of such.

Multiple Sclerosis

MS is a chronic progressive disease characterized by multiple sites of focal demyelination in the CNS as a result of inflammatory processes of assumed autoimmune origin in individuals with a susceptible genetic and environmental background. The focal inflammation gives rise to plaques scattered throughout the CNS, but with a preference for the periventricular white matter, the brain stem, and the spinal cord. The consequences of these plaques are a wide spectrum of neurological symptoms and signs, including motor, coordinative, sensory, autonomic, and cognitive abnormalities. One symptom—and an often neglected one—is pain. Central dysesthetic pain occurs, according to a study by , in 28% of cases, but it is rarely an initial symptom. The quality is variable; burning and aching types of pain are often reported, but they are not specific for the central pain in MS. There are only a few studies on the sensory abnormalities in MS patients with pain. In a random sample of 50 MS patients with pain, 50 MS patients without pain, and 50 controls, more than 50% of the patients had central pain. The presence of cold allodynia and abnormal temporal summation was higher in the MS group than in the non-painful MS group ( ). In this study no difference was found between the two groups in terms of abnormalities in dorsal column and spinothalamic function. , on the other hand, in a group of 62 MS patients with central pain and 16 MS patients without pain found sensory abnormalities in 97 and 81%, respectively. Abnormalities in spinothalamic function were more common in the pain group. Pain usually affects the legs and trunks uni- or bilaterally, and according to its origin it has a distribution that is compatible with a brain or spinal segmental localization. TN is more frequent and more often bilaterally localized when seen in MS patients than in the general population. Painful tonic seizures or tonic spasms, not to be confused with spasticity, are paroxysms of painful attacks lasting seconds and usually less than 2 minutes with pain in the face, arm, or leg associated with abnormal, often dystonic postures. They may start in one body part and spread either unilaterally in a segmental fashion or occasionally bilaterally. These attacks may last days or a few weeks and then disappear. The underlying mechanisms are unclear, but they have been related to an acute inflammatory lesion in the internal capsule or the cerebral peduncles ( ). Lhermitte’s sign refers to a sudden and short-lasting electrical sensation spreading down the spine from the cervical region, usually in response to flexion of the neck. These attacks are assumed to be due to acute demyelination or inflammation in plaques in the cervical cord. They generally occur for a period of days or weeks and then disappear. For TN in patients with MS, it has been suggested that it could be related both to intrapontine pathology of the trigeminal axons and to concomitant peripheral vascular compression ( ). As in other cases of CNS lesions associated with pain that is spasm related, pain is also seen with MS. Spasm-related pain represents a difficult problem in terms of classification. Spasms can be spontaneous or provoked by different stimuli, including tactile stimulation, urinary tract infection, a full bladder, or emotional factors. The frequency is variable, but in MS it has been estimated that about 10% have such spasms ( ). Flexor spasms are generally explained by disinhibition of the normal flexor withdrawal response ( ), and from that point of view it may be argued that spasm-related pain is a central pain phenomenon ( ). Others would argue that the pain with flexor spasms is related to repeated muscular contractions, movements, and postures and is therefore to be considered a musculoskeletal type of pain.