Age-related differences in the qualities but not the intensity of chronic pain

Pain 104 (2003) 597–608 www.elsevier.com/locate/pain

Age-related differences in the qualities but not the intensity of chronic pain Lucia Gagliesea,b,c,*, Ronald Melzackd a School of Kinesiology and Health Science, York University, 4700 Keele Street, Toronto, Ontario, M3J 1P3 Canada Department of Anesthesia and Pain Management, University Health Network, 200 Elizabeth Street, Toronto, Ontario, M5G 2C4 Canada c Department of Anesthesia, University of Toronto, Fitzgerald Building Rm. 131, 150 College Street, Toronto, Ontario, M5S 3E2 Canada d Department of Psychology, McGill University, Montreal, Quebec, Canada

b

Received 16 July 2002; received in revised form 24 February 2003; accepted 7 March 2003

Abstract Age differences in the experience of chronic pain remain unclear. A serious barrier to progress in the field of pain and aging arises from the lack of data regarding the psychometric properties of pain scales for use with the elderly. The present study was designed to assess age differences in pain intensity and quality and to compare the psychometric properties of the McGill Pain Questionnaire (MPQ) in young and elderly chronic pain patients. Young (n ¼ 139, mean age ¼ 42.93 ^ 9.41 years) and elderly (n ¼ 139, mean age ¼ 70.12 ^ 7.51 years) pain center patients, matched on primary diagnosis or pain location, duration, and sex, completed the MPQ, numeric ratings (0 –10) of pain intensity, a Pain Map, and the Hospital Anxiety and Depression Scale (HADS). A Pain Management Index (PMI) score was calculated for each patient. Age differences on the measure of pain qualities were found. The elderly group had significantly lower MPQ total and sensory scores and chose fewer words than the young group. However, there were no significant differences between the groups on numeric ratings of highest, usual, and lowest pain intensity. Similarly, there were no age differences on PMI, Pain Map, or the HADS Depression or Anxiety Subscales. Finally, the latent structure, internal consistency, and pattern of subscale correlations of the MPQ were very similar in the young and elderly groups. Possible explanations for the discrepancy in the pattern of age differences on measures of pain intensity and quality are explored. The implications of this pattern of age differences for basic pain mechanisms and pain management should be given serious empirical attention. q 2003 International Association for the Study of Pain. Published by Elsevier Science B.V. All rights reserved. Keywords: Pain and aging; Pain assessment; McGill Pain Questionnaire; Psychometrics; Reliability; Validity

1. Introduction Although up to 20% of patients referred to pain clinics are over 60 years old (Benbow et al., 1995; Gagliese and Melzack, 1997b; Kee et al., 1998), age differences in the experience of chronic pain remain unclear. Various studies have reported that scores on single-item measures of pain intensity increase (Puder, 1988; Wilkieson et al., 1993), decrease (Parker et al., 1988; Turk et al., 1995) or do not change (Middaugh et al., 1988; Sorkin et al., 1990) with age. Similarly, in some studies (Benbow et al., 1996; Lichtenberg et al., 1984), older people with chronic pain obtain lower scores than younger people on the McGill Pain * Corresponding author. Department of Anesthesia and Pain Management, Toronto General Hospital, 200 Elizabeth Street, Toronto, Ontario, Canada M5G 2C4. Tel.: þ 1-416-340-4296; fax: þ 1-416-3404739. E-mail address: [email protected] (L. Gagliese).

Questionnaire (MPQ) (Melzack, 1975), a measure of the sensory, affective, and evaluative qualities of pain. Other studies, however, do not find age differences in MPQ scores among chronic pain patients (Lichtenberg et al., 1986). Recently, it has been suggested that the pattern of age differences found may be dependent on the pain scales used (Gagliese and Melzack, 1997a). In a sample of individuals with arthritis pain, there were age-related decreases on both the Sensory and Affective Subscales of the Short Form MPQ (SF-MPQ) (Melzack, 1987) but no age differences in pain intensity whether measured with the Visual Analog Scale (VAS) (Huskisson, 1983), the Verbal Descriptor Scale (VDS) (Melzack, 1975), or the Behavioral Rating Scale (Linton and Gotestam, 1983). Although the VDS and BRS are made up of verbal descriptors of pain, these results suggest that these scales may not assess exactly the same dimension of pain as the MPQ. Specifically, the MPQ assesses the qualities of pain (how it feels) while the BRS

0304-3959/03/$20.00 q 2003 International Association for the Study of Pain. Published by Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-3959(03)00117-9

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and VDS measure intensity (how much it hurts). Therefore, the results of Gagliese and Melzack (1997a) suggest that the pain experienced by the elderly was of comparable intensity to that of younger people, but its qualitative characteristics may have differed. Several features of the study described above (Gagliese and Melzack, 1997a) limit the conclusions which may be drawn. The sample was small and included only individuals with chronic arthritis pain, limiting the generalizability of the results to the more heterogeneous pain clinic population. In addition, subjects completed the SF-MPQ. It is not clear whether this pattern of age differences would also be found using the full-scale MPQ. Clearly, the results described by Gagliese and Melzack (1997a) require replication in a larger sample, with a variety of painful disorders, and using the full-scale MPQ to enable a richer assessment of age differences in the experience of chronic pain. A more serious barrier to the interpretation of age differences in pain scales arises from the lack of data regarding the psychometric properties of these scales for use with the elderly. As such, results may reflect not only age differences in pain but also differences in the validity and reliability of the pain tools across age groups (Gagliese, 2001). Most of the scales in common use were developed and validated for young adult samples. The appropriateness of these scales for elderly people with pain has only recently begun to receive empirical attention. These studies suggest that intensity measures, especially those made up of verbal descriptors or numeric ratings, may be appropriate for use with the elderly (see review by Gagliese, 2001). On the other hand, there may be important problems for the use of the VAS. As many as 30% of cognitively intact elderly may be unable to complete this scale (Gagliese and Melzack, 1997a). Furthermore, among those who can complete the scale, intensity estimates may be significantly different from those obtained using other scales (Gagliese, 2001; Herr and Mobily, 1993). However, different scales designed to measure the same construct should give comparable estimates of that construct, referred to as convergent validity (Anastasi, 1988). The reasons for these potential threats to the validity of the VAS among the elderly remain to be elucidated. There is much evidence for the validity, reliability, and discriminative abilities of the MPQ when used with younger adults (see review by Melzack and Katz, 2001). However, comparable data regarding use with elderly patients are not yet available. There is some evidence to suggest that the short form of the MPQ (SF-MPQ) (Melzack, 1987) may be appropriate for use with the elderly. The proportion of subjects who are unable to complete this scale or who provide unscorable responses does not differ between age groups (Gagliese and Melzack, 1997a). Internal consistency, a measure of reliability, is high (LeFort et al., 1996), and the Sensory and Affective Subscales provide similar estimates of pain levels within elderly samples (Gagliese and Melzack, 1997a; Helme et al., 1989). Elderly people

endorse fewer words overall than younger people, but regardless of age, the same adjectives are chosen most frequently to describe the same type of pain (e.g. arthritis pain) (Gagliese and Melzack, 1997a). Although these findings are promising, they must be replicated in larger samples with more detailed psychometric analyses before conclusions can be drawn. Furthermore, data regarding the psychometric properties of the full-length MPQ for use with elderly people are urgently needed. The first objective of the present study was to assess age differences in pain intensity and quality. It was predicted that, in a heterogeneous sample of pain clinic patients, there would be no age differences in numeric ratings of pain intensity (NRS) but that older patients would obtain lower MPQ scores than younger patients. The second objective was to determine if the internal structure and other psychometric properties of the MPQ differed between young and elderly chronic pain patients.

2. Method 2.1. Subjects The files of all patients assessed at the McGill-Montreal General Hospital Pain Centre over a 5-year period were reviewed ðn ¼ 565Þ and those who completed a standard ‘intake’ questionnaire on their first visit were identified ðn ¼ 496Þ. In order to conduct a detailed analysis of age differences in the properties of the MPQ, a group of young (less than 60 years old) and a group of elderly (60 þ years old) patients were extracted from the complete sample. Because there were fewer elderly ðn ¼ 155Þ than young ðn ¼ 341Þ patients, the young subsample was chosen from the larger pool by matching each elderly patient to one young patient on the basis of the following criteria applied in order: primary diagnosis or pain location, duration (^ 6 months), and sex. If more than one potential match remained after the application of these criteria, the young patient was selected randomly. This matching was conducted to minimize the effects of these factors on the MPQ scores. Because there were no patients under 60 years of age with post-herpetic neuralgia, the elderly patients with this diagnosis ðn ¼ 16Þ were not included in the analysis. The results reported below are based on data provided by 139 young patients and 139 elderly patients. As shown in Table 1, only the most common pain problems experienced by young and elderly patients were represented in this sample. A much larger sample would be required to more adequately reflect the full spectrum of chronic pain disorders. 2.2. Measures 2.2.1. McGill Pain Questionnaire The MPQ (Melzack, 1975) is the most widely used

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system and criteria developed by Margolis et al. (1986). Pain Maps have been used across age groups (Gagliese, 2001).

Table 1 Characteristics of the matched age groups

Age (years) % Female Pain duration (months) Pain Map Pain disorder/site n (%) Low back pain Arthritis (osteo and rheumatoid) Headache Neuropathies Post-surgery Thoracic back Diffuse (not otherwise specified) Abdominal Post-traumatic/myofascial Fibromyalgia Complex regional pain syndrome Psychosomatic Pelvic Phantom limb Post-radiation Central pain

Young (n ¼ 139)

Elderly (n ¼ 139)

42.93 (9.41) 71.94 83.81 (94.59) 15.84 (10.72)

70.12 (7.51) 71.94 100.67 (120.73) 16.77 (12.90)

42 (30.22) 24 (17.27)

42 (30.22) 24 (17.27)

15 (10.79) 12 (8.63) 8 (5.76) 7 (5.04) 7 (5.04)

15 12 8 7 7

(10.79) (8.63) (5.76) (5.04) (5.04)

6 (4.32) 5 (3.60) 4 (2.88) 2 (1.44)

6 5 4 2

(4.32) (3.60) (2.88) (1.44)

2 (1.44) 2 (1.44) 2 (1.44) 1 (0.72) 1 (0.72)

2 2 2 1 1

(1.44) (1.44) (1.44) (0.72) (0.72)

599

n.s. n.s.

Values are means (standard deviation) unless otherwise indicated.

multidimensional pain inventory (Wilke et al., 1990). It is made up of 20 categories of adjectives that describe the sensory, affective, evaluative, and miscellaneous components of pain. Patients endorse the words which describe their feelings and sensations. The rank values of the words chosen are summed to obtain a Total Score (PRI-T) and separate scores for the Sensory (PRI-S), Affective (PRI-A), Evaluative (PRI-E), and Miscellaneous (PRI-M) Subscales. The number of words chosen (NWC) also is recorded. There is substantial evidence for the validity, reliability, and discriminative abilities of the MPQ when used with younger adults (Melzack and Katz, 2001). 2.2.2. Numeric rating scale (NRS) Patients were asked to rate the highest (NRS-H), lowest (NRS-L), and usual (NRS-U) intensity of pain which they had experienced in the last week on a scale from 0 to 10 where 0 was ‘no pain’ and 10 was ‘worst pain imaginable’. There is evidence to support the validity and reliability of this unidimensional measure for both younger (Jensen and Karoly, 2001) and older (Gagliese, 2001) patients. 2.2.3. Pain Map The Pain Map is an outline of the human body on which patients indicate the spatial distribution of their pain (Melzack, 1975). A clear plastic template was fit over the human figure and scores were calculated using the weighted

2.2.4. Pain Management Index (PMI) The PMI, as revised by Breitbart et al. (1996), was calculated for each patient. The PMI, a measure of the adequacy of analgesic therapy, compares the potency of prescribed and/or over-the-counter analgesics with the report of highest pain intensity. The potency of analgesics is classified according to the WHO analgesic ladder (World Health Organization, 1990). Prescriptions of strong opioids (e.g. morphine or hydromorphone) receive a score of 3, weak opioids (e.g. codeine) a score of 2, and non-opioid (e.g. NSAIDS) or adjuvant (e.g. tricyclic antidepressants) analgesics a score of 1. Patients not receiving any analgesic medication are assigned a score of 0. Analgesic score was based on the highest potency analgesic prescribed by the end of the initial evaluation at the Pain Centre. As such, analgesics being taken at the time of the initial visit and any prescribed in the course of the evaluation were included. Usual pain intensity was re-coded in a manner comparable to the analgesic scores. Severe pain intensity scores (8 – 10) are scored a 3, moderate intensity scores (4 –7) are scored a 2. Patients with mild intensity scores (1 – 3) receive a score of 1, and patients who report no pain receive a score of 0. The PMI is then calculated by subtracting the pain intensity score from the analgesic score. PMI scores range from 2 3 (indicating severe pain but no analgesics) to þ 3 (indicating a strong opioid prescription but no pain). Scores of 0 and above indicate adequate analgesia. 2.2.5. Hospital Anxiety and Depression Scale (HADS) The HADS (Zigmond and Snaith, 1983) was developed as a screening instrument for use with medical outpatient populations. It is a 14-item self-report measure with separate subscales for anxiety and depression. Scores on each subscale range from 0 to 21 with higher scores indicative of greater distress. The HADS is appropriate for use with medical populations because symptoms of emotional distress, which may also have a physical cause (e.g. insomnia and weight loss), are not included. The appropriateness of this scale across age groups (Spinhoven et al., 1997) and with chronic pain patients (Greenough and Fraser, 1991) has been demonstrated. 2.3. Procedure Patients completed the questionnaires as part of the standard intake process for the pain clinic. Questionnaire responses, demographic data, and medical information such as diagnosis and medications were extracted from the files. This study was approved by the McGill University Department of Psychology Ethics Committee.

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L. Gagliese, R. Melzack / Pain 104 (2003) 597–608 Table 2 Psychological factors, PMI, and prescribed analgesic by age group Young (n ¼ 139)

Fig. 1. MPQ scores for each age group. *P # 0:0001.

HADS – Depression HADS – Anxiety Correlation analysis (r)** HADS-D and PRI-T HADS-A and PRI-T HADS-D and NRS-U HADS-A and NRS-U PMI Analgesic %(n) None Non-opioid Weak opioid Strong opioid

Elderly (n ¼ 139)

9.49 (5.31) 10.27 (4.51)

8.61 (4.18) 9.67 (4.77)

n.s. n.s.

0.27 0.28 0.33 0.28 21.20 (0.92)

0.30 0.32 0.36 0.27 20.94 (0.85)

n.s. n.s. n.s. n.s. n.s.

4.32 (6) 47.48 (66) 26.62 (37) 21.58 (30)

2.88 (4) 41.73 (58) 31.65 (44) 23.74 (33)

n.s. n.s. n.s. n.s.

Values are means (standard deviation) unless otherwise indicated. HADS, Hospital Anxiety (HADS-A) and Depression (HADS-D) Scale, ** for all correlations P # 0:01.

3. Results 3.1. Patient characteristics Table 1 presents the average age of the groups and the variables used to match patients across the age groups. The matching was successful as the gender and pain disorder/ sites distributions in the two groups is identical and the duration of pain (which was matched on the basis of ^ 6 months) does not differ between the groups. 3.2. Age differences in pain scales and psychological measures T-Tests were conducted to determine if the two groups differed on the measures of pain, depression, or anxiety. Due to the large number of t-tests conducted, a Bonferroni correction was applied. As a result, P # 0:004 was required for statistical significance. Using this criterion, the elderly group had significantly lower MPQ PRI-T, PRI-S scores (see Fig. 1) and chose fewer words than the young group (young: 10.78 ^ 4.91; elderly: 8.42 ^ 4.69; P # 0:0001). There were no significant differences between the groups on

NRS-H, NRS-L, or NRS-U (Fig. 2). Similarly, there were no age differences on PMI, Pain Map, or the HADS Depression or Anxiety Subscales (Table 2). In order to assess age differences in the relationship between pain and distress, the correlation between HADS scores, PRI-T, and NRS-U were calculated separately for each age group. A series of Fisher’s Z tests showed that the magnitude of the correlations between the measures of distress and pain did not differ between the age groups (Table 2). Similarly, the correlation between NRS-U and PRI-T did not differ between the groups (young: r ¼ 0:23; elderly: r ¼ 0:28). 3.3. Detailed analysis of the MPQ Several steps were taken to compare the psychometric properties of the MPQ between the age groups. The internal consistency of the PRI-T, PRI-S, PRI-A, and PRI-M subscales was calculated in each age group using Cronbach’s alpha (Table 3). Because PRI-E is made up of

Fig. 2. NRS ratings of highest, lowest, and usual pain intensity for each age group. There were no significant age differences.

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PRI-M

solution accounted for 58% of the variance in the young group and 62% of the variance in the elderly group. Several steps were taken to compare the principal components (PC) across the age groups. Correlations of component loadings, coefficients of convergence, and Cattel’s salient similarity index of the loadings (Gorsuch, 1974; Pinneau and Newhouse 1964; Tabachnick and Fidell, 1983) were calculated for each possible pair of PC. Detailed descriptions of these tests are available (Gorsuch, 1974; Pinneau and Newhouse, 1964). These analyses suggested that each PC in the young group was significantly related to only one PC in the older group except for young group PC2 and older group PC1, which, although similar, did not reach statistical significance. Table 5 presents the results for the PCs that were significantly related. These data suggest that very similar PC were identified in each group. The component loadings and percent of the total variance explained by each component are presented in Table 6(A) and (B). The results of the analyses described above suggest that the psychometric properties of the MPQ are consistent in young and older chronic pain patients. In order to analyze age differences in the pattern of words chosen, patients with low back pain (LBP) (the most common pain problem presented by the elderly ðn ¼ 42Þ in this sample) were extracted from the two groups. Age differences in the frequency with which words in each MPQ category were chosen were tested with x 2 analysis (Fig. 3). As can be seen, the elderly patients with LBP were less likely to choose words from several sensory and miscellaneous categories of the MPQ, but there were no differences on the affective or evaluative subscales. Adjectives chosen by $ 33% of patients have been identified as characteristic of a particular type of pain (Dubuisson and Melzack, 1976). Using this criterion, seven words were identified in the young group but only four in the elderly group (Table 7). Interestingly, three of the four most frequent descriptors chosen by the elderly were also among the most frequently chosen by the young group. The final descriptor, intense, was chosen by over 30% of the young group, a rate that was not different from the older group. Nonetheless, there were some age-related

1.00

Table 5 Correlations of PC loadings (r), coefficients of convergence (s), and salient similarity index (P ¼ probability) of matched PC in the young and elderly groups

Table 3 Values of Cronbach’s alpha for the MPQ subscales in each age group

PRI-T PRI-S PRI-A PRI-M

Young

Elderly

0.80 0.64 0.58 0.36

0.82 0.67 0.53 0.35

MPQ scores: Total Score (PRI-T), Sensory Subscale (PRI-S), Affective Subscale (PRI-A), and Miscellaneous Subscale (PRI-M).

only one item, it is excluded from this analysis. As can be seen, the values of alpha are very similar in the two groups suggesting that the internal consistency of the MPQ does not differ between the young and elderly groups. In order to assess age differences in the relationship among the subscales, MPQ subscale correlations for each group were then calculated (Table 4). The significance of the difference of these correlations between the two age groups was tested using Fisher’s Z transformation. There were no significant differences in the pattern of correlations between the two groups. In fact, the magnitude of the correlations was remarkably similar in the young and elderly groups. Generally, these correlations suggest that the scales are moderately correlated. Age differences in the latent structure of the MPQ were then assessed using principal components analysis (PCA). The PCA was conducted separately for the young and elderly groups with each of the 20 descriptor categories treated as a variable. The analysis was conducted on the correlation matrix. In both age groups, a six-component analysis of the scree plots and the number of components having an eigenvalue greater than 1. The components were then rotated using Varimax rotation. This rotation was chosen in order to maximize the validity of the tests comparing the factor structures between the age groups (Gorsuch, 1974; Pinneau and Newhouse, 1964). The rotated Table 4 Correlation matrix of MPQ subscale scores PRI-S (A) Young group PRI-S 1.00 PRI-A 0.62 PRI-E 0.36 PRI-M 0.64 (B) Elderly group PRI-S 1.00 PRI-A 0.66 PRI-E 0.36 PRI-M 0.68

PRI-A

1.00 0.41 0.61

1.00 0.41 0.74

PRI-E

1.00 0.38

1.00 0.36

601

1.00

MPQ scores: Total Score (PRI-T), Sensory Subscale (PRI-S), Affective Subscale (PRI-A), Evaluative Subscale (PRI-E), and Miscellaneous Subscale (PRI-M).

Young group

Elderly group

r

s

P

PC1 PC2 PC3 PC4 PC5 PC6

PC2 PC1 PC6 PC4 PC3 PC5

0.56 0.11 0.43 0.13 0.36 0.49

0.80 0.47 0.60 0.60 0.64 0.68

0.0001 0.08 0.02 0.05 0.002 0.0001

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Table 6 Rotated PC loadings MPQ category

Principal component PC1

PC2

PC3

PC4

PC5

PC6

(A) Young group S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 A11 A12 A13 A14 A15 E16 M17 M18 M19 M20 Variance explained (%)

0.04 0.17 0.09 0.09 0.01 20.01 0.03 0.10 0.41 0.33 0.19 0.63 0.54 0.52 0.25 0.56 0.14 0.23 0.12 0.80 2.43 (12.14)

0.24 20.10 0.58 0.76 20.04 0.13 0.07 0.08 20.35 0.23 0.08 0.22 0.12 0.28 0.06 0.42 0.32 0.05 0.09 20.11 1.64 (8.22)

0.18 20.05 0.21 20.04 0.81 0.31 0.36 0.20 0.11 0.17 0.15 0.13 0.18 0.56 0.52 0.18 0.09 0.39 20.21 20.04 1.94 (9.71)

0.07 0.06 0.18 0.21 0.07 0.32 0.45 0.71 0.35 0.58 20.03 0.26 0.36 0.04 0.19 20.05 0.21 0.30 0.72 0.08 2.25 (11.27)

20.01 0.63 0.17 0.13 0.11 0.60 0.13 0.17 0.12 0.03 0.71 0.02 0.23 0.10 0.15 0.20 0.52 0.24 20.03 0.14 1.85 (9.24)

0.75 0.52 0.42 0.13 0.06 20.23 20.04 20.05 0.07 0.10 0.01 0.23 0.08 0.08 0.39 20.17 0.14 0.23 0.28 0.04 1.50 (7.48)

(B) Elderly group S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 A11 A12 A13 A14 A15 E16 M17 M18 M19 M20 Variance explained (%)

0.02 0.06 0.26 0.48 0.12 0.59 0.28 0.33 0.04 20.08 0.10 0.70 0.63 0.40 0.21 20.02 20.03 0.41 0.75 0.29 2.77 (13.84)

0.12 0.20 20.03 20.19 0.14 0.22 0.26 0.22 0.68 0.22 0.53 20.01 0.35 0.40 0.10 0.78 0.12 0.18 0.10 0.64 2.43 (12.14)

0.13 0.53 0.59 0.23 0.09 0.26 20.06 20.15 0.12 0.04 0.03 0.16 0.14 0.55 0.30 0.12 0.74 20.01 20.10 0.05 1.81 (9.07)

0.48 0.15 0.02 0.15 0.11 0.25 20.04 0.42 0.07 0.73 0.16 0.18 0.30 0.20 0.67 0.07 0.08 0.65 20.09 0.22 2.17 (10.87)

0.61 0.56 0.50 0.46 0.14 20.10 0.65 0.31 0.11 0.13 0.10 0.14 0.22 20.12 20.07 0.09 0.06 0.30 0.17 0.04 1.96 (9.79)

0.04 0.03 0.00 0.16 0.79 0.31 0.17 20.30 0.12 0.38 0.41 0.13 20.14 0.02 20.03 0.02 0.11 0.15 20.02 20.01 1.26 (6.30)

discrepancies in the pattern of most frequently chosen words. Young patients were significantly more likely than older patients to choose the descriptors ‘shooting’ and ‘burning’ to describe LBP. This may reflect the heterogeneous nature of LBP. In order to test this possibility, the pattern of words chosen by patients with a less frequent, but more homogeneous type of pain, arthritis ðn ¼ 24=groupÞ, was compared. Similar to the case with LBP, there was a high degree of similarity in the most frequently chosen descriptors, but older patients chose fewer of them. There

were no age differences in the proportion of patients in each group who chose the most common descriptors (Table 8).

4. Discussion Despite growing empirical attention, progress in our understanding of age-related issues in pain has been hindered by the lack of information regarding the psychometric properties of pain scales for the elderly. It is

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603

Fig. 3. Percent of patients in each age group choosing one of the descriptors in each MPQ category. a: P # 0:05; b: P # 0:01; c: P # 0:005; d: P # 0:0005.

impossible to reconcile the often discrepant age-related patterns reported in pain intensity and qualities when it is not yet clear that commonly employed scales are valid and reliable for use across the adult life span. In the present study, we have reported on age differences in chronic pain and in the psychometric properties of the MPQ. In a sample of younger and older pain center patients who were matched for gender, pain location/diagnosis, and duration, a discrepancy was found in the pattern of age differences obtained on different measures of pain. Elderly patients obtained lower total and sensory scores on the MPQ than younger patients, but there

were no age differences in NRS. There may be several possible explanations for these findings. The most important of these is that the differences reflect age-associated changes in the quality but not the intensity of chronic pain. However, before this possibility can be considered, we must rule out several alternate explanations including that the results are secondary to age differences in: (1) another pain-related variable; (2) the psychometric properties of the MPQ; and (3) pain language, or language abilities more generally. Each of these possibilities will be considered in turn.

Table 7 MPQ descriptors chosen by $33% of patients with LBP in either age group

Table 8 MPQ descriptors chosen by $33% of patients with arthritis pain in either age group

Descriptor

Descriptor

Sensory Throbbing Shooting Stabbing Sharp Burning Aching Affective Exhausting Evaluative Intense

Young (N ¼ 42)

Elderly (N ¼ 42)

P

(33.3) (54.8) (42.9) (35.7) (45.2) (40.5)

10 (23.8) 8 (19.1) 11 (26.2) 18 (42.9) 7 (16.7) 15 (35.7)

n.s. x 2 ¼ 11.51, P # 0.001 n.s. n.s. x 2 ¼ 8.02, P # 0.005 n.s.

24 (57.1)

17 (40.5)

n.s.

13 (31.0)

15 (35.7)

n.s.

14 23 18 15 19 17

Descriptors chosen by $33% of patients are shown in bold. Data are n (%).

Sensory Throbbing Shooting Stabbing Sharp Wrenching Burning Tender Affective Exhausting Evaluative Intense

Young (N ¼ 24)

Elderly (N ¼ 24)

P

9 (37.1) 10 (43.5) 7 (30.4) 11 (47.8) 9 (37.5) 8 (33.3) 10 (43.5)

4 (16.7) 9 (37.5) 9 (37.5) 8 (33.3) 4 (16.7) 8 (33.3) 5 (20.8)

n.s. n.s. n.s. n.s.

15 (62.5)

15 (62.5)

n.s.

10 (43.5)

9 (37.5)

n.s.

n.s. n.s.

Descriptors chosen by $33% of patients are shown in bold. Data are n (%).

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4.1. Age differences in pain and psychological distress The first explanation for the results reported in this study is that they are secondary to age differences in another pain-related variable. In order to minimize this possibility, patients were carefully matched for gender, pain duration, and pain location/diagnosis. Each of these factors may influence pain reports (Jensen and Karoly, 2001; Melzack and Katz, 2001). In addition, the matching process reduced the heterogeneity between the age groups making their reports of pain more directly comparable. Several other variables that may influence pain scores, including adequacy of analgesic therapy, depression, and anxiety, were not used as matching variables. Age differences in these variables could contribute to the pattern of differences seen on the pain scales. In order to minimize this possibility, each of these variables was compared between the groups. Several studies have suggested that adequacy of analgesic therapy decreases with age such that older people are especially vulnerable to undertreatment of pain (Bernabei et al., 1998; Cleeland et al., 1994). In the present sample, however, age was not related to the adequacy of analgesic therapy (PMI score). The difference between our results and those of other studies may be due to sample differences. This is the first comparison of PMI scores across age groups in a sample of patients with chronic pain seen at a multidisciplinary pain center. PMI scores are also reflective of the prescribing practices of the medical setting being studied making generalizations across settings difficult. The McGill-Montreal General Hospital Pain Centre has long advocated the judicious use of opioids for the management of chronic non-malignant pain (Jeans et al., 2000). PMI scores from this center therefore may be substantially different from those obtained in a center with a different philosophical approach to pain pharmacotherapy. As such, generalizations about the management of pain across the life span must be made with caution. For our purposes, we were only interested in the PMI as an index of age group comparability for this particular sample. The PMI is constructed using NRS ratings of pain intensity as compared to class of analgesic prescribed using the framework of the WHO analgesic ladder (Breitbart et al., 1996). The lack of age differences on PMI suggests that at a given level of pain intensity, younger and older patients are equally likely to receive the same type of analgesic. However, the negative mean value for the PMI in both age groups suggests that a considerable number of these patients, regardless of age, may not have been receiving adequate analgesia. This is not surprising given that data on pain intensity and management were collected at the first visit to a comprehensive multidisciplinary pain center. These patients were most likely referred because they had intractable pain. It would be interesting to follow patients to assess changes in PMI throughout the course of their care. Another consideration in the interpretation of PMI scores is

that this measure only assesses pharmacotherapy. It is unknown whether the groups differed in their use of nonpharmacological pain management strategies. To our knowledge, data regarding age differences in the use of non-pharmacological pain management strategies among chronic pain patients are not available. This would be an important consideration for future studies. Chronic pain can be associated with significant elevations on measures of psychological distress (Fishbain et al., 1997; Romano and Turner, 1985). Because increasing distress has been associated with higher scores on the MPQ (Kremer and Atkinson, 1983) and measures of pain intensity (Fishbain et al., 1997), it was important to compare the distress levels reported by the age groups. Consistent with several previous reports (Gagliese and Melzack, 1997c; Sorkin et al., 1990; Turk et al., 1995), there were no age differences in levels of anxiety and depression. In addition, the relationship between the distress and pain measures did not differ between the age groups. Although the HADS does not allow for clinical diagnosis (Zigmond and Snaith, 1983), these data support the conclusion that young and older people with chronic pain report comparable levels of distress and that they also may have similar need for intervention to relieve this distress. Importantly, these data suggest that differences in distress do not underlie the age-related pattern of pain scores. There are many other variables which may have contributed to the pattern of age differences that was found. However, it is not feasible to control or measure the entire spectrum of factors that might differ by age and also might contribute to pain reports (see Gagliese and Melzack, in press for a review). Instead, we have attempted to control or assess the most salient of these factors. As a result, it is possible to conclude that the age differences on the MPQ and the lack of age differences on the NRS are not directly related to age differences in pain duration, location or diagnosis, analgesic management, anxiety or depression. By maintaining equal proportions of men and women in each age group, the possible contribution of gender to the pattern of results has also been reduced. 4.2. Age differences in pain attributions Another possible explanation for the decreased MPQ scores with age may be secondary to age differences in response style. It has been shown that elderly people are more cautious than younger people about labeling experimentally applied stimulation as painful (Clark and Mehl, 1971; Harkins, 1977; Harkins and Chapman, 1976). Furthermore, healthy community-dwelling elderly people may be more reluctant to report painful symptom (Yong et al., 2001). This may be due, in part, to the belief that pain is a normal consequence of aging and therefore not amenable to treatment (Yong et al., 2001). However, age differences in beliefs about pain are less clear among clinical samples. For instance, no age differences in pain

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beliefs were found in a sample of people with arthritis pain (Gagliese and Melzack, 1997c). Among pain clinic patients, Strong et al. (1992) reported no age differences in pain beliefs, but Herda et al. (1994) found that increasing age was associated with a stronger belief that pain is a constant experience. Furthermore, among those referred for multidisciplinary pain management, there do not appear to be age differences in treatment expectations (Harkins and Price, 1992), acceptance, compliance, or drop out rates (Sorkin et al., 1990), which suggests that these older people do not believe that their pain is a natural, to-be-tolerated consequence of aging. Taken together, these data suggest that elderly people seeking treatment for pain may be different from those in the community in important ways which remain to be clarified. Clearly, more research is needed to elucidate the pattern of age differences in pain attributions among patients seen in multidisciplinary pain clinics. Importantly, if there were age differences in pain attributions and willingness to report pain in the present sample, the elderly group may have been more reluctant to endorse the ‘most intense’ adjectives on the MPQ leading to the age differences on this scale. However, it may be assumed that this would also cause the elderly to endorse lower levels of pain on the intensity measures, which the data show is not the case. As such, age-related reluctance to report pain does not appear to explain the present results. 4.3. Age differences in the psychometric properties of the MPQ Another possibility is that the pattern of results is due to changes in the psychometric properties of the MPQ when used with different age groups. Herr and Mobily (1991) have suggested that the MPQ may be too complex and timeconsuming for the elderly. They argue that this group may have difficulty understanding some of the pain descriptors and may be overwhelmed by the large number of choices. These authors do not present data to support this claim. To our knowledge, our analyses are the first direct comparison of the psychometric properties of the MPQ in younger and older people with chronic pain. We found that the latent structure, internal consistency, and pattern of subscale correlations were very similar in the young and elderly groups. These results suggest that the MPQ is as appropriate for use with elderly patients as younger patients. If this were not the case, these measures would have differed between the age groups. While the internal consistency of the total MPQ score was in the good range (Cronbach, 1984), the values for each subscale were lower. This is not surprising for the Miscellaneous Subscale given that it is not intended to measure a uniform component of pain (Melzack, 1975). However, the values for the Sensory and Affective Subscales are lower than has been reported previously (e.g. Graham et al., 1980). It is not clear why we obtained these discrepant results; however, the more important finding is that the values were

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very similar in both age groups. Similarly, although the exact factor structure of the MPQ remains controversial and open to debate (Melzack and Katz, 2001), our data suggest that the structure was the same in both age groups. As such, the MPQ appears to measure the same constructs in the same way in both young and elderly patients. It is noteworthy that the factor structure identified in this study is not the same as that originally proposed (Melzack, 1975). There have been various alternate structures identified for this scale, and this issue is far from resolved (see Melzack and Katz, 2001 for a review). The disparate crossstudy results may be secondary to differences in the analytic procedures employed and the populations studied (Melzack and Katz, 2001). This highlights the need to replicate the present analysis, ideally in a more homogeneous sample. Nonetheless, the results of the present analyses suggest that the lower MPQ scores obtained by the older patients are not due to age differences in the reliability or structure of this scale. Comparison of the MPQ categories and descriptors chosen by young and elderly patients with LBP and arthritis further supports this conclusion. Elderly patients were less likely than young patients to endorse several sensory and miscellaneous categories but the rates with which the affective and evaluative categories were endorsed did not differ between the groups. The lesser impact of age on PRI-A scores is consistent with the lack of association between age and scores on the depression and anxiety scales. Detailed analysis of the actual descriptors within each category which were endorsed suggested that the elderly used fewer words than young people to describe LBP, but that the words they chose most frequently were very similar to those used by younger people. This was also the case for the description of arthritis pain. This replicates our previous findings regarding age differences in the choice of adjectives from the short form of the MPQ when used to describe arthritis pain (Gagliese and Melzack, 1997a). These results further support our conclusion that the decreased MPQ scores are not due to random responding or early fatigue among the elderly. Instead, they appear to adopt a more parsimonious response style but choose the same adjectives as younger people most frequently. If they were choosing words randomly or only from the first few categories before they became too fatigued to continue, we would not expect to see this high level of similarity between age groups in word choice across the entire scale. These results also suggest that the discriminant properties of the MPQ, that is the well-known finding that characteristic constellations of words are chosen to describe different types of pain (Dubuisson and Melzack, 1976), may not be age-related. This possibility, however, requires formal testing. Taken together, these results suggest that the psychometric properties of the MPQ do not differ between young and elderly patients. As such, the age-related decline in scores on this measure is not due to changes in its reliability or validity with age.

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4.4. Age differences in pain language Another possibility is that the age differences on the MPQ may reflect age differences in the use of pain language, or language more generally, rather than reflecting a true difference in pain experience. Age differences in verbal abilities have been documented (Kemper and Sumner, 2001). However, these differences seem to be confined to discourse generation, verbal fluency (the generation of words within a given category in a set period of time, e.g. naming fruits), and word recognition (indicating whether a given word has been presented previously) or recall (generating lists of previously presented words) (Kemper and Sumner, 2001). None of these tasks parallels the cognitive demands of the MPQ. Specifically, when completing the MPQ, the patient is presented with all of the descriptors and selects those that he/she feels are appropriate to describe their pain. Within each category, a more fine-grained analysis to select the best descriptor is carried out. For instance, the patient first decides that the pain has a thermal quality and then must choose among various gradations (is it hot or burning or searing?). Unlike many formal tests of language abilities, there is no time limit on the completion of the MPQ nor are there right or wrong answers. We were unable to find studies reporting on age differences in a comparable but not pain-related descriptive task. This work would be an important contribution to our understanding of the effects of aging on verbal skills. The issue of pain language in the elderly has not received empirical attention. It would be very interesting to test for age differences in pain language tasks such as those employed by Reading et al. (1982) or Pearce and Morley (1989). Given the paucity of data regarding age differences in descriptor choice and pain language more specifically, it is difficult to entirely rule out this possible explanation of the age differences on the MPQ. However, this explanation is not likely given that the psychometric properties of the MPQ did not differ between the age groups suggesting that both young and older patients were using the scale in the same way. As well, the most frequently chosen adjectives were very similar across age groups (Gagliese and Melzack, 1997a; Gagliese and Melzack, submitted for publication) suggesting that the differences are not secondary to age differences in verbal ability. Although age differences in comprehension of the MPQ adjectives have yet to be tested, there is no compelling reason to believe that older people obtain lower scores on the MPQ because of age differences in pain language abilities. 4.5. Age differences in the qualities but not the intensity of pain Given the evidence that both the NRS (Gagliese, 2001) and the MPQ are valid and reliable measures of pain across the adult life span, the discrepancy in the age differences

on these scales may have implications for our understanding of basic pain mechanisms. It is now widely accepted that pain is a multidimensional experience made up of sensory, affective, and evaluative dimensions (Melzack and Casey, 1968), which are measured by the MPQ (Melzack, 1975). Melzack and Casey (1968) proposed that the different dimensions of pain may be subserved by different but interacting physiological substrates which are distributed throughout the central nervous system. Evidence from brain imaging studies supports this conceptualization (Casey, 1999) and has identified a network of multiple, distributed brain regions which interact in the creation of pain (Casey, 1999; Wiech et al., 2000). Perhaps, the pattern of age differences in the qualities of pain reflects a differential effect of age on the integrity or activity levels of these systems. There is evidence that age-related changes in the neurobiological substrates of pain are not uniform throughout the central nervous system (see review by Gagliese and Melzack, 2000). Furthermore, normal age-related changes in the brain, including neuronal loss and other degenerative changes are also not uniform throughout the brain (Anderton, 2002). Unfortunately, studies of age differences in brain imaging of pain have not been reported. However, it is possible that normal age-related changes of the central nervous system coupled with those due to reorganization secondary to chronic pain (Wiech et al., 2000) may lead to a change in the quality but not the intensity of pain with age. While the various components of pain are highly related and interdependent, there is some evidence that they can be dissociated (Rainville et al., 1997; Tolle et al., 1999). As such, it is possible that nonuniform changes in the brain with age (e.g. greater neuronal loss in cortical than non-cortical regions; Anderton, 2002) also could contribute to such a dissociation. The implications of this pattern of age differences for basic pain mechanisms and pain management should be given serious empirical attention. In summary, evidence has been presented that the MPQ is appropriate for use with elderly samples and that aging may be associated with a change in the quality but not the intensity of chronic pain. The same pattern is found across samples of people with chronic or acute pain and across settings including pain clinics, surgical wards, and community-dwelling adults (Gagliese and Katz, in press; Gagliese and Melzack, 1997a). The same pattern emerges whether pain intensity is assessed with the VAS, the NRS, or the Behavioral Rating Scale (Gagliese and Katz, in press; Gagliese and Melzack, 1997a). The results are also consistent whether the full or short form MPQ is used to assess the qualities of chronic pain (Gagliese and Katz, in press; Gagliese and Melzack, 1997a). The changes appear to be more robust in the sensory than the affective dimension of pain. Reasons why the sensory qualities but not the intensity of pain may change with age have been proposed and should be studied further.

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Acknowledgements This work was supported by a CIHR Studentship and Fellowship Award and a University of Toronto, Faculty of Medicine Dean’s Fund Grant to L.G. and National Science and Engineering Research Grant A7891 to R.M.

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