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Cognitive functioning in chronic fatigue syndrome: a meta-analysis

Published online by Cambridge University Press:  05 January 2010

S. J. Cockshell  and
J. L. Mathias
S. J. Cockshell
Affiliation:
School of Psychology, The University of Adelaide, South Australia, Australia
J. L. Mathias*
Affiliation:
School of Psychology, The University of Adelaide, South Australia, Australia
*
*Address for correspondence: Dr J. L. Mathias, School of Psychology, The University of Adelaide, Adelaide SA 5005, Australia (Email: [email protected])
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Abstract

Background

Cognitive problems are commonly reported in persons with chronic fatigue syndrome (CFS) and are one of the most disabling symptoms of this condition. A number of cognitive deficits have been identified, although the findings are inconsistent and hindered by methodological differences. The current study therefore conducted a meta-analysis of research examining cognitive functioning in persons with CFS in order to identify the pattern and magnitude of any deficits that are associated with this condition.

Method

A comprehensive search of the PubMed and PsycINFO databases for studies that examined cognitive functioning in CFS between 1988 and 2008 identified 50 eligible studies. Weighted Cohen's d effect sizes, 95% confidence intervals and fail-safe Ns were calculated for each cognitive score.

Results

Evidence of cognitive deficits in persons with CFS was found primarily in the domains of attention, memory and reaction time. Deficits were not apparent on tests of fine motor speed, vocabulary, reasoning and global functioning.

Conclusions

Persons with CFS demonstrate moderate to large impairments in simple and complex information processing speed and in tasks requiring working memory over a sustained period of time.

Keywords

Chronic fatigue syndromecognitivemeta-analysisreview
Type
Review Article
Information
Psychological Medicine , Volume 40 , Issue 8 , August 2010 , pp. 1253 - 1267
Copyright
Copyright © Cambridge University Press 2010

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References

Abbey, SE, Garfinkel, PE (1991). Chronic fatigue syndrome and depression: cause, effect or covariate. Reviews of Infectious Disease 13, S73–S83.CrossRefGoogle ScholarPubMed
Altay, HT, Toner, BB, Brooker, H, Abbey, SE, Salit, IE, Garfinkel, PE (1990). The neuropsychological dimensions of postinfectious neuromyasthenia (chronic fatigue syndrome): a preliminary report. International Journal of Psychiatry in Medicine 20, 141–149.CrossRefGoogle ScholarPubMed
*Blackwood, SK, MacHale, SM, Power, MJ, Goodwin, GM, Lawrie, SM (1998). Effects of exercise on cognitive and motor function in chronic fatigue syndrome and depression. Journal of Neurology, Neurosurgery, and Psychiatry 64, 541–546.CrossRefGoogle Scholar
*Busichio, K, Tiersky, LA, DeLuca, J, Natelson, BH (2004). Neuropsychological deficits in patients with chronic fatigue syndrome. Journal of the International Neuropsychological Society 10, 278–285.CrossRefGoogle ScholarPubMed
*Capuron, L, Welberg, L, Heim, C, Wagner, D, Solomon, L, Papanicolaou, DA, Craddock, RC, Miller, AH, Reeves, WC (2006). Cognitive dysfunction relates to subjective report of mental fatigue in patients with chronic fatigue syndrome. Neuropsychopharmacology 31, 1777–1784.CrossRefGoogle ScholarPubMed
*Caseras, X, Mataix-Cols, D, Giampietro, V, Rimes, KA, Brammer, M, Zelaya, F, Chalder, T, Godfrey, EL (2006). Probing the working memory system in chronic fatigue syndrome: a functional magnetic resonance imaging study using the n-back task. Psychosomatic Medicine 68, 947–955.CrossRefGoogle ScholarPubMed
*Chiaravalloti, ND, Christodoulou, C, Demaree, HA, DeLuca, J (2003). Differentiating simple versus complex processing speed: influence on new learning and memory performance. Journal of Clinical and Experimental Neuropsychology 25, 489–501.CrossRefGoogle ScholarPubMed
Christodoulou, C, DeLuca, J, Lange, G, Johnson, SK, Sisto, SA, Korn, L, Natelson, BH (1998). Relation between neuropsychological impairment and functional disability in patients with chronic fatigue syndrome. Journal of Neurology, Neurosurgery, and Psychiatry 64, 431–434.CrossRefGoogle ScholarPubMed
*Claypoole, K, Mahurin, R, Fischer, ME, Goldberg, J, Schmaling, KB, Schoene, RB, Ashton, S, Buchwald, D (2001). Cognitive compromise following exercise in monozygotic twins discordant for chronic fatigue syndrome: fact or artifact? Applied Neuropsychology 8, 31–40.CrossRefGoogle ScholarPubMed
*Claypoole, KH, Noonan, C, Mahurin, RK, Goldberg, J, Erickson, T, Buchwald, D (2007). A twin study of cognitive function in chronic fatigue syndrome: the effects of sudden illness onset. Neuropsychology 21, 507–513.CrossRefGoogle ScholarPubMed
Cohen, J (1988). Statistical Power Analysis for the Behavioural Sciences. Academic Press: New York.Google Scholar
Cooper, HM (1979). Statistically combining independent studies: a meta-analysis of sex differences in conformity research. Journal of Personality and Social Psychology 37, 131–146.CrossRefGoogle Scholar
*Creswell, C, Chalder, T (2002). Underlying self-esteem in chronic fatigue syndrome. Journal of Psychosomatic Research 53, 755–761.CrossRefGoogle ScholarPubMed
*Davey, NJ, Purib, BK, Nowickyc, AV, Maind, J, Zaman, R (2001). Voluntary motor function in patients with chronic fatigue syndrome. Journal of Psychosomatic Research 50, 17–20.CrossRefGoogle ScholarPubMed
de Lange, FP, Koers, A, Kalkman, JS, Bleijenberg, G, Hagoort, P, van der Meer, JWM, Toni, I (2008). Increase in prefrontal cortical volume following cognitive behavioural therapy in patients with chronic fatigue syndrome. Brain 131, 2172–2180.CrossRefGoogle ScholarPubMed
*DeLuca, J, Christodoulou, C, Diamond, BJ, Rosenstein, ED, Kramer, N, Natelson, BH (2004 a). Working memory deficits in chronic fatigue syndrome: differentiating between speed and accuracy of information processing. Journal of the International Neuropsychological Society 10, 101–109.CrossRefGoogle ScholarPubMed
*DeLuca, J, Christodoulou, C, Diamond, BJ, Rosenstein, ED, Kramer, N, Ricker, JH, Natelson, BH (2004 b). The nature of memory impairment in chronic fatigue syndrome. Rehabilitation Psychology 49, 62–70.CrossRefGoogle Scholar
*DeLuca, J, Johnson, SK, Beldowicz, D, Natelson, BH (1995). Neuropsychological impairments in chronic fatigue syndrome, multiple sclerosis, and depression. Journal of Neurology, Neurosurgery, and Psychiatry 58, 38–43.CrossRefGoogle ScholarPubMed
*DeLuca, J, Johnson, SK, Ellis, SP, Natelson, BH (1997 a). Cognitive functioning is impaired in patients with chronic fatigue syndrome devoid of psychiatric disease. Journal of Neurology, Neurosurgery, and Psychiatry 62, 151–155.CrossRefGoogle ScholarPubMed
*DeLuca, J, Johnson, SK, Ellis, SP, Natelson, BH (1997 b). Sudden vs gradual onset of chronic fatigue syndrome differentiates individuals on cognitive and psychiatric measures. Journal of Psychiatric Research 31, 83–90.CrossRefGoogle ScholarPubMed
*DeLuca, J, Johnson, SK, Natelson, BH (1993). Information processing efficiency in chronic fatigue syndrome and multiple sclerosis. Archives of Neurology 50, 301–304.CrossRefGoogle ScholarPubMed
*DiClementi, JD, Schmaling, KB, Jones, JF (2001). Information processing in chronic fatigue syndrome: A preliminary investigation of suggestibility. Journal of Psychosomatic Research 51, 679–686.CrossRefGoogle ScholarPubMed
*Dobbs, BM, Dobbs, AR, Kiss, I (2001). Working memory deficits associated with chronic fatigue syndrome. Journal of the International Neuropsychological Society 7, 285–293.CrossRefGoogle ScholarPubMed
Easterbrook, PJ, Berlin, JA, Gopalan, R, Matthews, DR (1991). Publication bias in clinical research. Lancet 337, 867–872.CrossRefGoogle ScholarPubMed
Farmer, A, Chubb, H, Jones, I, Hillier, J, Smith, A, Borysiewicz, L (1996). Screening for psychiatric morbidity in subjects presenting with chronic fatigue syndrome. British Journal of Psychiatry 168, 354–358.CrossRefGoogle ScholarPubMed
*Fiedler, N, Kipen, HM, DeLuca, J, Kelly-McNeil, K, Natelson, B (1996). A controlled comparison of multiple chemical sensitivities and chronic fatigue syndrome. Psychosomatic Medicine 58, 38–49.CrossRefGoogle ScholarPubMed
*Fuentes, K, Hunter, MA, Strauss, E, Hultsch, DF (2001). Intraindividual variability in cognitive performance in persons with chronic fatigue syndrome. Clinical Neuropsychologist 15, 210–227.CrossRefGoogle ScholarPubMed
Fukuda, K, Straus, SE, Hickie, I, Sharpe, MC, Dobbins, JG, Komaroff, AL, the International Chronic Fatigue Syndrome Study Group (1994). The chronic fatigue syndrome: a comprehensive approach to its definition and study. Annals of Internal Medicine 121, 953–959.CrossRefGoogle ScholarPubMed
*Gaudino, EA, Coyle, PK, Krupp, LB (1997). Post-lyme syndrome and chronic fatigue syndrome: neuropsychiatric similarities and differences. Archives of Neurology 54, 1372–1376.CrossRefGoogle ScholarPubMed
*Grafman, J, Schwartz, V, Dale, JK, Scheffers, M, Houser, C, Straus, SE (1993). Analysis of neuropsychological functioning in patients with chronic fatigue syndrome. Journal of Neurology, Neurosurgery, and Psychiatry 56, 684–689.CrossRefGoogle ScholarPubMed
Hedges, LV (1982). Estimation of effect size from a series of independent experiments. Psychological Bulletin 92, 490–499.CrossRefGoogle Scholar
Henderson, M, Tannock, C (2005). Use of depression rating scales in chronic fatigue syndrome. Journal of Psychosomatic Research 59, 181–4.CrossRefGoogle ScholarPubMed
Higgins, JP, Deeks, JJ (2008). Selecting studies and collecting data, chapter 7. In Cochrane Handbook for Systematic Reviews of Interventions, version 5.0.1 (updated September 2008) (ed. Higgins, J. and Green, S.). The Cochrane Collaboration. (www.cochrane-handbook.org) Accessed 18 December 2008.Google Scholar
Holmes, GP, Kaplan, JE, Gantz, NM, Komaroff, AL, Schonberger, LB, Straus, SE, Jones, JF, Dubois, RE, Cunningham-Rundles, C, Pahwa, S, Tosato, G, Zegans, S, Purtilo, DT, Brown, N, Schooley, RT, Brus, I (1988). Chronic fatigue syndrome: a working case definition. Annals of Internal Medicine 108, 387–389.CrossRefGoogle ScholarPubMed
*Hou, R, Moss-Morris, R, Bradley, BP, Peveler, R, Mogg, K (2008). Attentional bias towards health-threat information in chronic fatigue syndrome. Journal of Psychosomatic Research 65, 47–50.CrossRefGoogle ScholarPubMed
Jason, LA, Richman, JA, Rademaker, AW, Jordan, KM, Plioplys, AV, Taylor, RR, McCready, W, Huang, C-F, Plioplys, S (1999). A community-based study of chronic fatigue syndrome. Archives of Internal Medicine 159, 2129–2137.CrossRefGoogle ScholarPubMed
*Johnson, SK, DeLuca, J, Diamond, BJ, Natelson, BH (1996). Selective impairment of auditory processing in chronic fatigue syndrome: a comparison with multiple sclerosis and healthy controls. Perceptual & Motor Skills 83, 51–62.CrossRefGoogle ScholarPubMed
*Johnson, SK, DeLuca, J, Diamond, BJ, Natelson, BH (1998). Memory dysfunction in fatiguing illness: examining interference and distraction in short-term memory. Cognitive Neuropsychiatry 3, 269–285.CrossRefGoogle Scholar
*Johnson, SK, DeLuca, J, Fiedler, N, Natelson, BJ (1994). Cognitive functioning of patients with chronic fatigue syndrome. Clinical Infectious Diseases 18, S84–S85.CrossRefGoogle ScholarPubMed
*Johnson, SK, Lange, G, DeLuca, J, Korn, LR, Natelson, B (1997). The effects of fatigue on neuropsychological performance in patients with chronic fatigue syndrome, multiple sclerosis, and depression. Applied Neuropsychology 4, 145–153.CrossRefGoogle ScholarPubMed
*Joyce, E, Blumenthal, S, Wessely, S (1996). Memory, attention, and executive function in chronic fatigue syndrome. Journal of Neurology, Neurosurgery, and Psychiatry 60, 495–503.CrossRefGoogle ScholarPubMed
*Kane, RL, Gantz, NM, DiPino, RK (1997). Neuropsychological and psychological functioning in chronic fatigue syndrome. Neuropsychiatry, Neuropsychology, & Behavioral Neurology 10, 25–31.Google ScholarPubMed
*Krupp, LB, Sliwinski, M, Masur, DM, Friedberg, F, Coyle, PK (1994). Cognitive functioning and depression in patients with chronic fatigue syndrome and multiple sclerosis. Archives of Neurology 51, 705–710.CrossRefGoogle ScholarPubMed
*LaManca, JJ, Sisto, SA, DeLuca, J, Johnson, SK, Lange, G, Pareja, J, Cook, S, Natelson, BH (1998). Influence of exhaustive treadmill exercise on cognitive functioning in chronic fatigue syndrome. American Journal of Medicine 105, 59S–65S.CrossRefGoogle ScholarPubMed
*Lawrie, SM, MacHale, SM, Cavanagh, JTO, O'Carroll, RE, Goodwin, GM (2000). The difference in patterns of motor and cognitive function in chronic fatigue syndrome and severe depressive illness. Psychological Medicine 30, 433–442.CrossRefGoogle ScholarPubMed
Lezak, MD, Howieson, DB, Loring, DW (2004). Neuropsychological Assessment. Oxford University Press: Oxford.Google Scholar
Lipsey, MW, Wilson, DB (2001). Practical Meta-analysis. Sage Publications: Thousand Oaks.Google ScholarPubMed
*McCue, P, Scholey, AB, Herman, C, Wesnes, KA (2002). Validation of a telephone cognitive assessment test battery for use in chronic fatigue syndrome. Journal of Telemedicine and Telecare 8, 337–343.CrossRefGoogle ScholarPubMed
McDonald, E, Cope, H, David, A (1993). Cognitive impairment in patients with chronic fatigue: a preliminary study. Journal of Neurology, Neurosurgery, and Psychiatry 56, 812–815.CrossRefGoogle ScholarPubMed
*Mahurin, RK, Claypoole, KH, Goldberg, JH, Arguelles, L, Ashton, S, Buchwald, D (2004). Cognitive processing in monozygotic twins discordant for chronic fatigue syndrome. Neuropsychology 18, 232–239.CrossRefGoogle ScholarPubMed
*Majer, M, Welberg, LAM., Capuron, L, Miller, AH, Pagnoni, G, Reeves, WC (2008). Neuropsychological performance in persons with chronic fatigue syndrome: Results from a population-based study. Psychosomatic Medicine 70, 829–836.CrossRefGoogle ScholarPubMed
*Marcel, B, Komaroff, AL, Fagioli, LR, Kornish, J II, Albert, MS (1996). Cognitive deficits in patients with chronic fatigue syndrome. Biological Psychiatry 40, 535–541.CrossRefGoogle ScholarPubMed
*Marshall, PS, Forstot, M, Callies, A, Peterson, PK, Schenck, CH (1997). Cognitive slowing and working memory difficulties in chronic fatigue syndrome. Psychosomatic Medicine 59, 58–66.CrossRefGoogle ScholarPubMed
Matt, GE, Cook, TD (1994). Threats to the validity of research synthesis. In The Handbook of Research Synthesis (eds. Cooper, H. and Hedges, L. V.), pp. 503–520. Russell Sage Foundation: New York.Google Scholar
*Metzger, F, Denney, DR (2002). Perception of cognitive performance in patients with chronic fatigue syndrome. Annals of Behavioral Medicine 24, 106–112.CrossRefGoogle ScholarPubMed
Michiels, V, Cluydts, R (2001). Neuropsychological functioning in chronic fatigue syndrome: a review. Acta Psychiatrica Scandinavica 103, 84–93.CrossRefGoogle ScholarPubMed
*Michiels, V, Cluydts, R, Fischler, B (1998). Attention and verbal learning in patients with chronic fatigue syndrome. Journal of the International Neuropsychological Society 4, 456–466.CrossRefGoogle ScholarPubMed
*Michiels, V, Cluydts, R, Fischler, B, Hoffmann, G, Le Bon, O, de Meirleir, K (1996). Cognitive functioning in patients with chronic fatigue syndrome. Journal of Clinical and Experimental Neuropsychology 18, 666–677.CrossRefGoogle ScholarPubMed
*Michiels, V, de Gucht, V, Cluydts, R, Fischler, B (1999). Attention and information processing efficiency in patients with chronic fatigue syndrome. Journal of Clinical and Experimental Neuropsychology 21, 709–729.CrossRefGoogle ScholarPubMed
Morriss, K, Wearden, AJ (1998). Screening instruments for psychiatric morbidity in chronic fatigue syndrome. Journal of the Royal Society of Medicine 91, 365–368.CrossRefGoogle ScholarPubMed
*Moss-Morris, R, Petrie, KJ (2003). Experimental evidence for interpretive but not attention biases toward somatic information in patients with chronic fatigue syndrome. British Journal of Health Psychology 8, 195–208.CrossRefGoogle ScholarPubMed
Moss-Morris, R, Petrie, KJ, Large, RG, Kydd, RR (1996). Neuropsychological deficits in chronic fatigue syndrome: artifact or reality? [Editorial]. Journal of Neurology, Neurosurgery, and Psychiatry 60, 474–477.CrossRefGoogle ScholarPubMed
Prasher, D, Smith, A, Findley, L (1990). Sensory and cognitive event related potentials in myalgic encephalomyelitis. Journal of Neurology, Neurosurgery, and Psychiatry 53, 247–253.CrossRefGoogle ScholarPubMed
*Ray, C, Phillips, L, Weir, WRC (1993). Quality of attention in chronic fatigue syndrome: subjective reports of everyday attention and cognitive difficulty, and performance on tasks of focused attention. British Journal of Clinical Psychology 32, 357–364.CrossRefGoogle ScholarPubMed
Reeves, WC, Lloyd, A, Vernon, SD, Klimas, N, Jason, LA, Bleijenberg, G, Evengard, B, White, PD, Nisenbaum, R, Unger, ER, the International CFS Study Group (2003). Identification of ambiguities in the 1994 chronic fatigue syndrome research case definition and recommendations for resolution. BMC Health Services Research, volume 3, issue 25.CrossRefGoogle ScholarPubMed
Riccio, M, Thompson, C, Wilson, B, Morgan, DJR, Lant, AF (1992). Neuropsychological and psychiatric abnormalities in myalgic encephalomyelitis: a preliminary report. British Journal of Clinical Psychology 31, 111–120.CrossRefGoogle ScholarPubMed
Rosenthal, R (1995). Writing meta-analytic reviews. Psychological Bulletin 118, 183–192.CrossRefGoogle Scholar
*Ross, S, Fantie, B, Straus, SF, Grafman, J (2001). Divided attention deficits in patients with chronic fatigue syndrome. Applied Neuropsychology 8, 4–11.CrossRefGoogle ScholarPubMed
*Sargent, C-A, Anderson, S, Budek, M (1997). Psychomotor functioning in chronic fatigue syndrome. Journal of the Chronic Fatigue Syndrome 3, 53–68.CrossRefGoogle Scholar
*Scheffers, MK, Johnson, R, Grafman, J, Dale, JK, Straus, SE (1992). Attention and short-term memory in chronic fatigue syndrome patients: an event-related potential analysis. Neurology 42, 1667–1675.CrossRefGoogle ScholarPubMed
Schluederberg, A, Straus, SE, Peterson, P, Blumenthal, S, Komaroff, AL, Spring, SB, Landay, A, Buchwald, D (1992). Chronic fatigue syndrome: definition and medical outcome assessment. Annals of Internal Medicine 117, 325–331.CrossRefGoogle ScholarPubMed
Schmaling, KB, DiClementi, J, Cullum, M, Jones, JF (1994). Cognitive functioning in chronic fatigue syndrome and depression: a preliminary comparison. Psychosomatic Medicine 56, 383–388.CrossRefGoogle ScholarPubMed
*Schmaling, KB, Lewis, DH, Fiedelak, JI, Mahurin, R, Buchwald, DS (2003). Single-photon emission computerized tomography and neurocognitive function in patients with chronic fatigue syndrome. Psychosomatic Medicine 65, 129–136.CrossRefGoogle ScholarPubMed
Sharpe, MC, Archard, LC, Banatvala, JE, Borysiewicz, LK, Clare, AW, David, A, Edwards, RHT, Hawton, KEH, Lambert, HP, Lane, RJM, McDonald, EM, Mowbray, JF, Pearson, DJ, Peto, TEA, Preedy, VR, Smith, AP, Smith, DG, Taylor, DF, Tyrrell, DAJ, Wessely, S, White, PD (1991). A report – chronic fatigue syndrome: guidelines for research. Journal of the Royal Society of Medicine 84, 118–121.CrossRefGoogle ScholarPubMed
*Short, K, McCabe, M, Tooley, G (2002). Cognitive functioning in chronic fatigue syndrome and the role of depression, anxiety, and fatigue. Journal of Psychosomatic Research 52, 475–483.CrossRefGoogle ScholarPubMed
*Smith, A, Pollock, J, Thomas, M, Llewelyn, M, Borysiewicz, L (1996). The relationship between subjective ratings of sleep and mental functioning in healthy subjects and patients with chronic fatigue syndrome. Human Psychopharmacology 11, 161–167.3.0.CO;2-5>CrossRefGoogle Scholar
Smith, AP, Behan, PO, Bell, W, Millar, K, Bakheit, M (1993). Behavioural problems associated with the chronic fatigue syndrome. British Journal of Psychology 84, 411–423.CrossRefGoogle ScholarPubMed
*Smith, AP, Borysiewicz, L, Pollock, J, Thomas, M, Perry, K, Llewelyn, M (1999). Acute fatigue in chronic fatigue syndrome patients. Psychological Medicine 29, 283–290.CrossRefGoogle ScholarPubMed
Smith, GA, Carew, M (1987). Decision time unmasked: individuals adopt different strategies. Australian Journal of Psychology 39, 339–351.CrossRefGoogle Scholar
*Starr, A, Scalise, A, Gordon, R, Michalewski, HJ, Caramia, MD (2000). Motor cortex excitability in chronic fatigue syndrome. Clinical Neurophysiology 111, 2025–2031.CrossRefGoogle ScholarPubMed
Strauss, E, Sherman, EMS, Spreen, O (2006). A Compendium of Neuropsychological Tests: Administration, Norms, and Commentary. Oxford University Press: Oxford.Google Scholar
Thomas, M, Sadlier, M, Smith, A (2006). The effect of multi convergent therapy on the psychopathology, mood and performance of chronic fatigue syndrome patients: a preliminary study. Counselling & Psychotherapy Research 6, 91–99.CrossRefGoogle Scholar
*Tiersky, LA, Cicerone, KD, Natelson, BH, DeLuca, J (1998). Neuropsychological functioning in chronic fatigue syndrome and mild traumatic brain injury: a comparison. Clinical Neuropsychologist 12, 503–512.CrossRefGoogle Scholar
Tiersky, LA, Johnson, SK, Lange, G, Natelson, BH, DeLuca, J (1997). Neuropsychology of chronic fatigue syndrome: a critical review. Journal of Clinical and Experimental Neuropsychology 19, 560–586.CrossRefGoogle ScholarPubMed
*Tiersky, LA, Matheis, RJ, DeLuca, J, Lange, G, Natelson, BH (2003). Functional status, neuropsychological functioning, and mood in chronic fatigue syndrome (CFS): relationship to psychiatric disorder. Journal of Nervous and Mental Disease 191, 324–331.CrossRefGoogle ScholarPubMed
*Vercoulen, JHMM, Bazelmans, E, Swanink, CMA, Galama, JMD, Fennis, JFM, van der Meer, JWM, Bleijenberg, G (1998). Evaluating neuropsychological impairment in chronic fatigue syndrome. Journal of Clinical and Experimental Neuropsychology 20, 144–156.CrossRefGoogle ScholarPubMed
*Wearden, A, Appleby, L (1997). Cognitive performance and complaints of cognitive impairment in chronic fatigue syndrome (CFS). Psychological Medicine 27, 81–90.CrossRefGoogle ScholarPubMed
Wolf, FM (1986). Meta-Analysis: Quantitative Methods for Research Synthesis. Sage Publications: Beverly Hills.CrossRefGoogle Scholar
Zakzanis, KK (2001). Statistics to tell the truth, the whole truth, and nothing but the truth: formulae, illustrative numerical examples, and heuristic interpretation of effect size analyses for neuropsychological researchers. Archives of Clinical Neuropsychology 16, 653–667.CrossRefGoogle ScholarPubMed