Table 3

Concussion assessment and outcome measurements among American football players (US and Canadian) in the included studies

Ref.Study designMethod of assessing concussion and its definitionMeasurement of outcome(s)Prevalence of outcomeInternal comparisonBetween sport comparisonExternal comparison
Baker JG, et al 201824 Cross-sectional study with between sports comparison group (non-contact master athletes)

  • Information on concussion not collected because deemed to be too inaccurate

  • Neurocognitive tests

  • Mild cognitive impairment as defined by Jak and coll.38

  • Depression

  • Vascular risk factors

 Prevalence of cognitive impairment as defined by Jak and coll.38 among contact sport athletes: 38%

  • The contact sport athletes scored significantly lower on this measure of estimated IQ, but did not significantly differ from non-contact sport athletes on most of the primary scores in the five domains (executive function, attention, memory, language, perceptual motor skills). They only performed worse in letter fluency and immediate recall

  • Eight contact sport athletes (38%) versus three non-contact controls (14%) met the criteria for cognitive impairment

Misiquitta K et al, 201833 Cross-sectional study with two external comparison groups (‘population’ controls and controls from the Cambridge Centre for Ageing and Neuroscience)

  • Self-reported

  • Concussion defined according to the Zurich consensus statement62

Median number of concussion among former CFL players (IQR): 4 (3–8.5)

  • Neurocognitive tests

  • Neuroimaging

  • Personality Assessment Inventory

  • No difference in neurocognitive tests between former footballers and study controls

Alosco et al, 201722 Cross-sectional study with external comparison groupConcussion not measured

  • Neurocognitive tests

  • Neuroimaging

  • Olfactory function

  • Depression

  • Behaviour and mood

  • NFL players exhibited significantly worse Performance across most of the cognitive and behavioural/mood measures (attention, executive function, psychomotor speed, visual and verbal episodic memory, language, motor and visuospatial functions)

Multani et al. 201634 Cross-sectional with external comparison group

  • Self-report

Definition: ‘injuries caused by a blow to the head or body that resulted in concussion symptoms, including at least one of the following: headache, nausea, vomiting, dizziness/balance problems, fatigue, trouble sleeping, drowsiness, sensitivity to light or noise, blurred vision, difficulty remembering, and trouble concentrating’61
Prevalence of concussion impossible to calculate due to study design (concussion is an inclusion criterion)

  • Neurocognitive tests

  • Neuroimaging

  • No difference in Visuospatial learning and memory, or premorbid intellectual functioning between former players and healthy controls

Hart et al. 201330 Cross-sectional with external comparison group

  • Concussion history was obtained retrospectively from participants and informants

Definition of the 1997 American Academy of Neurology practice parameter guidelines for grading concussion: ‘trauma-induced alteration in mental status that may or may not involve loss of consciousness. Confusion and amnesia are the hallmarks of concussion’63
Prevalence of concussion: 94%
Mean concussions: 4 (range 1–13)

  • Neurocognitive tests

  • Neuroimaging

  • Depression

 Research doctor diagnosis:

  • Prevalence of fixed cognitive deficit: 4/34 to 12%

  • Prevalence of cognitive impairment: 8/34 to 24%

  • Prevalence of dementia: 6/34 to 6%

  • No difference between neuropsychological measures and concussion or the number of years played in the NFL

Randolph et al., 201335 Cross-sectional with two external comparison groups (one of patients diagnosed with MCI) and population norms Exposure to concussion not assessed

  • Neurocognitive tests

Prevalence of cognitive impairment as defined by AD8 (39): 35%

  • Among players with cognitive impairment (score 2+on AD8), length of career was not associated with cognitive test performance (r=0.016)

Amen et al. 201123 Cross-sectional with external comparison group

  • Self-reported at interview

  • Definition: Centre for Diseases Control and Prevention (CDC) definition of concussions: ‘conditions of temporarily altered mental status as a result of head trauma’ that may or may not involve a loss of consciousness64

Prevalence of episodes of ‘loss of consciousness’: 63%

  • Neurocognitive tests

  • Overall general and mental health (DSM-IV)

  • Cognitive Impairment as defined by the Mild Cognitive Impairment Screen40

Prevalence of cognitive impairment as defined by the Mild Cognitive Impairment Screen 40 : 19% (increasing with increasing age)

  • Players scored in the bottom half of the percentile placements on all cognitive measures except spatial processing and reaction-time, which were both in the top half of the percentile placements compared with population norms

Guskiewicz et al 200529 Cross-sectional with external comparison group

  • Self-reported by questionnaire

  • Definition: ‘injury resulting from a blow to the head that caused an alteration in mental status and one or more of the following symptoms: headache, nausea, vomiting, dizziness/balance problems, fatigue, trouble sleeping, drowsiness, sensitivity to light or noise, blurred vision, difficulty remembering, and difficulty concentrating’

Prevalence of concussion: 61%

  • Mental Component Score (MCS) of SF-36

  • Self- or spouse- reported MCI defined according to the American Academy of Neurology Practice Parameter

 Prevalence of self-reported doctor diagnosed AD: 1.3%
AD age-adjusted prevalence ratio 1.37 (95% CI 0.98 to 1.56) in former footballers compared with the general population
Prevalence of self-reported doctor-diagnosed MCI:3%
Prevalence of spouse reported cognitive impairment: 12%

  • Retired players with a history of concussion, especially recurrent concussion, scored lower (worse) on the MCS than those without a history of recurrent concussion (p<0.001).

  • Recurrent concussion significantly associated with MCI diagnosis in athlete population (p=0.002), self-report memory impairment (p=0.001), and spouse/relative reported memory impairment (p=0.04)

  • Dose–response relationship between number of concussion and memory impairment

  • MCS scores on the SF-36 were similar between the NFL retirees and population-based normative values for all age groups

  • AD, Alzheimer disease; CFL, Canadian Football League; DSM-IV, Diagnostic and Statistical Manual of Mental disorders – IV revision; MCI, Mild Cognitive impairment; NFL, National Football League; SF-36, Short form Health survey; SPECT, single-photon emission CT.