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What Definition Is Used to Describe Second
Impact Syndrome in Sports? A Systematic and
Critical Review
Steven D. Stovitz, MD, MS, FACSM
1
; Jonathan D. Weseman
2
; Matthew C. Hooks
2
;
Robert J. Schmidt
2
; Jonathan B. Koffel
3
; and Jon S. Patricios, MD, FACSM
4,5
Abstract
Concern about what has been termed, ‘‘second impact syndrome’’ (SIS)
is a major factor determining return-to-play decisions after concussion.
However, definitions of SIS vary. We used Scopus to conduct a sys-
tematic review and categorize the definitions used to describe SIS. Of
the 91 sources identified, 79 (87%) clearly specified that SIS involved
either cerebral edema or death after a concussion when a prior concus-
sion had not resolved. Twelve articles (13%) could be interpreted as
merely the events of two consecutive concussions. Among the articles
that listed mortality rates, nearly all (33/35, 94%) said the rate of death
was ‘‘high’’ (e.g., 50% to 100%). Our review found that most articles
define SIS as a syndrome requiring catastrophic brain injury after con-
secutive concussive episodes. Given that it is unclear how common it is
to have a second concussion while not fully recovered from a first con-
cussion, the actual mortality rate of SIS is unknown.
‘‘Most controversies would soon be ended, if those
engaged in them would first accurately define their
terms, and then rigidly adhere to their definitions.’’
Tryon Edwards, American theologian (1809Y1894)(28)
Introduction
Concussions, generally described as a traumatically in-
duced disturbance of brain function involving a complex
pathophysiological process, are a major concern in a num-
ber of contact and collision sports and
have assumed increasing prominence in
sports medicine. Researchers from the
CDC estimate that between 1.6 and 3.8
million sports related concussions occur
intheUnitedStateseachyear(52).There
is worry about both acute and chronic
ramifications of concussions, especially
if repetitive (37). Those involved in ath-
lete care must decide when athletes may
return to play after a concussion. Among
the factors influencing this decision is the
desire to avoid ‘‘second impact syn-
drome’’ (SIS), a poorly defined term that
is not universally accepted.
SIS is a controversial issue with some
questioning its existence in sports-related
injuries (63,65). The mechanisms are
theoretical. The purpose of this article is not to rehash the
debate on its existence nor the theoretical mechanisms. Rather,
we are interested in reviewing the definitions that have ap-
peared in the peer-reviewed literature when discussing the
possible syndrome.
The impetus for this study came after one of the authors
(S.D.S.), a parent of youth soccer players, attended a con-
cussion education program before the players received
preseason baseline neurocognitive testing as part of a con-
cussion management system. The parents were told that
part of the reason for the neurocognitive testing was to
prevent SIS. The instructor informed the parents that SIS
was defined as a concussion suffered by an individual who
has had a recent concussion that has not completely re-
solved. The instructor went on to say that the mortality rate
associated with SIS was ‘‘about 50%.’’ Similar references to
SIS regularly occur in both the lay and medical literature,
often used as a warning to avoid contact after a sports-
related concussion (61).
Depending on the definition of SIS, the risk and ramifi-
cations may vary widely, affecting personal and public
health decisions. How likely is it that an athlete who is still
recovering from a previous concussion suffers a second
SPECIAL COMMUNICATION
50 Volume 16 &Number 1 &January/February 2017 Special Communication
1
Department of Family Medicine and Community Health, University of
Minnesota, Minneapolis, MN;
2
University of Minnesota Medical School,
Minneapolis, MN;
3
Bio-Medical Library, University of Minnesota, Minneapolis,
MN;
4
Section of Sports Medicine, Faculty of Health Sciences, University of
Pretoria, Pretoria, South Africa; and
5
Department of Emergency Medicine,
Faculty of Health Sciences, University of the Witwatersrand, Johannesburg,
South Africa
Address for correspondence: Steven D. Stovitz, MD, MS, FACSM,
University of Minnesota, Department of Family Medicine and Community
Health, Minneapolis, MN; E-mail: stovitz@umn.edu.
1537-890X/1601/50Y55
Current Sports Medicine Reports
Copyright *2017 by the American College of Sports Medicine
Copyright © 2016 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.
concussion? The answer is unknown, but probably occurs
with some regularity. Consider that concussion is a common
sports-related injury (recently estimated at 13.2% of all
sports injuries in 20 different high school sports) (58), often
unreported (estimates are that as many of 50% of concus-
sions are not reported) (62), and many (10% to 20%) last
beyond a week (57). Thus, it is likely that many youth sport
participants experience a concussion when a prior concus-
sion has not resolved. Because concussions are common,
and second concussions while still symptomatic likely occur
with some regularity, it is incomprehensible that a concus-
sion atop of an unresolved primary concussion results in
death ‘‘50% of the time.’’ The primary purpose of this study
was to systematically review the literature that includes a
definition of sports-related SIS with the goal of more clearly
defining the concept. Secondarily, we reviewed the sources
for the mortality rates cited by the studies.
Methods
To identify articles that defined SIS, our author team for-
mulated the literature search parameters. Our biomedical li-
brarian (J.K.) searched Scopus on December 29, 2014 (Scopus
is an interface that allows searches of both Medline, 1946-
December 29, 2014 and Embase, 1946-December 29, 2014),
to identify all articles that mentioned, ‘‘second impact
syndrome’’ or ‘‘recurrent traumatic brain injury’’ in the title
or abstract, or were indexed as containing MeSH or Embase
terms on the concept ‘‘second impact syndrome.’’ No limits
were applied to any searches and both keywords and
indexing terms (when available) were used. The complete
search strategy for Scopus was TITLE-ABS-KEY(Wsecond
impact syndromeW) OR TITLE-ABS-KEY(Wrecurrent trau-
matic brain injuryW). Three additional authors (J.W., R.S.,
M.H.) examined reference lists of included articles, articles
citing included articles and review articles to identify addi-
tional articles and books, which were then screened for in-
clusion. Conference abstracts were not reviewed.
Articles and books were included if they used the term
‘‘second impact syndrome’’ and provided what seemed to be a
definition or cited a definition from another source. To decide
on what the articles used as a definition, articles were searched
for the letters ‘‘defin’’ to capture the words ‘‘define,’’ ‘‘defined’’
and ‘‘definition.’’ If none of these terms were present, our re-
viewers made best estimates based upon other terminology,
for example, ‘‘second impact syndrome isI’’. Only English-
language sources were included. All reports of SIS were eligi-
ble regardless of patient age. Definitions in sources unrelated
to sports or in journalistic/newspaper sources were excluded.
All article screening and data extraction were performed in-
dividually by three authors and disagreements resolved by
consensus or examination by a fourth author.
Definitions and mortality rates were extracted from each
article and categorized to measure the prevalence of differ-
ent definitions and reported mortality rates.
Results
The literature searches identified 144 articles with an ad-
ditional eight articles and one textbook identified through
examination of references or citing articles (Fig.). Ninety-one
articles met inclusion criteria.
Definition of SIS
As seen in the Table, of the 91 sources, 80 (88%) clearly
specified that SIS involved either cerebral edema or death
after a second concussion when a prior concussion had not
resolved. As an example, Schunk and Schutzman (82)
wrote, ‘‘Second-impact syndrome refers to a very rare, but
usually fatal diffuse cerebral edema as a consequence of a
mild head injury. This term is applied typically when an
athlete develops diffuse cerebral edema from a second head
injury while still symptomatic from a first concussion.’’ Of
note, only a few studies noted that the concussions must
occur within a certain time period, generally in the range of
a few weeks’’ (26,33,85,97).
Twelve articles (13%) included a statement where the
definition could be interpreted as merely two consecutive
concussions, that is, a concussion in an individual who has
had a prior concussion that has not completely resolved. In
these articles, many had the following statement, ‘‘The sec-
ond impact syndrome has been defined as occurring when an
athlete who has sustained an initial head injury, most often a
concussion, sustains a second injury before symptoms asso-
ciated with the first have fully cleared’’ (64,91,92,95). These
articles generally led back to a 1995 article by Cantu (20)
which, interestingly, did not have the term ‘‘define’’ or any
related term in the description. While these articles contain
the above statement, they all subsequently describe the se-
quela of more serious brain injury (generally, cerebral ede-
ma). Of note, a 1992 article by Cantu (18) presented a
definition that was less ambiguous as follows: ‘‘rapid brain
swelling and herniation following a second head injury.’’
Mortality Rates
Mortality rates were mentioned in 35 of the 91 articles
(38%). The vast majority were review articles repeating
rates cited by case reports. Four manuscripts described these
cases and discussed causes of death (20,40,69,95). Among
the articles that listed mortality rates, nearly all (33/35,
94%) said the rate of death was either ‘‘high,’’ ‘‘about
50%,’’ ‘‘50% to 100%,’’ or ‘‘100%.’’
Discussion
The main purpose of this study was to describe the defi-
nition of SIS by systematically reviewing the peer-reviewed
literature. Secondarily, we reported the mortality rates listed
within these articles. Most articles defined SIS as a syn-
drome requiring catastrophic brain injury (e.g., cerebral
edema) in a person who suffers a head trauma while still
recovering from the effects of a recent concussion. There
were a small number of articles that could be interpreted as
merely involving a second concussion when a first concus-
sion had not resolved.
How SIS Is Defined: Historical Origins and Controversy
Our review found an interesting sequence of interpretations
that may provide insight into some of the discrepancy and
confusion. The term, ‘‘second impact syndrome’’ can be traced
back to 1984 when Saunders and Harbaugh wrote an article
entitled, ‘‘The Second Impact in Catastrophic Contact-Sports
Head Trauma’’ (81). The article described a case report of a
footballplayerwhodied4daftersufferingaheadinjury.He
had returned to play on the day of death, whereby he collapsed
www.acsm-csmr.org Current Sports Medicine Reports 51
Copyright © 2016 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.
and died after a presumed second head injury. Although they
never used the exact phrase, ‘‘second impact syndrome,’’ Cantu
in 1992 (18) and subsequently in several other articles
(3,14,15,19) attributed to Saunders and Harbaugh the phrase,
‘‘the second impact syndrome of catastrophic head injury.’’
Subsequently, others removed the phrase ‘‘of catastrophic
head injury,’’ and shortened the term to merely ‘‘second im-
pact syndrome,’’ occasionally without immediate mention
that the second head injury causes a catastrophic outcome.
Why might someone interpret the definition of SIS to be
merely the events of two concussions without the require-
ment of significant brain damage? While our review found
this interpretation to be in the minority, a close read of these
articles reveals how one could come to this conclusion.
Consider the following statement taken from a 2005 review
article, ‘‘The second impact syndrome has been defined as a
sustained head injury after an initial head injury, usually a
concussion, where symptoms associated with the first injury
have not fully cleared’’ (92). The following sentence states,
‘‘It has been postulated that this second impact leads to
rapid development of cerebral vascular congestion and in-
creased intracranial pressure, resulting in brainstem herni-
ation and death’’ (92). A literal interpretation of the first
sentence without consideration of the next sentence might
lead one to believe that it is merely the events of two con-
secutive concussions.
However, definitions often require context and interpre-
tation beyond a single sentence. In addition, there may be
some misunderstanding of the word, syndrome. In medi-
cine, a ‘‘syndrome,’’ defines an outcome when a number of
signs and symptoms occur together (e.g., ‘‘patellofemoral
syndrome’’ or ‘‘shoulder impingement syndrome’’). This is
consistent with the definitions of SIS that involved cata-
strophic brain injury (e.g., cerebral edema) with or without
death after a second concussion.
While trying to settle on a definition of SIS, it is essential
to understand that some question the existence of a unique
syndrome of second impacts as any concussion can result in
brain edema and death (63). In 1998, McCrory and
Berkovic reviewed 17 published cases attributed to SIS.
However, there was no evidence of a second impact in the
majority of the cases (65).
Death Associated With SIS
What about the rate of death associated with SIS? The
high death rates come from case reports of severe outcomes.
For example, in 1995, Cantu (20) described six case reports
of death (one from ice hockey and five from boxing) that
Figure: PRISMA flow diagram.
Table.
Categorization of articles defining ‘‘second impact syndrome.’’
A second concussion
a
(n = 12) (20,35,101),(64)D,(63)D,(95)D,(65)D,(91)D,(92)D,(51)D,(72)D,(15)
A second concussion
a
with
edema +/jdeath (n = 71)
(30,33,66), (78), (25), (7,19), (84), (93), (45), (26), (90), (87), (98), (89), (82), (75), (1), (83),
(44), (99), (73), (11), (41), (86), (22), (85), (31), (34), (4), (40), (94), (55), (59), (17), (29),
(12), (2), (32), (100), (10), (23), (16), (97), (47), (5), (36), (48), (71), (21), (77),(8)D, (49),
(56), (27), (70), (46), (68,80), (96), (69), (42),(76)D, (43), (3,6,14,18,38,60,79)
A second concussion
a
resulting in
death (n = 8)
(88),(50)D, (24), (67), (54), (74)D, (53), (13)
a
The second concussion occurs when a first concussion has not resolved.
‘‘D’’ signifies that the word ‘‘define’’ or ‘‘definition’’ was specifically used for SIS.
52 Volume 16 &Number 1 &January/February 2017 Special Communication
Copyright © 2016 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.
apparently resulted from minor head trauma in athletes
who may have had symptoms from a previous concussion.
Thus, a death rate of 100%. In 2006, Mori reviewed pub-
lished papers with reports of ‘‘severe sports related head
injury’’ (69). There were eight cases with CT scans. All eight
had cerebral edema and two died; a 25% death rate. Of
interest, in Jordan’s report of 316 cases of traumatic brain
injury in boxers, there was one death (40).
If one were to follow the articles that define it as ‘‘death’’
after two consecutive concussions, then, by definition the
rate of death would be 100%. Taking the most common
definition with the outcome being cerebral edema with or
without death, the death rate is unknown, and likely un-
knowable. To our knowledge, no one has ever tracked
random cases of cerebral edema after single or consecutive
concussions. When 2 impacts occur in very close succession
(during the same game or within a few days), it is impossible
to determine whether it was the initial injury that may have
given rise to the catastrophic consequences.
The Importance of an Accurate Definition
This issue would not be important if it were not affecting
decision making in the field of sports medicine. However, it
is. The vignette that led to this review is likely not an iso-
lated incident. A public advertisement from a major sports
medicine unit has a headline that states, ‘‘A second hit could
be the last.’’ It then states, ‘‘Second impact syndrome, when
someone incurs a second concussion while still recovering
from the initial injury, may result in rapid brain swelling
that’s often fatal’’ (61). Advertisements like this instill fear
in the public and concussion testing has assumed a high
profile. If occult brain injury puts a child at risk for death
should further injury occur, and if neurocognitive testing
can detect occult brain injury, then it follows that demand
for more frequent and advanced methods of testing will
continue to grow.
There is further concern that repetitive concussions may
lead to chronic traumatic encephalopathy (CTE) (66), an
irreversible brain injury, although there are others who be-
lieve that the epidemiology does not clearly establish the
concussions as the causative factor (39). There is evidence
from both clinical studies and animal models that head in-
jury in a concussed brain may prolong symptoms and
neurocognitive recovery (9,37). As summarized by Harmon
et al. (37), a concussion decreases cognitive ability and re-
action time, theoretically increasing the risk of subsequent
injury, including another concussion.
Strengths and Limitations
Our librarian-designed search allowed us to systemati-
cally locate scholarly articles on SIS. Like all searches, there
is the potential for missing articles. Articles that did not
have ‘‘second impact syndrome’’ in either the title, abstract
or indexing terms may have not been located. We sought to
include articles picked up by references of our search to
minimize missed articles. Another limitation is the impre-
cision of definitions within the articles and difficulty deter-
mining which elements were stated and which were merely
implied. In an effort to minimize bias, all retrieved articles
were independently screened and had their definitions
extracted by multiple reviewers.
Conclusion
Our systematic review on the definition of SIS found that
the vast majority of articles define SIS as a syndrome re-
quiring catastrophic brain injury (e.g., cerebral edema) in a
person who suffers a head trauma while still recovering
from the effects of a recent concussion. The definition does
not include diffuse cerebral edema resulting from a single
significant impact. It is unlikely that we will ever know the
true mortality rate from SIS because that would entail
knowing an estimate of the number of concussed partici-
pants who developed cerebral edema and who had been
playing with an unresolved previous concussion. There are
many reasons that parents, coaches, and health care pro-
viders should seek to reduce the risk of sports-related con-
cussions, and disallow a concussed athlete to return to play
in a sport with a high risk of head injury. It is not necessary
to instill unsubstantiated fear. We must align public health
messages with the proper definition and, to the best of our
ability, the actual rate of serious consequences.
The authors declare no conflict of interest and do not
have any financial disclosures.
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