Estimating Premorbid
Intelligence
Determining Change in Cognition Following Brain
Injury
Richard A. Lanham, Jr., PhD and Thomas Misukanis,
PhD
Brain Injury Source, Pediatric Issue, Volume 3, No.
3, Summer 1999
Introduction
Neuropsychology, a
specialty discipline within clinical psychology, is
devoted to the study of brain-behavior
relationships, the assessment of neurobehavioral
impairment and its effects on functioning and
methods of treatment. In cases of traumatic brain
injury (TBI), where an individual is suspected of
having experienced a disruption in cognitive
functioning, a referral for neuropsychological
evaluation often will be made in order to determine
the nature and extent of any cognitive changes that
may have occurred. Such information is valuable for
tracking the course of recovery, guiding treatment
and compensatory strategies and assisting in
discharge planning. This article discusses an
essential part of the neuropsychological evaluation:
the methods and associated problems of estimating
premorbid level of functioning against which
post-injury test results are compared in an effort
to establish the resulting cognitive impairments of
the TBI.
Methods of Evaluation
In evaluating an
individual with a suspected brain injury, the
neuropsychologist makes use of a variety of
psychometric tests. These tests either have been
specifically developed to assess for cerebral
impairment (e.g. Halstead-Reitan Neuropsychological
Test Battery, HRB) or have been routinely employed
in such a manner as to allow for the determination
of deficient cognitive functioning (e.g. Wechsler
Adult Intelligence Scale, now in its third edition).
Without some standard of comparison, however, test
results in and of themselves would be little more
than meaningless numbers. In general, two standards
of comparison are employed: 1) a normative or
extra-individual standard of comparison where an
individual’s performance is compared to that of
others with whom some important attributes are
shared and 2) a self-standard or intra-individual
standard of comparison, where an individual’s
performance is compared to pre-injury expectations
regarding that individual’s level of ability.
Normative Standard
In employing the
normative standard of comparison, an individual’s
test scores are compared to the performance of a
large group of people of interest, permitting score
adjustment for such factors as age, sex or level of
education. These comparisons give the
neuropsychologist information about the individual’s
current cognitive functioning relative to a
particular normative group. For example, knowing
that an individual performed at the 2nd percentile –
thus performing worse than 98% of the sample against
which the score is being compared – may be a strong
indicator of cerebral impairment. The adequacy of
the comparison between the individual being
evaluated and the normative group being employed is
crucial and places limitations on the type of
conclusions that may be drawn and the certainty one
may have in them. For instance, knowing that the
above 2nd percentile performance occurred on the
entrance examination for Mensa and that the
"normative" comparison group was all current and
past Mensa members, might alter the interpretation
of the score as a reflection of brain impairment.
Self-Standard
The second standard
of comparison, the self-standard, compares an
individual’s post-injury performance with some
measure of pre-injury functioning. Ideally, one
would directly compare post-injury test results
against results obtained from those very same tests
administered some time prior to the injury. While
this situation potentially does exist for military
personnel participating in the Defense and Veterans
Head Injury Program (DVHIP), it is a rare occurrence
for pre-injury testing to exist in nonmilitary
settings. Even for ex-military persons, access to
the Armed Services Vocational Aptitude Battery
(ASVAB), the current version of the military
classification test taken by all noncommissioned
personnel upon entry into service, is restricted to
protect the test’s integrity, much like the various
civilian entrance examinations (e.g., SAT, MCAT,
GRE). While the DVHIP is able to administer the
ASVAB, with the permission of the military, six
months post-injury as part of its ongoing clinical
and research program (e.g., Grafman et al., 1988),
in general clinical practice it would not be
available for re-administration.
Most commonly, the
self-standard of comparison involves estimating an
individual’s pre-injury level of ability.
Unfortunately, this standard is frequently applied
incorrectly or neglected entirely. Lezak (1995)
considers it critical that the neuropsychologist
obtains or makes an educated estimate about the
individual’s cognitive functioning prior to his/her
injury. Similarly, Matarazzo (1990) wrote that too
often the psychological practitioner relies solely
on data gathered from test results and fails to
consider and integrate other important sources of
data. Using only the normative standard of
comparison can result in erroneous conclusions.
For example, it is
not unusual to assess an individual with a superior
pre-injury level of cognitive ability whose
post-injury scores now fall within the average
range. While such results may be considered "within
normal limits" from a statistical standpoint, these
average scores clearly represent a loss in
functioning for that individual. The impact of this
change in functioning will ultimately depend on a
variety of factors, including the interaction of the
person’s constellation of cognitive strengths and
weaknesses with the variety of occupational and
environmental demands present in his/her life.
To illustrate,
"average" ability in the area of information
processing speed may be adequate for a person
employed in a position that is repetitive or where
rapid appraisal, integration and utilization of new
information is not required. It may not be adequate,
however, for someone working as an air traffic
controller or a trauma surgeon. Because of the
limitations evident in utilizing a strictly
normative approach, it is important that the
neuropsychologist also gather extra-test information
in order to make an estimation of the individual’s
premorbid level of cognitive functioning against
which to compare current test performance.
Methods of Premorbid
Estimation
Vanderploeg (1994)
divides the methods of estimating premorbid
functioning into four basic approaches: one based on
historical data reflecting past achievements, two
based on an individual’s post-injury test
performance and one based upon demographic
information.
The first of these
methods involves information collected through
clinical interview and review of available records.
Typically, the clinical interview will cover such
areas as educational achievements and work
accomplishments; marriage, family and social
functioning; and medical and psychological
histories. While this information is often based
upon self-report, which should be corroborated, it
provides the neuropsychologist with the individual’s
perspective on how his/her life has changed as a
result of the injury. Given that it is not unusual
for persons with brain injury to lack awareness of
and insight into their cognitive and psychological
difficulties, it is extremely beneficial and
frequently crucial to speak with others who interact
with the individual on a regular basis. Family
members, co-workers and personal friends can provide
valuable supplemental and corroborative information
on how the individual’s functioning has changed
since the TBI, as well as provide information on the
individual’s level of awareness and insight.
Historical records,
such as school transcripts, military C-file and
employment records, particularly job performance
evaluations, are helpful in establishing the
capabilities the individual had prior to the injury
(Kay, 1992; Matarazzo, 1990; McCaffrey et al., 1993;
Sbordone, 1991). Academic records, such as school
transcripts and results from standardized
achievement tests, are another source of valuable
information in establishing an individual’s
premorbid level of cognitive ability. The patterns
of performance present within an individual’s grades
may reflect prior cognitive strengths or weaknesses.
Caution should be exercised when reviewing school
records. While good grades and high scores on
academic tests are generally associated with average
to above average cognitive ability, poor performance
on these same measures does not necessarily reflect
lower premorbid functioning. Other factors such as
motivation, attitudes towards academic pursuits, the
lack of a stable supportive home environment and
other life events can result in actual academic
achievement being lower than intellectual
capability.
In reviewing records
pertaining to work performance, the
neuropsychologist attempts to determine the
cognitive abilities that were required for the
successful performance of a particular person. If
the records indicate a history of successful work
performance, then this information provides some
indication of the types of capabilities the
individual may have possessed. Military records
often can provide similar job skills related
information. Ideally, all of the collected
historical data would converge to present a unified
picture of how the individual functioned prior to
their TBI. Some of the difficulties in estimating
premorbid intellectual ability from historical data
are the myriad of factors, other than intellectual
ability, that influence an individual’s life
achievements and the more subjective nature of the
information being collected. Standardized
achievement test results can assist greatly in
making the above estimation process more objectively
based. Unfortunately, obtaining such information is
more the exception rather than the rule.
The remaining three
methods of estimation attempt to address the need
for a more objectively based procedure. Two of these
methods make use of current, post-injury
neuropsychological test results to estimate an
individual’s prior level of cognitive functioning
(Ciplotti & Warrington, 1995; Lezak, 1995). The
first of these is referred to as the "hold" method
of estimation. This approach is named from the
deterioration quotient devised by Wechsler (1958).
In this approach, the neuropsychologist makes use of
test scores which are resistant to brain injury and
frequently well preserved (thus the name "hold"
tests) in order to estimate prior level of
functioning. Abilities such as vocabulary (Yates,
1956) and reading (Nelson, 1978) are examples of
cognitive functions, shown to be more resilient to
disrupting effects of brain injury, that are
typically used to predict premorbid intellectual
ability.
As Ciplotti and
Warrington (1995) point out, however, the use of
these abilities to predict prior functioning may be
problematic in cases where the individual has a
long-standing learning problem such as dyslexia or
where there exists damage to the parts of the brain
directly involved in the performance of those
functions. For example, performance on tests of
reading or vocabulary would likely underestimate the
prior ability of an individual who sustained left
temporal lobe damage resulting in aphasia. In this
instance, the estimation of the individual’s
premorbid abilities would be skewed by a deficiency
that was result of the injury. It also should be
pointed out that implicit in the "hold" test method
is the assumption that a single level of performance
accurately reflects an individual’s pre-injury
state. Research has shown that this assumption may
not necessarily be true, at least not for segments
of the population at the extremes of intellectual
functioning (Matarazzo et al., 1988). While
normative data do indicate significant variability
in performance (i.e. strengths and weaknesses)
across a range of cognitive abilities, research also
has shown that a high degree of variability in
performance may reflect the presence of brain
impairment (Black, 1974; Simpson & Vega, 1971).
The second of these
methods utilizing current test performance to
estimate premorbid cognitive ability, the "best
performance method" is an offshoot of the "hold"
test approach. Rather than using tests which are
pre-determined to be resistant to brain injury, the
entire testing record is reviewed and the highest
scores, in combination with historical data, are
used to make predictions concerning the individual’s
prior level of functioning (Lezak, 1995). While at
times providing useful information, caution should
be exercised using this approach in that I often
results in an overestimation of pre-injury level of
cognitive ability (Vanderploeg, 1994).
The last of the four
methods employs an actuarial approach to the
estimation of pre-injury ability, utilizing known
relationships between demographic variables and
performance on intelligence testing. Variables such
as age, education and occupation are put into one of
a variety of regression formulas that exist in the
research literature to yield a predicted "IQ" score.
This score is then compared to the individual’s
actual test performance. In the event that current
performance levels are significantly below predicted
performance, such discrepancies may be suggestive of
impaired cognitive functioning. There are numerous
studies utilizing the actuarial approach to predict
IQ scores (Barona, Reynolds & Chastain, 1984;
Eppinger et al., 1987; Karzmark et al., 1985;
Wilson, Rosenbaum & Brown, 1979) and performance on
other neuropsychological measures (Karzmark et al.,
1984). The actuarial approach has its limitations,
particularly when the individual’s premorbid
functioning lies at the extremes of ability where
there exists much less accuracy in prediction
(Ciplotti & Warrington, 1995; Sweet, Moberg &
Tovian, 1990).
Discussion
Clearly, the
preferred method of determining alteration in
functioning would be to reassess the individual on a
battery of standardized tests that encompass a
sufficient range of cognitive abilities administered
prior to the injury and then examine any differences
in performance. Unfortunately, this possibility
rarely exists. As a result, a number of approaches
have been developed over the years which
neuropsychologists use to estimate a premorbid level
of functioning against which to compare test
results. It should be emphasized that none of these
methods are without criticism. Consequently, it is
ill advised to rely too heavily on any single
method. While the above methods have their
limitations, when used in combination they can
provide fairly accurate estimations of premorbid
functioning that allow the neuropsychologist to
better determine what changes in cognitive function
may have occurred as a result of the individual’s
brain injury.
In conclusion, the
following example is given to illustrate the
combined use of the above methods. An individual who
was involved in a motor vehicle crash is referred
for a neuropsychological evaluation to determine the
nature and extent of any cognitive impairment. Prior
to the injury, this person had graduated college
with excellent grades and had established a
successful work history as a financial broker. Since
the accident, she reports experiencing difficulties
with forgetfulness, inattention, distractibility and
irritability that are detrimentally affecting her
work performance. The results of all the
neuropsychological testing reveal that the vast
majority of scores fall around the upper end of the
average to high average range (75th percentile).
None of the scores fall into what is commonly
considered the impaired range. If a strictly
normative interpretive approach were applied to the
data, the results would be considered reflective of
normal cognitive functioning. Upon closer
inspection, however, the neuropsychologist notes
that tests of attention and vigilance, speed of
information processing, new verbal learning and
executive functioning generally fall in the low
average to the low end of the average range (15th to
30th percentiles). Further, school grades,
standardized test scores such as the Scholastic
Aptitude Test (SAT) and several actuarial methods of
premorbid IQ estimation all suggest premorbid
functioning falling within the high average to
superior range of ability. Utilizing this
information as the standard of comparison suggests
that this individual is experiencing diminished
functioning in attention, memory, processing speed
and executive functioning relative to her other
cognitive abilities – a pattern consistent with the
known sequelae of TBI.
Estimating an
individual’s pre-injury level of cognitive
functioning is a complex but essential part of the
neuropsychological evaluation. As in the above
example, going from a high average/superior level of
ability to an average level of ability reflects a
loss of cognitive function which can have dire
effects for an individual’s wage earning potential
and his/her overall quality of life. Proper
estimation of premorbid functioning ensures more
accurate diagnosis of cognitive changes, which
ultimately increases the chances of individuals with
brain injury getting the services they need.
Richard A. Lanham,
Jr., PhD, is the clinical and research
neuropsychologist for the Traumatic Brain Injury
Clinic at the Minneapolis VA Medical Center and a
co-investigator in the defense and Veterans Head
Injury Program (DVHIP). He holds a clinical
assistant professor appointment with the Department
of Psychology at the University of Minnesota and is
in private practice with Minnesota Clinical and
Neuropsychological Associates, P.A., Bloomington,
MN.
Thomas Misukanis,
PhD, is a clinical neuropsychologist in private
practice with Minnesota Clinical and
Neuropsychological Associates, P.A., Bloomington,
MN. He also serves as an adjunct professor in the
Department of Psychology at the University of Saint
Thomas in St. Paul, MN.
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