VOLUME 11 NUMBER 1 • MARCH 2014
5
SA JOURNAL OF DIABETES & VASCULAR DISEASE
REVIEW
Specific cognitive functions affected by acute
hypoglycaemia
Attention
Attention is a fundamental and complex cognitive ability, with
multiple definitions and subcategories.
Studies of attention can be grouped into three major themes
3
:
• Alertness, which is the maintenance of attention when per-
forming long, boring tasks and maintaining optimal sensitivity
to external stimulation
• Selective attention, which is the ability to select information
from one source, or of one kind rather than another
• Limited central processing capacity, which makes it difficult to
handle two tasks simultaneously.
Insulin-induced hypoglycaemia affected performance in several
tests of attention, in people with type 1 diabetes mellitus (type 1
diabetes), and in healthy non-diabetic controls.
4-7
In 16 volunteers
with type 1 diabetes, different aspects of attention were tested
during hypoglycaemia.
4
Deterioration in performance occurred in
visual selective attention, auditory selective attention, attentional
switching (where attention has to alternate between competing
percepts) and divided attention (attention is divided between dual
tasks). Sustained attention (vigilance or alertness) was preserved,
but as most of the people undertaking this test achieved a maximum
possible score, this probably represented a “ceiling effect”, and was
not therefore a sufficient measure of how hypoglycaemia affected
sustained attention.
4
Impaired attentional processes can have a
profound impact on many everyday tasks such as driving, and can
have a detrimental effect on many occupational activities.
Memory
Memory is the process of storing, encoding and retrieving
information,
8
and can be classified into several types, including
short-term, long-term, working and sensory memory. Short-
term memory is the temporary retention (less than 20 seconds)
of stimuli that have just been perceived, and can be translated
through repetition into long-term memory by a process known as
consolidation. Long-term memory has no known limit to capacity
and is relatively durable. Working memory is the short-termmemory
function that allows concurrent retention and manipulation of
information. Sensory memory is the representation of a stimulus
that is stored for a very brief period of time – less than 1 second.
Memory in all its forms is a fundamental cognitive function, and
working memory is particularly important for intellectual activity
in humans as it correlates very highly with general problem solving
ability and intelligence.
9
Short-term, delayed and working memories are all profoundly
impaired during controlled, experimental hypoglycaemia.
8,10
Working memory is particularly sensitive, and is completely disabled
in people with and without type 1 diabetes
9
(Fig. 1). Recent studies
have tried to isolate the processes involved inmemory by dividing the
learning and recall tasks between euglycaemia and hypoglycaemia,
as demonstrated in Fig. 2. Recall of a prospective memory task was
impaired during hypoglycaemia,
11
despite learning having taken
place normally during euglycaemia. Hypoglycaemia also adversely
affected learning by reducing the ability to recall information,
immediately, and 50 minutes after hypoglycaemia.
11
Consolidation was studied by inducing hypoglycaemia (blood
glucose 2.5mmol/l) within 20 to 40minutes of completing a learning
task,
12
and appeared to be unaffected by hypoglycaemia as recall
remained intact after euglycaemia had been restored. However,
consolidation may have been completed before the hypoglycaemia
was induced, and because of the simplicity of the task, it is possible
that recognition was being tested rather than memory. In another
study subjects had to remember a series of word pairs before going
to sleep, during which a period of hypoglycaemia or euglycaemia
was induced.
13
Fewer word pairs were recalled after exposure to
hypoglycaemia than after euglycaemia.
Sensory memory and sensory processing also deteriorate during
hypoglycaemia in adults with type 1 diabetes and in matched, non-
diabetic subjects – affecting visual
14,15
and auditory pathways.
16,17
Psychomotor and spatial ability
Psychomotor function (dexterity, fine-motor function and
coordination) deteriorated during acute hypoglycaemia in subjects
with and without type 1 diabetes, although to a lesser degree in
those with type 1 diabetes;
18
the results of two of these tests are
shown in Fig 3. Those with type 1 diabetes had previous exposure
to hypoglycaemia, so their autonomic response to hypoglycaemia
may have been attenuated, reducing tremor and preserving fine-
motor control. Previous experience of hypoglycaemia may also
Fig 1.
Performance on Kyllonen’s four-term order working memory task during
euglycaemia and hypoglycaemia in 32 adults (16 with type 1 diabetes mellitus),
shown as mean ± SD. (Derived from data in Deary
et al
.
9
)
Fig 2.
Outline of study by Warren et al
11
indicating how the effect of hypogly-
caemia on learning and recall was investigated by dividing the tests between the
two study conditions.
