Attention deficit hyperactivity disorder (ADHD) is a very com-
mon psychiatric disorder in children and adolescents. ADHD
is associated with impairments in different domains of sensory
function.1-6 Understanding sensory problems in ADHD may
help us better understand the spectrum of problems included
within the broad label of ADHD.
Unlike to other sensory systems, few synapses are located be-
tween the olfactory cortex and the peripheral olfactory receptors
150 Copyright © 2012 Korean Neuropsychiatric Association
leading to a direct relation between brain and environment.2,7
Regarding these facts, olfactory function is increasingly be-
ing used as a marker in neurodegenerative and neuropsychiat-
ric disorders. Olfactory dysfunction is reported in many neuro-
logic and psychiatric disorders such as Alzheimer’s disease,
Parkinson’s disease (PD), Schizophrenia, Autism and Obsessive
compulsive disorder (OCD).8-10 For example, odor identifica-
tion, odor detection threshold and odor memory are impaired
in patients with schizophrenia.11 The ability of children and ad-
olescents with autism is less than healthy matched controls in
identifying common odors.10
Olfactory function in patients with ADHD has been a topic
of increasing interest. It is reported that processing of smells is
mediated by a set of neuroanatomical and neurochemical st-
ructures that have been implicated in the pathophysiology of
Greater errors in odor identification are found in adults with
ADHD comparing to healthy control group; although the dif-
Smell Identification Function in Children
with Attention Deficit Hyperactivity Disorder
Ahmad Ghanizadeh1 , Maryam Bahrani1, Ramin Miri2 and Ali Sahraian1
1Research Center for Psychiatry and Behavioral Sciences, Department of Psychiatry, School of Medicine, Shiraz University of Medical Sciences,
2Medicinal & Natural Products Chemistry Research Center, Shiraz University of Medical Sciences, Shiraz, Iran
ObjectiveaaDeficits in olfactory function are common features in neurodegenerative and neuropsychiatric disorders. Olfactory process-
ing is related to dopamine metabolism and orbitofrontal cortex functioning, both known to be involved in the neurobiology of ADHD.
Some investigations suggested alterations in olfactory processing (identification and detection threshold) in patients with ADHD. Despite
increasing knowledge, controversy about this topic still exists regarding children with ADHD. This study was conducted to help elucidate
some of this controversy.
Methodsaa50 participants (8-15 years, mean=10.70±1.77) with ADHD were compared to 50 controls. The two groups were well matched
for age, gender and Mean School Scores (MSS). We assessed odor identification and threshold through a smell test composed of two tests of
identification and detection threshold. Odor detection threshold was assessed with the odorant phenyl ethyl alcohol solved in propylene
glycol using a single staircase method. Odor identification was assessed with chemical essences of five common odorants.
ResultsaaThe mean Sensory Identification Score for children with ADHD and the control groups were 3.76 (1.06) and 4.46 (0.76), respec-
tively (p<0.001). The mean for Sensory Threshold Score for ADHD and control group was 6.4 (3.35) and 9.75 (2.16), respectively (p<0.001).
ConclusionaaThis study replicated altered olfactory performance in ADHD. Substantial olfactory deficits across the two domains of iden-
tification and detection threshold are observed in children with ADHD. These deficits do not seem to be a result of olfactory task difficulty
and are not influenced by age, gender and MSS. Further studies are required to investigate whether olfactory function can be used as a bi-
ological marker for early diagnosis, treatment and prognosis of ADHD.
Psychiatry Investig 2012;9:150-153
Key Wordsaa ADHD, Olfaction, Identification, Threshold, Smell test.
Received: June 7, 2011 Revised: December 11, 2011
Accepted: December 16, 2011 Available online: April 30, 2012
Correspondence: Ahmad Ghanizadeh, MD
Research Center for Psychiatry and Behavioral Sciences, Department of Psychi-
atry, School of Medicine, Shiraz University of Medical Sciences, Shiraz, Iran
Tel: +98-711-627 93 19, Fax: +98-711-627 93 19
cc This is an Open Access article distributed under the terms of the Creative Commons
Attribution Non-Commercial License (http://creativecommons.org/licenses/by-
nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduc-
tion in any medium, provided the original work is properly cited.
Print ISSN 1738-3684 / On-line ISSN 1976-3026
A Ghanizadeh et al.
ference was not significant anymore when the two groups mat-
ched for IQ.2 In another study Scheckelman and colleagues
reported no altered olfactory performance in adult ADHD pa-
tients but they found an association of cortical olfactory pro-
cesses with hyperactivity and impulsivity in adult ADHD.14
Gansler reported that olfactory identification in adults with
ADHD is less than the normal controls.15
There was found only two studies included children and ad-
olescents with ADHD. Romanos et al reported significant im-
proved odor sensitivity (threshold) in un-medicated ADHD chil-
dren and adolescents compared to odor sensitivity of patients
medicated with methylphenidate and healthy controls which
are similar to each other.16 Others reported that olfactory iden-
tification ability of children with ADHD is significantly poorer
than healthy controls.17
Moreover, a study compared odor identification between
ADHD adults and a control group. There was no significant
interaction between group and gender.2 Therefore, we hypoet-
ized that there is no gender differences in olfactory process-
ing between boys and girls with ADHD.
Overall, olfactory processing function is not typically stud-
ied in ADHD patients and the few existing researches on this
topic have yielded inconsistent results.14,16-18 In general, olfac-
tory processing in children with ADHD, but not in adults with
ADHD,19 seems to be impaired. Therefore, we performed the
current study to assess the olfactory function of ADHD children
and compare it to healthy controls. Regarding the similarity of
the pathways involved in the pathophysiology of ADHD and
those of olfactory processing, we hypothesized that olfaction is
diminished in ADHD patients in both genders. If this comes
true, future studies may wish to make exploration into whether
systematic clinical assessment of olfactory function can provide
valuable information for the early detection and classification
of ADHD patients or whether this sensory domain can be used
to predict course of the illness or to monitor treatment response.
The participants were 100 children and adolescents includ-
ing 50 individuals with ADHD and 50 children and adolescents
as the control group. The two groups were match for age, gen-
der and mean school scores (MSS). This study involved a com-
parison of two groups of children between the ages 8-15 (SD=
1.7) years. There were 15 girls and 35 boys in each group. All
the patients did not take stimulants for at least seven hours pri-
or to testing.
Inclusive criteria were:
1- Chronological age between 8-15 years
2- No evidence or history of recent upper respiratory tract
infection, sinusitis or nasal allergies.
3- No past history of severe head trauma, hypothyroidism,
neurologic disorder or gestational and birth problems.
ADHD was diagnosed using the kiddie schedule for affec-
tive disorders and schizophrenia (KSADS) according to Di-
agnostic and Statistical Manual of Mental Disorders, 4th.
Edition (DSM-IV).20 The ADHD children and their parents
were interviewed face to face. The parents of the patient partici-
pants joined the test but the healthy one’s did not. Olfactory
identification and threshold were assessed through a smell-test
kit provided by the Center of Medicinal and Natural Products
Chemistry Research affiliated to Shiraz University of Medical
Sciences. The test was composed of two different tasks that were
initially explained for the participants by detail. All of the chil-
dren understood what they had to do. One task was to examine
smell identification ability and the other one to assess smell th-
reshold ability. The smell identification part consisted of five
glass sniff-bottles, each one containing 4 mL of a specific odor-
ant. Five common odorants including essences of apple, banana,
lemon, orange and rose were chosen in respect to their famil-
iarity to almost all individuals (16). The participant’s task was
to sniff the bottle for 3 seconds and choose the answer from a
list of 4 descriptors reported by examiner. The number of cor-
rect answers was registered as the sensory identification score
of the participant.
The olfactory threshold was assessed by serving a perfume
grade Phenyl Ethyl Alcohol (PEA) as the odorant. The diluent
used in this study was Propylene Glycol (PG) a relatively odor-
less and non-toxic material in which PEA is soluble (17). De-
tection thresholds were determined using a single staircase pro-
cedure (18). 16 triplets of bottles were prepared based on 16 dif-
ferent dilutions of PEA in PG in a geometric series with dilution
ratio of 2/1. Each triplet was composed of three glass sniff-bot-
tles: one of them containing 4 mL of a particular concentration
of the odorant (PEA) dissolved in the diluent (PG), the other two
bottles containing 4 mL of the diluents (PG) alone. The partici-
pant’s task was to sniff each bottle for 3 seconds and identify the
bottle that contained the odorant (or the bottle that had smell).
If a correct answer was given in 2 successive trials, it would trig-
ger the reversal of the staircase to the next lower concentration.
If a wrong answer was given on any trial the next higher con-
centration was presented. There was a total 4 staircase rever-
sals. The geometric mean of the last 3 reversal points was de-
fined as the threshold estimate of the participant. The testing
procedure lasted about 25 minutes. This study was approved by
Shiraz university of Medical sciences. Participation was volun-
tary. The Univariate Analysis of Variance were used to compare
the smell identification score (SIS) and smell threshold score
(STS) of the two groups considering age and mean school scores
covariates. Pearson correlation was performed to assess test- re-
152 Psychiatry Investig 2012;9:150-153
ADHD of Olfactory Function
The two groups of participants did not differ in terms of
age, gender and MSS. Their age ranged from 8-15 (Mean=10.8,
SD±1.7). The mean (SD) of age for the patients and control
group was 10.7 (1.7) and 10.7 (1.6), respectively. The mean of
age was not different between the two groups (t=0.11, df=98,
p=0.9). 30% (n=15) of all participants in each group were girls
and 70% were boys. The mean (SD) of MSS for patients and
control group was 18.1 (2.2) and 18.8 (2.0), respectively (t=1.6,
The pearson correlation between SIS results during the first
and the second sessions was (r=0.55, p=0.001). Regarding the
STS the correlation results between the 2 sessions was (r=0.28,
The mean (SD) of SIS and STS are showed in Table 1. Sig-
nificant differences between the two groups were evident in the
two domains of olfactory function. After adjusting for age and
Mean school score, both STS and SIC in ADHD group were
statistically less than the control group (p<0.001, p<0.001, re-
These deficits were not associated with age and Mean
School Scores (MSS). None of SIS and STS was related to gen-
der in the ADHD group (Table 2).
The findings from this clinical study show that children with
ADHD have significant impairments on odor identification
and odor threshold tasks compared to healthy control partici-
pants. Such findings corroborate past theoretical predictions
that ADHD should be associated with olfactory dysfunction
due to their neuro-anatomical and neuro-chemical similari-
ties. Because the prefrontal lobe that is long-standing assumed
to have diminished functioning in ADHD, contains primary
and secondary areas for olfactory processing. These olfactory
deficits are not associated with age, gender and MSS totally
and are not a result of olfactory task difficulty. Another expla-
nation for these impairments could be simply inattention of
ADHD children during the olfactory task.
Prior research findings for olfactory function of children with
ADHD have been quite inconsistent. The findings of this study
lend confidence to one earlier study by Karsz et al.17 showing
that patients with ADHD performed significantly worse than
control participants on the olfactory identification test, howev-
er, they did not evaluate olfactory thresholds in that study and
gender effects on odor identification ability could not be eval-
uated due to small sample size for girls. Two other studies re-
ported no differences in olfactory function among ADHD pa-
tients compared to typical controls.2,14 One recent study sug-
gested improved olfactory threshold in un-medicated ADHD
patients compared to medicated ones and healthy controls.16
Our result is inconsistent with that study.16 We could not find
any study reported this result.
Determining whether some children with ADHD have ol-
factory dysfunction has implications for recommending reha-
bilitation treatment approaches. Suggestion that olfactory dys-
function may predict other behavioral problems or psycho-
pathy in ADHD should be explored further. Although our
sample size was small to divide the group with ADHD into in-
tact and impaired on olfactory tasks, a larger sample size may
confirm that two distinct subgroups exist within the popula-
tion with ADHD regarding olfactory function and may these
subgroups are related to the traditional three subgroups men-
tioned for ADHD.
The current study did not support a diagnosis specific gen-
der difference in olfactory testing. There was no differential
deficit in odor identification and threshold performance be-
tween boys and girls in both patient and control groups. It is
in agreement with a recent study of patients with ADHD re-
ported that gender was not a predictor for smell detection abili-
ty in ADHD patients according to parent reports.18 It is never-
theless contrary to results of one previous study done by
Murphy et al.2 which reported that girls with ADHD perform-
ed better on olfactory identification tasks compared to boys,
however, that study tested adult-age ADHD patients and cur-
rent study included small sample size .
The impairment in olfactory function in ADHD patients
does not seem to be a result of olfactory task difficulty, as some
of participants with high MSS performed poorer on both iden-
tification and threshold tasks than some of those with low MSS.
Moreover, the MSS of participants did not correlate with either
SIS or STS totally; suggesting that the decline in olfactory func-
tion demonstrated in ADHD is relatively independent of the
Table 1 .The comparison of Smell Identification Score and Smell
Threshold Score between patients and controls
GroupMean (SD) Minimum Maximum
Control9.75 (2.16) 13.6
Table 2 .The comparison of Smell Identification Score & Smell
Threshold Score between genders within attention deficit hyper-
activity disorder group
Smell identification score
Smell threshold score0.78
A Ghanizadeh et al.
difficulty of the tasks. the smell test administered here prepared
in accordance with the standard method described by Doty et
al and Hummel et al.21-23
In sum, our results suggest olfactory deficits are present in
ADHD in both domains of smell identification and smell detec-
tion threshold. These deficits are not associated with age, gen-
der and MSS totally.
The MSS used here could better be replaced by intelligence
test but it was not possible due to limitations in time and fund.
The discrepant findings about olfactory function in ADHD pa-
tients that have been reported in all respective studies may be
due to differences in the measures and procedures employed.
Therefore, establishment of an easy administrable, worldwide
affordable, and standard test of olfactory function seems war-
ranted. In hopes that true evaluation of olfactory functioning in
patients with ADHD enables us to use such assessments in ear-
ly diagnosis, treatment and prognosis of the disorder. Further
studies with larger sample sizes need to be conducted before a
differential deficit or intact ability in these olfactory functions
can be confidently declared. Future studies are required to ob-
tain more evidence with regard to diagnostic sensitivity, speci-
ficity and validity to establish olfactory function as a biological
marker of ADHD.
This study was the undergraduate thesis of Dr. Maryam Bahrani. This
study was supported by a grant (No. 1943) from Shiraz University of Medi-
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