JCPSLP Vol 18 no 2 2016

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JCPSLP Volume 18, Number 2 2016 95
Gender
There are conflicting findings regarding gender as a
prognostic factor. Yairi and Ambrose (2013) concluded that
only small differences have been found between males and
females for stuttering onset, whereas Reilly and colleagues
(2013) reported that being male was a significant risk factor
for stuttering onset by age 4.
Neurology
Neurological studies of factors associated with stuttering
onset are emerging. To illustrate, brain scans of preschool
aged children, taken soon after the onset of stuttering, have
revealed deficiencies in left grey matter volume when
compared to fluent controls (Chang, Erickson, Ambrose,
Hasegawa-Johnson, & Ludlow, 2005).
Maternal education
Reilly et al. (2013) reported that by age 4 (n = 1619), higher
maternal education was an additional significant prognostic
variable for stuttering onset using logistic regression
analysis. The authors speculated that this may be due to
more reports of stuttering in children from mothers who are
more highly educated and aware of stuttering.
Persistent stuttering
The difference between the incidence and prevalence rates
has been attributed to the rate of natural recovery in a large
proportion of people who stutter. Some disagreement as to
the definition of natural recovery exists (Bloodstein &
Bernstein Ratner, 2008), but it is generally considered to be
recovery without clinical intervention (Yairi & Ambrose,
2013). Alternatively, treatment assisted recovery is
sometimes referred to, and the distinction between this and
natural recovery should be made clear in studies. At times it
is not, and this can be problematic in defining relevant
prognostic factors to predict those who recover or persist
with stuttering without clinical treatment.
It has been suggested that up to 74% to 83% of
children who start to stutter recover naturally (Ambrose &
Yairi, 1999; Dworzynski et al., 2007), but this also means
that approximately 1 in 5 of these children will develop a
persistent stutter. It is of interest to be able to predict if
a child who has started to stutter will naturally recover in
order to accurately prioritise therapy services to those at
greater risk.
Genetics
Persistent stuttering has been found to relate to positive
familial history of stuttering (Yairi & Ambrose, 2013) with
individuals who stutter and have a family member who
stuttered but recovered being more likely to recover
themselves (Dworzynski et al., 2007; Yairi, Ambrose,
Paden, & Throneburg, 1996).
Gender
The sex ratio of stuttering indicates that gender in itself is a
risk factor for persistent stuttering. Ambrose and Yairi
(1999) conducted a longitudinal study of 147 children,
collecting data from when the children were close to
stuttering onset. They found that 84% females recovered
versus 77% males, and that females who recovered did so
at a younger age than males who recovered. Contrary to
findings from Yairi and colleagues, Reilly et al. (2013)
reported a higher rate of recovery within 12 months for
males compared to females. However, as the number of
recovered children was low (n = 9), the authors stated that
they could not examine predictors of recovery appropriately.
Aims of this review
A comprehensive systematic review of the prognostic and
predictive factors of stuttering is beyond the scope of this
article. Instead, the aim is to provide a synopsis of
prognostic and predictive factors, and to present an
argument for why there is a need for a comprehensive
systematic review of the topic to be conducted. There are
no previously published reviews of predictive factors of
treatment outcomes of stuttering. In contrast, there are
some previous published reviews of stuttering prognostic
factors, for stuttering onset and for persistence of stuttering
without treatment. For example, Yairi and Ambrose (2013)
discuss the incidence, prevalence, natural recovery and
persistency, and subtypes of stuttering in light of recent
research advances. While the literature in this area has not
changed dramatically in the past 25 years, more recent
studies, including a prospective, community cohort study
by Reilly and colleagues (Reilly et al., 2013), are contributing
new insights into prognostic factors, outlined below.
Prognostic factors associated with
stuttering onset and persistent
stuttering
Stuttering onset
The onset of stuttering usually occurs between 2 and 5
years of age (Bloodstein & Bernstein Ratner, 2008). The
prevalence of stuttering, or percentage in a particular
population at a given time, is just below at 0.72% (Craig,
Hancock, Tran, Craig, & Peters, 2002). Recent incidence
data of the lifetime risk for stuttering indicate a rate of
between 8% (Dworzynski, Remington, Rijsdijk, Howell, &
Plomin, 2007) and 11% (Reilly et al., 2013). Accurately
predicting who will stutter is challenging given there is no
single known cause. However, in recent years, there has
been converging evidence to indicate that stuttering is a
complex neurological disorder of speech motor control with
genetic influences (Dworzynski et al., 2007).
Genetics
Regarding prognostic factors of stuttering onset, there is a
tendency for stuttering to run in families, with approximately
30%–50% of people who stutter reporting a positive family
history (Bloodstein & Bernstein-Ratner, 2008). Stuttering is
more common in monozygotic twins (52%) than in dizygotic
twins (12%) (Dworzynski et al., 2007), but the specific role
of genetics in stuttering onset is still somewhat unclear.
Linkage studies have analysed the genetic marker(s) for
stuttering in affected and unaffected members in families
(Yairi & Ambrose, 2013). Such studies have reported
multiple genes that could be related to stuttering. However,
the findings are inconsistent and need replication. In Reilly
et al.’s (2013) study, logistic regression analysis found that
being a twin was a significant prognostic variable and family
history of stuttering was close to significant ( p = .07) for
predicting stuttering onset by 4 years of age.
Age
Age is a relevant prognostic factor for onset of stuttering.
The older a child is, the less risk they have of beginning to
stutter. Reilly et al. (2013) reported that the incidence for
stuttering onset slowed markedly after 3.6 years of age.
Similarly, Craig et al. (2002) found that a child of 6 years or
older is 75% less likely to start stuttering when compared
with younger children.