======================================================================
= Anhedonia =
======================================================================
Introduction
======================================================================
Anhedonia is a diverse array of deficits in hedonic function,
including reduced motivation or ability to experience pleasure. While
earlier definitions of anhedonia emphasized the inability to
experience pleasure, anhedonia is used by researchers to refer to
reduced motivation, reduced anticipatory pleasure (wanting), reduced
consummatory pleasure (liking), and deficits in reinforcement
learning. In the DSM-5, anhedonia is a component of depressive
disorders, substance related disorders, psychotic disorders, and
personality disorders, where it is defined by either a reduced ability
to experience pleasure, or a diminished interest in engaging in
pleasurable activities. While the ICD-10 does not explicitly mention
anhedonia, the depressive symptom analogous to anhedonia as described
in the DSM-V is a loss of interest or pleasure.
Definition
======================================================================
While anhedonia was originally defined in 1896 by Théodule-Armand
Ribot as the reduced ability to experience pleasure, it has been used
to refer to deficits in multiple facets of reward.
Re-conceptualizations of anhedonia highlight the independence of
"wanting" and "liking". "Wanting" is a component of anticipatory
positive affect, mediating both the motivation (i.e. incentive
salience) to engage with reward, as well as the positive emotions
associated with anticipating a reward. "Liking", on the other hand,
is associated with the pleasure derived from consuming a reward. The
consciousness of reward-related processes has also been used to
categorize reward in the context of anhedonia, as studies comparing
implicit behavior versus explicit self-reports demonstrate a
dissociation of the two. Learning has also been proposed as an
independent facet of reward that may be impaired in conditions
associated with anhedonia, but empirical evidence dissociating
learning from either "liking" or "wanting" is lacking.
Anhedonia has also been used to refer to "affective blunting",
"restricted range of affect", "emotional numbing", and "flat affect",
particularly in the context of post-traumatic stress disorders. In
PTSD patients, scales measuring these symptoms correlate strongly with
scales that measure more traditional aspects of anhedonia, supporting
this association.
'Social anhedonia' is defined as a disinterest in social contact and a
lack of pleasure in social situations, and is characterized by social
withdrawal. This characteristic typically manifests as an indifference
to other people. In contrast to introversion, a nonpathological
dimension of human personality, social anhedonia represents a deficit
in the ability to experience pleasure. Additionally, social anhedonia
differs from social anxiety in that social anhedonia is predominantly
typified by diminished positive affect, while social anxiety is
distinguished by both decreased positive affect and exaggerated
negative affect. This trait is currently seen as a central
characteristic to, as well as a predictor of, schizophrenia spectrum
disorders, as it is seen as a potential evolution of most personality
disorders, if the patient is above age 24, when prodromal
schizophrenia may be excluded.
Causes
======================================================================
Studies in clinical populations, healthy populations, and animal
models have implicated a number of neurobiological substrates in
anhedonia. Regions implicated in anhedonia include the prefrontal
cortex as a whole, particularly the orbitofrontal cortex (OFC), the
striatum, amygdala, anterior cingulate cortex (ACC), hypothalamus, and
ventral tegmental area (VTA). Neuroimaging studies in humans have
reported that deficits in consummatory aspects of reward are
associated with abnormalities in the ventral striatum and medial
prefrontal cortex, while deficits in anticipatory aspects of reward
are related to abnormalities in hippocampal, dorsal ACC and prefrontal
regions. These abnormalities are generally consistent with animal
models, except for inconsistent findings with regard to the OFC. This
inconsistency may be related to the difficulty in imaging the OFC due
to its anatomical location, or the small number of studies performed
on anhedonia; a number of studies have reported reduced activity in
the OFC in schizophrenia and major depression, as well as a direct
relationship between reduced activity and anhedonia.
Researchers theorize that anhedonia may result from the breakdown in
the brain's reward system, involving the neurotransmitter dopamine.
Anhedonia can be characterised as "impaired ability to pursue,
experience and/or learn about pleasure, which is often, but not always
accessible to conscious awareness".
The conditions of akinetic mutism and negative symptoms are closely
related. In akinetic mutism, a stroke or other lesion to the anterior
cingulate cortex causes reduction in movement (akinetic) and speech
(mutism).
Major depressive disorder
===========================
Anhedonia occurs in roughly 70% of people with a major depressive
disorder. Anhedonia is a core symptom of major depressive disorder;
therefore, individuals experiencing this symptom can be diagnosed with
depression, even in the absence of low/depressed mood. The Diagnostic
and Statistical Manual of Mental Disorders (DSM) describes a "lack of
interest or pleasure", but these can be difficult to discern given
that people tend to become less interested in things which do not give
them pleasure. The DSM criterion of weight loss is probably related,
and many individuals with this symptom describe a lack of enjoyment of
food. They can portray any of the non-psychotic symptoms and signs of
depression.
Schizophrenia
===============
Anhedonia is commonly listed as one component of negative symptoms in
schizophrenia. Although five domains are usually used to classify
negative symptoms, factor analysis of questionnaires yield two
factors, with one including deficits in pleasure and motivation.
People with schizophrenia retrospectively report experiencing fewer
positive emotions than healthy individuals. However, "liking" or
consummatory pleasure, is intact in schizophrenics, as they report
experiencing the same degree of positive affect when presented with
rewarding stimuli. Neuroimaging studies support this behavioral
observation, as most studies report intact responses in the reward
system (i.e. ventral striatum, VTA) to simple rewards. However,
studies on monetary rewards sometimes report reduced responsiveness.
More consistent reductions are observed with regard to emotional
response during reward anticipation, which is reflected in a reduced
responsiveness of both cortical and subcortical components of the
reward system. Schizophrenia is associated with reduced positive
prediction errors (a normal pattern of response to an unexpected
reward), which a few studies have demonstrated to be correlated with
negative symptoms. Schizophrenics demonstrate impairment in
reinforcement learnings tasks only when the task requires explicit
learning, or is sufficiently complex. Implicit reinforcement
learning, on the other hand, is relatively intact. These deficits may
be related to dysfunction in the ACC, OFC and dlPFC leading to
abnormal representation of reward and goals.
Substance related disorders
=============================
Anhedonia is common in people who are dependent upon a wide variety of
drugs, including alcohol, opioids, and nicotine. Although anhedonia
becomes less severe over time, it is a significant predictor of
relapse.
Post traumatic stress disorder
================================
While PTSD is associated with reduced motivation, part of the
anticipatory "wanting", it is also associated with elevated sensation
seeking, and no deficits in physiological arousal, or self reported
pleasure to positive stimuli. PTSD is also associated with blunted
affect, which may be due to the high comorbidity with depression.
Parkinson's disease
=====================
Anhedonia occurs frequently in Parkinson's disease, with rates between
7%-45% being reported. Whether or not anhedonia is related to the
high rates of depression in Parkinson's disease is unknown.
Sexual anhedonia
======================================================================
Sexual anhedonia in males is also known as 'ejaculatory anhedonia'.
This condition means that the man will ejaculate with no accompanying
sense of pleasure.
The condition is most frequently found in males, but women can suffer
from lack of pleasure when the body goes through the orgasm process as
well.
Sexual anhedonia may be caused by:
* Hyperprolactinaemia
* Hypoactive sexual desire disorder (HSDD), also called inhibited
sexual desire
* Low levels of the hormone testosterone
* Spinal cord injury
* Multiple sclerosis
* Use of SSRI antidepressants or having used SSRI antidepressants in
the past.
* Use (or previous use) of antidopaminergic neuroleptics
(anti-psychotics)
* Fatigue
* Physical illness
It is very uncommon that a neurological examination and blood tests
can determine the cause of a specific case of sexual anhedonia.
Patients may be prescribed sustained-release bupropion to aid in
treatment, which has been shown to relieve sexual dysfunction even in
patients without depression.
Signs and symptoms
====================
* Decreased ability to experience interpersonal pleasure
* Social withdrawal/isolation
* Decreased need for social contact
* Lack of close friends and intimate relationships, and decreased
quality of those relationships
* Poor social adjustment
* Decreased positive affect
* Flat affect
* Depressed mood
* State-related anxiety
Background and early clinical observation
===========================================
The term 'anhedonia' is derived from the Greek 'an-', "without" and