Diagnosis and treatment of mitochondrial myopathies
GERALD PFEFFER 1,2 & PATRICK F. CHINNERY 1
1 Institute of Genetic Medicine, Newcastle University, Newcastle, United Kingdom, and 2 Clinician Investigator Program,
University of British Columbia, Vancouver, Canada
Mitochondrial disorders are a heterogeneous group of disorders resulting from primary dysfunction of the respiratory
chain. Muscle tissue is highly metabolically active, and therefore myopathy is a common element of the clinical presenta-
tion of these disorders, although this may be overshadowed by central neurological features. This review is aimed at a
general medical and neurologist readership and provides a clinical approach to the recognition, investigation, and treatment
of mitochondrial myopathies. Emphasis is placed on practical management considerations while including some recent
updates in the fi eld.
Key words: Diagnosis , mitochondrial disorders , mitochondrial myopathy , treatment
Mitochondrial myopathies (MM) comprise a large
heterogeneous group of disorders resulting from pri-
mary dysfunction of the mitochondrial respiratory
chain and causing muscle disease. These disorders
are characterized by dysfunction in multiple organ
systems, extensive variability in clinical presentation,
and generally poor genotype – phenotype correlation.
Several characteristics of mitochondria produce
features which differentiate mitochondrial disorders
from other genetic diseases and are relevant for clini-
cal practice. Mitochondria are intracellular organelles
which contain their own genetic material (mtDNA),
in the form of a 16.5-kb genome. However, most of
the mitochondrial proteins are encoded by the nuclear
genome (nDNA). Therefore, mitochondrial myopa-
thies can result from abnormalities of mtDNA or
nDNA. Abnormalities of mtDNA include point muta-
tions, single large-scale deletions, mtDNA depletion,
and multiple deletions. Point mutations of mtDNA
affect the protein-coding regions of the genome and
tRNA genes which alter intramitochondrial protein
synthesis. Single large-scale deletions are typically due
to sporadic events and are usually not inherited,
although inherited deletions have been described (1).
Depletion (loss of mtDNA) and multiple deletions
are typically secondary effects of faulty mtDNA
maintenance, from mutation of nDNA-encoded
Each cell contains several mitochondria, and
each mitochondrion contains numerous genomes.
In this setting, the phenomenon of genetic hetero-
plasmy arises, where a proportion of genomes con-
tain a mutation, and a proportion are wild-type
(normal). The degree of heteroplasmy affects the
likelihood and severity of the disease phenotype.
The major factor affecting the inherited level of
heteroplasmy of mutations occurs during oogenesis
and is referred to as the ‘ bottle-neck ’ effect, in
which the organism ’ s entire repertoire of mitochon-
dria is replicated from a small pool of genomes.
However, the transmission of mtDNA mutations is
complex and incompletely understood; women with
mtDNA mutations pass their mutations forward at
a level of heteroplasmy which is unpredictable, and
apparently random (2).
Correspondence: Professor Patrick F. Chinnery, Institute of Genetic Medicine, Central Parkway, Newcastle, NE13BZ, United Kingdom. E-mail: p.f.chinnery@
(Received 1 August 2011; accepted 15 August 2011)
Annals of Medicine, 2013; 45: 4–16
ISSN 0785-3890 print/ISSN 1365-2060 online © 2013 Informa UK, Ltd.
Although the focus of this article is on primary
mitochondrial disorders which feature myopathy, it
is important to mention that in many of these syn-
dromes the myopathy component is overshadowed
by other aspects of the clinical presentation. Disor-
ders which will not be considered in this review
include primary mitochondrial disorders which do
not have myopathy as a clinical feature (for example,
Leber optic neuropathy), and diseases due to sec-
ondary mitochondrial dysfunction, or which other-
wise involve the mitochondria in their pathogenesis
(several inherited and acquired muscle diseases, and
numerous common neurodegenerative and other
genetic diseases (3)). The reader should also be cau-
tioned that syndromes resembling primary mito-
chondrial disorders may be produced from secondary
mitochondrial dysfunction and acquired mitochon-
drial toxicity. Furthermore, acquired mitochondrial
toxicity and medication effects may also affect
the muscle biopsy or other test results to mimic
those seen in a primary mitochondrial disorder, as
may the effects of healthy ageing (4).
The prevalence of mitochondrial disorders as a
whole is approximately 1 in 10,000 (5), although the
carrier frequency of mtDNA mutations is about
1 in 200 (6). Onset can occur at any age, although
typically the more severe phenotypes present ear-
lier in life, and milder phenotypes present later in
life. As a prototype example of this, the so-called
deletion syndromes (caused by sporadic, large-scale
deletions of mtDNA) exist on a disease spectrum in
which the most severe syndrome presents in infancy
(Pearson syndrome), a more moderate syndrome in
early childhood or adolescence (Kearns – Sayre
syndrome (KSS)), and a milder syndrome in child-
hood up to late adulthood (progressive external
ophthalmoplegia (PEO)). MM are usually progressive
conditions which produce signifi cant disability and, in
some instances, premature death, often due to non-
muscle involvement such as cardiac conduction
defects or seizures (7).
Because mitochondria are the main source of
energy production in mammalian cells, clinical fea-
tures typically involve tissues with the highest energy
requirements. Furthermore, the presence of mtDNA
in all human tissues means that dysfunction occurs
in multiple organ systems. The most commonly
affected organ systems are the nervous system (cen-
tral, peripheral, autonomic, as well as optic nerve
and retina), muscles (and in particular extra-ocular
muscle), cardiac, and endocrine systems. The clinical
presentation is highly variable with regard to onset
age, symptoms, signs, severity, and prognosis. The
clinician should consider a diagnosis of MM when
myopathy is accompanied by clinical features of
multi-organ dysfunction, which are summarized in
Figure 1. It is common for MM to present with con-
stellations of symptoms, which allow them to be cat-
egorized into one of several syndromes. Ocular
myopathy, which is manifested by ptosis and oph-
thalmoparesis, is an important feature in various
MM syndromes, which are summarized in Table I.
In this sense, PEO is both a syndrome on its own
but also a component of other syndromes when
certain combinations of other features are present.
Mitochondrial myopathies frequently pres-
ent with multi-system dysfunction and have
a broad variety of phenotypes and genetic
Although no disease-modifying therapy
exists, it is important to address disease
complications which are often treatable and
have an important impact on patient care.
Further study is required to assess the effi -
cacy of various treatments, but a trial of
coenzyme Q 10 is reasonable as it may be use-
ful for the rare patients with coenzyme Q 10
coenzyme Q 10 , also known as ubiquinone
cytochrome c oxidase
myopathy, encephalopathy, lactic acidosis
and stroke-like episodes
myoclonic epilepsy myopathy sensory ataxia
myoclonus, epilepsy, and ragged red fi bres
mitochondrial recessive ataxia syndrome
nicotinamide adenine dinucleotide
nuclear (or chromosomal) DNA
progressive external ophthalmoplegia
respiratory chain enzyme
ragged red fi bres
sensory ataxic neuropathy, dysarthria,
spinocerebellar ataxia and epilepsy
6 G. Pfeffer & P . F . Chinnery
The ocular muscle weakness develops gradually, and
presentation may be delayed by years or decades
until the signs are noticed by family members. Iso-
lated PEO, which is the mildest syndrome, may still
carry signifi cant visual and other disability (8) and
often presents with multiple features of mitochon-
drial disease (9). PEO deserves special mention
because of its extensive genetic heterogeneity, which
has implications for the risk to other family members
of inheriting the disease. PEO may be sporadic
(due to single deletions), maternally inherited (due
to mtDNA mutation), autosomal dominant, or
recessive (due to nDNA mutations). These are often
clinically indistinguishable, emphasizing the impor-
tance of obtaining a molecular diagnosis. The
phenotypic variability should also be emphasized,
since this condition may present at any age, the
ophthalmoparesis may be anywhere along a spec-
trum of subtle to complete, and associated features
of the condition may appear in any combination.
For the ataxia-neuropathy syndromes (ANS),
this includes an overlapping group of disorders,
about half of which have PEO as part of their clini-
cal presentation. These syndromes are variably
referred to as spinocerebellar ataxia with epilepsy
(SCAE), myoclonic epilepsy myopathy sensory ataxia
(MEMSA), sensory ataxia, neuropathy, dysarthria,
ophthalmoplegia (SANDO), or mitochondrial reces-
sive ataxia syndrome (MIRAS). These syndromes
have in common the presence of an axonal sensory
Neurologic - central
• Movement disorder
• Stroke-like episodes
• Cognitive impairment
• Myopathy: ophthalmoplegia
• Optic atrophy
• Pigmentary retinopathy
• Conduction abnormalities
• Cardiomyopathy: hypertrophic>
Skeletal muscle: ocular >
axial/proximal > bulbar > distal
Smooth muscle: dysphagia
• Axonal polyneuropathy
• Sensory ataxia
• Sensorineural hearing loss
• Autonomic dysfunction
• Short stature
• Spontaneous abortion
• Dysmotility: gastroparesis,
diarrhoea, constipation, and/or
• Hepatic failure
• Renal tubular defects
• Diabetes mellitus
• Gonadal failure
• Growth hormone
Figure 1. Clinical features of mitochondrial myopathies, by organ system.
neuropathy, affecting proprioceptive function in
combination with variable degrees of cerebellar
ataxia, making them part of a disease spectrum
which is caused by nDNA mutations affecting
Other syndromes present with multi-organ dys-
function, without ocular myopathy, and these are
summarized in Table II. In myoclonus, epilepsy,
and ragged red fi bres (MERRF), myopathy, enceph-
alopathy, lactic acidosis, and stroke-like episodes
(MELAS), and some of ANS, central nervous sys-
tem dysfunction predominates on a background of
dysfunction in other organ systems. For these syn-
dromes, genotype – phenotype correlations are some-
what better, where the majority of MELAS and
MERRF patients have common tRNA mutations
(respectively, m.3243A ? G and m.8344A ? G, and
ANS are most often caused by mutations in the gene
encoding the mtDNA polymerase gamma, POLG ).
Isolated MM typically presents with axial and
proximal weakness, variable age of onset or severity,
and variable co-occurrence of other features of
mitochondrial dysfunction. As in PEO, genotype –
phenotype correlation is poor, and as it stands this
condition is already diffi cult to distinguish from
other types of acquired or genetic myopathy, due to
its fairly non-specifi c presentation.
The infant-onset mitochondrial myopathies have
a severe clinical presentation, although it is impor-
tant to be aware of a subset of patients with infantile
cytochrome c oxidase (COX)-defi ciency myopathy
with reversible disease, whose molecular defect has
recently been described (10,11).
Another rare but important subgroup of patients
with MM are due to defects in coenzyme Q 10
(CoQ 10 ) biosynthesis. These disorders are important
to recognize because of their partial responsiveness
to CoQ 10 supplementation. The infantile-onset form
of CoQ 10 defi ciency is a multi-systemic disorder with
encephalopathy and nephropathy. Typically this is
steroid-resistant and may progress to renal failure
(12,13). In adults, CoQ 10 defi ciency manifests as
adult-onset myopathy or ataxia with variable myopathy,
peripheral neuropathy, and/or seizures (14 – 16).
Numerous patients with MM do not fi t into the
described syndromes. Many features of MM are
uncommon, or recently described (such as distal
myopathy from certain POLG mutations (17)).
These ill-defi ned syndromes may have a novel or
unique molecular basis, or they may be due to muta-
tions previously described to cause specifi c syn-
dromes. For example, the m.3243A ? G mutation
has been implicated in a broad variety of atypical
clinical presentations (18).
For patients with suspected MM, diagnostic tests fall
into two broad categories. The fi rst category of test-
ing confi rms the presence of dysfunction in various
organ systems (summarized in Table III) and does
not as such confi rm a diagnosis of MM. These tests
Table I. Mitochondrial myopathy syndromes presenting with ocular myopathy.
Syndrome Clinical symptoms/signs Onset ageGenetics
Proximal myopathy often
present. Various other
clinical features variably
PEO, ptosis, pigmentary
ataxia, CSF elevated
protein, diabetes mellitus,
sensorineural hearing loss,
SANDO: PEO, dysarthria,
Other ANS: variable presence
of PEO and/or myopathy
PEO, ptosis, GI dysmotility,
proximal myopathy, axonal
Any age of onset.
Typically more severe
mtDNA single deletions; mtDNA
point mutations (including
m.3243A ? G, m.8344A ? G);
nDNA mutations ( POLG, ANT,
PEO1, OPA1 )
mtDNA single deletions Kearns – Sayre syndrome (KSS)
? 20 years
Ataxia neuropathy syndromes
(ANS): Including MIRAS,
SCAE, SANDO, MEMSA
Teen or adultnDNA mutations ( POLG , PEO1 )
Childhood to early
nDNA mutations in (TP )
MEMSA?myoclonic epilepsy myopathy sensory ataxia; MIRAS?mitochondrial recessive ataxia syndrome; SANDO?sensory ataxia
neuropathy dysarthria ophthalmoplegia; SCAE?spinocerebellar ataxia with epilepsy.
8 G. Pfeffer & P . F . Chinnery
are nonetheless important to defi ne the extent of
the phenotype, to exclude other disorders, and to
increase or decrease the clinical suspicion of a MM
diagnosis. The tests selected are guided by the pat-
tern of organ involvement in each individual patient.
Some of these tests deserve special mention, because
of their potential to alter patient management. Car-
diac investigations are of particular importance
because cardiac conduction defects can be fatal if
not identifi ed and are treatable with cardiac pace-
makers. Endocrine investigations may identify dia-
betes mellitus, hypothyroidism, or growth hormone
defi ciency, all of which are treatable. Patients with
hearing or visual symptoms should be investigated
in order to obtain appropriate aids if required. Dys-
phagia is common in some mitochondrial syndromes
and can be managed with dietary modifi cation.
The second category of tests defi nitively addresses
whether the patient is affected by a MM, and these
mainly include muscle biopsy and molecular genetic
studies. These studies are performed in combination,
since the assessment of mtDNA should ideally be
done from DNA extracted from muscle (the high
replication rate of blood cells selects against patho-
genic mtDNA abnormalities, therefore many mtDNA
abnormalities are not detectable in blood).
Muscle biopsy is typically performed from a limb
muscle, such as quadriceps femoris or deltoid, and
examples of characteristic abnormalities are pro-
vided in Figure 2. The testing should include a
variety of histochemical functional assays and be
performed in a centre with experience in mitochon-
drial disease diagnosis. The major diagnostic feature
is the presence of fi bres defi cient for COX activity,
Table II. Mitochondrial myopathy typically presenting without PEO.
Syndrome Clinical symptoms/signsOnset ageGenetics
Childhood or adult onset
lactic acidosis, stroke like
Stroke-like episodes with
seizures. Variable presence
A minority of patients have
generalized seizures, ataxia,
cardiomyopathy. A minority
of patients have PEO
Sensory axonal neuropathy
with variable degrees of
sensory and cerebellar
ataxia. PEO in 50%.
Epilepsy and dysarthria are
present in some
Axial/proximal myopathy. May
have other features of
Typically ? 40 years of age but
childhood more common
mtDNA point mutations
(m.3243A ? G in 80%)
Myoclonus, epilepsy, and
ragged red fi bres
ChildhoodmtDNA point mutations
(m.8344A ? G most
Adult onsetnDNA mutations ( POLG ,
TWINKLE , OPA1 )
Any age of onset mtDNA point mutations
Congenital or infant-onset
Diffuse myopathy or
Congenital or infantile
weakness, and death within
few years of life. Infantile
COX-defi ciency myopathy
occasionally reverses after
fi rst year of life
Fatal in fi rst year, or
reversible after fi rst year in
Infantile myopathy with
Diffuse myopathy, lactic
(m.14674T ? C) in the
MEMSA?myoclonic epilepsy myopathy sensory ataxia; MIRAS?mitochondrial recessive ataxia syndrome; SANDO?sensory ataxia
neuropathy dysarthria ophthalmoplegia; SCAE?spinocerebellar ataxia with epilepsy.
which represents poor activity of complex IV of the
respiratory chain (and is encoded by both mtDNA
and nDNA genes). However, a low frequency of
COX-defi cient fi bres is a normal fi nding in healthy
aged individuals. In general, the detection of any
COX-defi cient fi bres in individuals ? 50 years of age,
or a higher frequency of COX-defi cient fi bres at any
age ( ? 5%), is strongly suggestive of a mitochondrial
disorder. The identifi cation of COX-defi cient fi bres
is greatly helped by serially staining muscle for COX
followed by succinate dehydrogenase (SDH), which
stains for complex II (and is encoded entirely by
nuclear genes). The demonstration of COX- defi cient,
SDH-positive muscle fi bres is thought to have the
best sensitivity and specifi city for MM (19). The
sub-sarcolemmal accumulation of mitochondria is a
classic feature of MM, and can be demonstrated by
SDH histochemistry (so-called ‘ raggedblue fi bres ’ ),
or the Gomori trichrome stain (so-called ‘ ragged red
fi bres ’ or RRF). Again, a low frequency of RRFs
( ? 5%) can be seen in healthy aged individuals.
However, the detection of RRFs in individuals ? 50
years of age, or ? 5% RRF at any age, is highly sug-
gestive of MM, although even high levels can be sec-
ondary to other pathologies, such as inclusion body
Electron microscopy (EM) may also be performed
on muscle specimens and demonstrate a variety of
abnormalities associated with MM, although these
are rarely specifi c for mitochondrial diseases. These
Table III. Confi rmatory tests for organ dysfunction in mitochondrial myopathy.
Symptom/sign/disorderTests Possible abnormalitiesExamples of treatments
EEG Epileptiform abnormality,
High-signal T2 abnormality not
conforming to vascular
Axonal sensory or sensorimotor
Normal or slightly elevated.
May be very high in CoQ 10
Myopathic changes or normal
Decreased FVC. Apnoeic
episodes during sleep
Abnormalities consistent with
type 2 diabetes mellitus, and/
or hypothyroidism, and/or
May indicate cognitive
Sensorineural-type hearing loss
MRI brain L-arginine a possible therapy
Sensory neuropathy Nerve conduction studiesSymptomatic therapy
PFTs, sleep studies Respiratory failureCPAP or BiPAP
Fasting glucose, glucose
tolerance test, HgBA1c,
TSH, calcium, PTH,
cortisol, synacthen test
agents and/or insulin;
Cognitive dysfunctionMental status testing
Hearing lossAudiography Auditory aids, cochlear
Corrective lenses, surgery for
strabismus or ptosis
Ocular symptoms/signs Ophthalmology referral Oculomotor abnormalities,
optic atrophy, pigmentary
Cricopharyngeal achalasia or
Normal, or elevated
Normal, or elevated
Normal, or elevated protein
Normal, or basal ganglia
calcifi cations ? atrophy
Basal ganglia signal
abnormalities, non-specifi c
white matter abnormalities,
stroke-like lesions, cerebellar
or brain-stem atrophy, or
normal. MR spectroscopy
may demonstrate elevated
DysphagiaSwallowing studies (video
fl uoroscopy or manometry)
Dietary modifi cation
Other general tests
10 G. Pfeffer & P . F . Chinnery
include enlarged pleiomorphic mitochondria and
paracrystalline inclusions. At present EM is thought
to provide minor criteria for the diagnosis of MM
(20). However, EM may provide minor diagnostic
criteria for mitochondrial disease in some patients
with normal histochemistry (9), therefore EM may
contribute additional information in selected cases.
Some caveats to diagnosis with muscle biopsy
should be discussed. Muscle histochemistry and/or
EM may be normal even in the context of genetically
proven mitochondrial syndromes (21), particularly
early in the disease course or when the biochemical
defect does not involve complex IV (COX). Further-
more, for certain MM syndromes (mainly PEO),
unconventional muscle biopsy sites have been stud-
ied for their utility in providing a diagnosis of MM.
These include levator palpebrae superioris (22) and
orbicularis oculi (23,24), which are easily accessible
muscles during corrective ocular surgery for ptosis
in PEO. For patients requiring ocular surgery,
biopsy of ocular muscle may be able to provide a
diagnosis and avoid a separate procedure for limb
muscle biopsy — however, limited information from
healthy controls can limit interpretation from these
Another test available from muscle tissue is
respiratory chain enzyme (RCE) analysis. This test-
ing must be done either on fresh or snap-frozen
muscle samples. RCE is technically diffi cult to per-
form, even in specialist laboratories (19,25,26),
and the results should be interpreted in the context
of the other investigations. Demonstrating a RCE
defect is a crucial diagnostic step in patients with
normal or near-normal muscle histochemistry,
The genetic tests should be guided based on the
muscle biopsy fi ndings, the MM syndrome which is
suspected, and, if present, the inheritance pattern.
Figure 2. Abnormalities on skeletal muscle biopsy in mitochondrial myopathy. Serial sections through vastus lateralis in a patient with
mitochondrial myopathy showing: (A) haematoxylin and eosin, (B) cytochrome c oxidase histochemistry (COX) (note the COX defi cient
fi bres), (C) succinate dehydrogenase histochemistry (SDH) (note the sub-sarcolemmal accumulation of mitochondria analogous to a
ragged red fi bre), and (D) sequential COX-SDH histochemistry showing a mosaic COX defect as seen in patients with mtDNA
As a general rule, mosaic appearance of COX-
negative fi bres suggests a mtDNA mutation (due to
the variable degrees of heteroplasmy between muscle
cells), whereas uniformly decreased COX activity
suggests a nDNA mutation (which would be equally
present in all muscle cells). If only a single respira-
tory chain complex has decreased activity, this sug-
gests a mutation in a structural gene for the relevant
complex, which may be in mtDNA or nDNA, or a
specifi c complex assembly factor in nDNA. How-
ever, these general principles are not invariably true,
because patients with mtDNA depletion may have
isolated complex defi ciencies early in the disease
course. Other characteristic features include the
presence of strongly succinate dehydrogenase-
positive blood vessels (SSVs) seen in patients with
MELAS harbouring m.3243A ? G (27). If the
patient fi ts into a particular clinical syndrome, this
can be helpful in deciding testing, and common
mutations for different phenotypes are listed in
Tables I and II. Cases in which genetic testing may
precede muscle biopsy include syndromes and/or
inheritance history that implicate nDNA mutations,
which may be tested in blood. Characteristic exam-
ples include syndromes caused by POLG mutations
(autosomal dominant or recessive PEO, the ANS,
and hepatocerebral syndromes such as Alpers syn-
drome) (28). The m.3243A ? G mutation is easily
detected in urine, may cause a variety of mitochon-
drial syndromes (MELAS, maternally inherited dia-
betes and deafness, PEO, isolated MM, and
cardiomyopathy), and its mutation load may even
provide prognostic information (29). Elevations of
plasma and urine thymidine are seen in myopathy,
neuropathy, gastro intestinal
(MNGIE) syndrome due to mutations in TP .
If a genetic diagnosis is not reached after elimi-
nating common molecular defects, more extensive
testing should be carried out. This may involve
sequencing the mitochondrial genome and/or known
nuclear disease genes. In the case of mtDNA genome
sequencing, previously described mutations may
be identifi ed in this manner, or novel mutations,
although distinguishing mutations from the high
level of variability in the mtDNA sequence in the
general population is a challenging exercise (30).
Tissues aside from muscle may be biopsied to sup-
port a diagnosis of MM. Skin biopsy is a non-invasive
procedure which is used to obtain fi broblasts for RCE
and DNA for genetic studies. However, the RCE defect
or molecular genetic defect may not be present in fi bro-
blasts in all patients, and as a result this method has
lower sensitivity than muscle biopsy (31). Liver biopsy
is appropriate in selected situations with an important
component of hepatic failure, providing there is no
coagulopathy. In these situations the biopsy is helpful
to exclude other disorders and is a tissue source for
histological, EM, RCE, and DNA analysis.
Diagnosis of MM due to CoQ 10 biosynthetic
defects are made by the demonstration of CoQ 10
defi ciency in muscle tissue and may be supported by
decreased levels in other tissues such as fi broblasts
and white blood cells (12,26). Plasma levels have a
broad reference range and may be normal in this
condition (26). RCE analysis may demonstrate the
combination of either complex I ? III defi ciency or
complex II ? III defi ciency, since these complexes are
CoQ 10 -dependent (16).
Another category of diagnostic tests for MM
includes exercise testing. There are numerous des-
cribed protocols for testing using cycle ergometry or
treadmill exercise (32). The diagnostic usefulness of
these investigations is controversial, given reports
revealing low specifi city (33,34) and sensitivity (35).
Protocols for measuring venous pO 2 during handgrip
testing have demonstrated excellent specifi city, and
for practical purposes these work well as non- invasive
screening tests for MM (36,37).
There is currently no available disease-modifying
therapy for MM. Several agents (mostly nutritional
supplements) have been investigated with double-blind,
placebo-controlled studies. These include carnitine
(38), creatine (39 – 41), CoQ 10 (42,43), cysteine (44),
dichloroacetate (42,45 – 48), dimethylglycine (49), and
the combination of creatine, CoQ 10 , and lipoic acid
(50). None has demonstrated effi cacy in clinical disease
end-points, although numerous non-blinded studies
and case reports have suggested effi cacy. Examples of
treatments with reported benefi t in MM that have not
yet been evaluated in placebo-controlled trials are
summarized in Table IV. Further study is required to
identify whether any of these agents have therapeutic
Although extremely rare, MM caused by CoQ 10
defi ciency will sometimes respond to CoQ 10 supple-
mentation (51,52), therefore a trial on this agent is
appropriate for patients who have a possible pheno-
type of these conditions. Anecdotal and open-labelled
case series report improvements with the CoQ 10 ana-
logue idebenone in mitochondrial myopathy (53 – 55).
Otherwise, due to the lack of available treatments,
numerous experimental treatments are in develop-
ment, and these were reviewed recently (56). These
include the PPAR/PGC-1 α activator bezafi brate,
which increased mitochondrial biogenesis and
delayed the onset of myopathy in transgenic mice
with a COX defect (57), and the mitochondrially
targeted antioxidant MitoQ which has been used
safely in several common human diseases (58).
12 G. Pfeffer & P . F . Chinnery
There has been great interest in exercise pro-
grammes and their benefi t on both biochemical
and clinical end-points in MM. Aerobic (59,60),
endurance (61,62), and resistance (63) training pro-
grammes have been studied. It is currently not clear
whether the benefi ts of exercise in MM are simply
reversing the de-conditioning, which is a common
feature of many muscle diseases, or whether the
exercise affects the underlying pathology. In any
event, evidence is mounting that exercise programmes
are safe and benefi cial for numerous end-points,
including strength, fatigue, and quality of life.
Treatment of MM concentrates on the manage-
ment of disease complication. A diseasecomplica-
tion which is particular to MM is the stroke-like
episodes of MELAS. A non-blinded study of 24
MELAS patients compared L-arginine with placebo
as acute treatment for stroke-like episodes (64).
Symptoms improved 30 minutes and 24 hours after
administration. A portion of the study also followed
six patients on daily treatment with L-arginine for
18 months. The frequency and severity of stroke-like
episodes were signifi cantly decreased. However,
these fi ndings have not been replicated, and the non-
blinded nature of the study may have biased the
results. Recently, a small study suggested a benefi t
of L-arginine in cardiomyopathy due to MM (65),
and so further study of this agent would be of
interest. Finally, status epilepticus was successfully
treated with intravenous magnesium in two teenage
girls with juvenile-onset Alpers syndrome due to POLG
mutations (66). One died 2 weeks after treatment from
pneumonia. The other remained seizure-free 8 months
Other disease complications of MM are due to
dysfunction of various organ systems, which are
important to recognize and treat because they are
potentially preventable causes of death and disability
in MM patients (67). Table III provides a summary
of these as well as examples of possible treatments.
Cardiac dysfunction will be discussed in further
detail because it is common, frequently asymptom-
atic, and potentially fatal. Cardiac dysfunction can
take many forms, namely cardiac conduction defects
and cardiomyopathy. Abnormalities of cardiac con-
duction are common, even among asymptomatic
patients (68), and although they are a central feature
of KSS they may occur in any MM. Cardiomyopathy
may be hypertrophic or dilated and is most com-
monly present in MELAS, MERRF, and KSS (69).
Figures are not available for adults, although one
study in children demonstrated hypertrophic cardio-
myopathy to be present in 17%, often asymptomatic,
and associated with higher mortality than MM
patients without cardiomyopathy (70). Patients with
MM should therefore be screened with 12-lead ECG
and transthoracic echocardiogram irrespective of
whether they are symptomatic for cardiac disease.
Guidelines do not exist as to whether investigations
should be repeated in the event that they are normal,
although repeating investigations every 1 – 3 years may
be reasonable. Other cardiac investigations which
may be considered include Holter monitoring in
patients with symptoms suggestive of arrhythmia.
Cardiac MRI is a new imaging modality, but from
case reports the fi ndings in MM may be characteristic
Endocrinopathy is another common fi nding in
MM patients and is treatable. The classic endocrine
manifestation is diabetes mellitus, which is particu-
larly associated with KSS and MELAS (73). Hypo-
thyroidism may also be a common endocrinopathy
Table IV. Treatments with reported benefi t in mitochondrial myopathy which may benefi t from further study in blinded placebo-controlled
AgentReported benefi t Evidence
High-fat diet with
vitamins and CoQ 10
Improvement on 31 PNMR and symptomaticSingle case reports in complex III defi ciency
Open-label study of 15 paediatric patients (94) Short-term improvement in neurodevelopment,
seizure control, level of consciousness
Biochemical improvements; delayed disease
progression; improvement of respiratory
Reduction in acute symptoms of stroke-like
episodes, reduction of incidence of stroke-like
episodes in MELAS; improvement of TCA
metabolic rate on C11-PET in cardiomyopathy.
Resolution of refractory status epilepticus in
Biochemical improvements. Reduced
encephalopathy and stroke-like episodes in
Improvement of respiratory muscle weakness
decrease in stroke-like episodes
Single case reports (53 – 55)
L-arginine Non-blinded, placebo-controlled study of 24
MELAS patients (64); C11-PET study in
6 patients with MELAS (65)
Magnesium Two patients (66)
Nicotinamide Six-month open label trial of seven MELAS
patients (95), and single case reports (96,97)
Succinate Single case reports (98,99)
in milder syndromes (9), and the clinician should be
aware that any type of endocrine abnormality is pos-
sible, particularly growth hormone defi ciency, which
Respiratory dysfunction can have serious rami-
fi cations (74), although it has received limited study
in MM. Recent clinical series have queried the pres-
ence of respiratory symptoms but found them to
be similar to controls in a large series of MELAS
patients (73), and present in only 1 patient in a
series of 40 PEO patients (9). The only major study
on this subject investigated respiratory parameters
during sleep in eight patients with PEO (75).
Although all had no respiratory symptoms, during
the sleep studies four of the patients had central
apnoeic episodes and/or poor responsivity to CO 2 .
This was postulated to be due to the chronic adap-
tation to respiratory and laryngeal muscle weakness,
or due to a central mechanism. While it is unclear
what effect these asymptomatic respiratory distur-
bances had on these patients, clinicians should have
a low threshold to investigate respiratory- and sleep-
related symptoms in MM patients. Another series
reporting more severe respiratory disturbances sug-
gested a central mechanism that could be episodic
and exacerbated by metabolic stressors, such as
infection or anaesthesia (76).
Other management considerations in MM
include the avoidance of agents which may worsen
the patient ’ s condition. While the list of medications
with theoretical toxicity is massive (77), a few agents
of clinical importance will be discussed here. Statin
medications are thought to cause toxic effects on
skeletal muscle through a disturbance of mitochon-
drial function (78), although the precise mechanisms
remain unclear. Statins appear to cause muscle
symptoms in 10% of patients who receive the drugs
(79), have been reported to unmask symptoms of
metabolic myopathies in patients who were previ-
ously asymptomatic (80), and occasionally these
agents are associated with syndromes resembling
PEO (81,82). They should therefore be used cau-
tiously in MM, with careful monitoring of symptoms
and the serum creatine kinase. Antiretroviral agents
are known to cause reversible and dose-dependent
mitochondrial toxicity (83). If necessary for the
treatment of HIV, it appears that certain agents have
less mitochondrial toxicity (84) and should be used
preferentially. Small series have documented the
development of PEO-like syndromes in patients on
antiretrovirals (85 – 87), although whether this is due
to an unmasking effect or whether the disease is
caused by cumulative mitochondrial toxicity is
unknown. This is especially the case for nucleotide
reverse transcriptase inhibitors. Valproic acid is
known to interfere with mitochondrial function and
in clinical practice can aggravate symptoms in
patients with MM (88), and valproate-induced hepa-
totoxicity may be more common in MM patients
(89,90). Genetic variation in POLG is strongly asso-
ciated with increased risk of hepatotoxicity due to
valproic acid (91).
In summary, the identifi cation of patients with
possible MM depends upon the investigation of
multiple organ dysfunction in the clinical history,
examination, and clinical tests. Although there is no
disease-modifying therapy for MM, there are numer-
ous points of clinical relevance that can reduce mor-
bidity and improve quality of life for patients with
G.P. is the recipient of funding from the Clinician
Investigator Program from the University of British
Columbia, and from a Bisby Fellowship from the
Canadian Institutes of Health Research.
P.F.C. is an Honorary Consultant Neurologist at
Newcastle upon Tyne Foundation Hospitals NHS
Trust. He is a Wellcome Trust Senior Fellow in Clin-
ical Science and a UK NIHR Senior Investigator
who also receives funding from the Medical Research
Council (UK), the Association Fran ç aise contre les
Myopathies, and the UK NIHR Biomedical Research
Centre for Ageing and Age-related disease award to
the Newcastle upon Tyne Foundation Hospitals
Declaration of interest: No competing interests
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