Neuropathy and mitochondrial myopathy

Nerve excitability changes related to muscle weakness in chronic progressive external ophthalmoplegia

OBJECTIVE

To explore potential spreading to peripheral nerves of the mitochondrial dysfunction in chronic progressive external ophthalmoplegia (CPEO) by assessing axonal excitability.

METHODS

CPEO patients (n=13) with large size deletion of mitochondrial DNA and matching healthy controls (n=22) were included in a case-control study. Muscle strength was quantified using MRC sum-score and used to define two groups of patients: CPEO-weak and CPEO-normal (normal strength). Nerve excitability properties of median motor axons were assessed with the TROND protocol and changes interpreted with the aid of a model.

RESULTS

Alterations of nerve excitability strongly correlated with scores of muscle strength. CPEO-weak displayed abnormal nerve excitability compared to CPEO-normal and healthy controls, with increased superexcitability and responses to hyperpolarizing current. Modeling indicated that the CPEO-weak recordings were best explained by an increase in the 'Barrett-Barrett' conductance across the myelin sheath.

CONCLUSION

CPEO patients with skeletal weakness presented sub-clinical nerve excitability changes, which were not consistent with axonal membrane depolarization, but suggested Schwann cell involvement.

SIGNIFICANCE

This study provides new insights into the spreading of large size deletion of mitochondrial DNA to Schwann cells in CPEO patients.

Peripheral neuropathy in patients with CPEO associated with single and multiple mtDNA deletions

Objective:

To characterize peripheral nerve involvement in patients with chronic progressive external ophthalmoplegia (CPEO) with single and multiple mitochondrial DNA (mtDNA) deletions, based on clinical scores and detailed nerve conduction studies.

Methods:

Peripheral nerve involvement was prospectively investigated in 33 participants with CPEO (single deletions n = 18 and multiple deletions n = 15). Clinically, a modified Total Neuropathy Score (mTNS) and a modified International Cooperative Ataxia Rating Scale (mICARS) were used. Nerve conduction studies included Nn. suralis, superficialis radialis, tibialis, and peroneus mot. Early somatosensory evoked potentials were obtained by N. tibialis stimulation.

Results:

Participants with multiple deletions had higher mTNS and mICARS scores than those with single deletions. Electrophysiologically in both sensory nerves (N. suralis and N. radialis superficialis), compound action potential (CAP) amplitudes and nerve conduction velocities were lower and mostly abnormal in multiple deletions than those in single deletions. Early somatosensory evoked potentials of N. tibialis revealed increased P40 latencies and decreased N35-P40 amplitudes in multiple deletions. Both sensory nerves had higher areas under the receiver operating characteristic curves for the decreased CAP amplitudes than the 2 motor nerves. The N. suralis had the best Youden index, indicating a sensitivity of 93.3% and a specificity of 72.2% to detect multiple deletions.

Conclusions:

Peripheral nerve involvement in participants with multiple mtDNA deletions is an axonal type of predominant sensory neuropathy. This is clinically consistent with higher mTNS and mICARS scores. Sensory nerve involvement in participants with multiple deletions was not correlated with age at onset and duration of disease.

Neuromuscular and systemic presentations in adults: diagnoses beyond MERRF and MELAS

Mitochondrial diseases are a diverse group of inherited and acquired disorders that result in inadequate energy production. They can be caused by inheritable genetic mutations, acquired somatic mutations, and exposure to toxins (including some prescription medications). Normal mitochondrial physiology is responsible, in part, for the aging process itself, as free radical production within the mitochondria results in a lifetime burden of oxidative damage to DNA, especially the mitochondrial DNA that, in turn, replicate the mutational burden in future copies of itself, and lipid membranes. Primary mitochondrial diseases are those caused by mutations in genes that encode for mitochondrial structural and enzymatic proteins, and those proteins required for mitochondrial assembly and maintenance. A number of common adult maladies are associated with defective mitochondrial energy production and function, including diabetes, obesity, hyperthyroidism, hypothyroidism, and hyperlipidemia. Mitochondrial dysfunction has been demonstrated in many neurodegenerative disorders, including Alzheimer's disease, Parkinson disease, amyotrophic lateral sclerosis, and some cancers. Polymorphisms in mitochondrial DNA have been linked to disease susceptibility, including death from sepsis and survival after head injury. There is considerable overlap in symptoms caused by primary mitochondrial diseases and those illnesses that affect mitochondrial function, but are not caused by primary mutations, as well as disorders that mimic mitochondrial diseases, but are caused by other identified mutations. Evaluation of these disorders is complex, expensive, and not without false-negative and false-positive results that can mislead the physician. Most of the common heritable mitochondrial disorders have been well-described in the literature, but can be overlooked by many clinicians if they are uneducated about these disorders. In general, the evaluation of the classic mitochondrial disorders has become straightforward if the clinician recognized the phenotype and orders appropriate confirmatory testing. However, the majority of patients referred for a mitochondrial evaluation do not have a clear presentation that allows for rapid identification and testing. This article provides introductory comments on mitochondrial structure, physiology, and genetics, but will focus on the presentation and evaluation of adults with mitochondrial symptoms, but who may not have a primary mitochondrial disease.

Peripheral neuropathy in genetic mitochondrial diseases

Peripheral neuropathy is an underrecognized but common occurrence in genetic mitochondrial disorders. To gain insight into the frequency and clinical presentation of this complication, nerve conduction studies were performed on 43 subjects with congenital lactic acidosis enrolled in a controlled clinical trial of oral dichloroacetate. Median and peroneal motor conduction studies and median and sural sensory conduction studies were performed on each patient. The mean amplitude of the peroneal motor nerve (P < 0.001) and the conduction velocities of the median (P < 0.001) and peroneal (P < 0.001) motor nerves were uniformly lower in our subjects than in healthy literature control subjects. There were no significant differences in sensory nerve conduction studies. A generalized reduction in motor nerve conduction velocity was the dominant electrophysiological abnormality in the patients in this study and was independent of age, sex, or congenital mitochondrial disorder. We postulate that cellular energy failure is the most likely common cause of peripheral neuropathy in patients with genetic mitochondrial diseases, owing to the high demand for adenosine triphosphate via aerobic carbohydrate metabolism by nerve tissue.

Les encéphalomyopathies mitochondriales

Mitochondrial encephalomyopathies include various syndromes involving both muscles and the nervous system. They are characterized by morphological and/or functional mitochondrial abnormalities. Relevant histological modifications in muscle are ragged-red fibers with or without cytochrome C oxidase (COX) activity. Neuropathological alterations in the brain are not specific. They consist of spongiosis with or without preferential involvement of territories of "system degeneration", neuronal loss, focal necrosis, capillary proliferation and mineral deposits. Their topographic patterns are characteristic of each syndrome. Mitochondrial encephalomyopathies are due to defects in mitochondrial DNA, sporadic, with maternal inheritance or defects in nuclear DNA with mendelian inheritance. The first group is more frequent including MERRF, MELAS, KEARNS-SAYRE, and some LEIGH syndromes. LEIGH syndrome is also the most frequent in the second group. However, in accordance with the progress in molecular genetics, these syndromes might be reclassified.

Pathology of mitochondrial encephalomyopathies

Muscle biopsy provides the best tissue to confirm a mitochondrial cytopathy. Histochemical features often correlate with specific syndromes and facilitate the selection of biochemical and genetic studies. Ragged-red fibres nearly always indicate a combination defect of respiratory complexes I and IV. Increased punctate lipid within myofibers is a regular feature of Kearns-Sayre and PEO, but not of MELAS and MERRF. Total deficiency of succinate dehydrogenase indicates a severe defect in Complex II; total absence of cytochrome-c-oxidase activity in all myofibres correlates with a severe deficiency of Complex IV or of coenzyme-Q10. The selective loss of cytochrome-c-oxidase activity in scattered myofibers, particularly if accompanied by strong succinate dehydrogenase staining in these same fibres, is good evidence of mitochondrial cytopathy and often of a significant mtDNA mutation, though not specific for Complex IV disorders. Glycogen may be excessive in ragged-red zones. Ultrastructure provides morphological evidence of mitochondrial cytopathy, in axons and endothelial cells as well as myocytes. Abnormal axonal mitochondria may contribute to neurogenic atrophy of muscle, a secondary chronic feature. Quantitative determinations of respiratory chain enzyme complexes, with citrate synthase as an internal control, confirm the histochemical impressions or may be the only evidence of mitochondrial disease. Biological and technical artifacts may yield falsely low enzymatic activities. Genetic studies screen common point mutations in mtDNA. The brain exhibits characteristic histopathological alterations in mitochondrial diseases. Skin biopsy is useful for mitochondrial ultrastructure in smooth erector pili muscles and axons; skin fibroblasts may be grown in culture. Mitochondrial alterations occur in many nonmitochondrial diseases and also may be induced by drugs and toxins.

Relationship of primary mitochondrial respiratory chain dysfunction to fiber type abnormalities in skeletal muscle

Variation in the size and relative proportion of type 1 and type 2 muscle fibers can occur in a number of conditions, including structural myopathies, neuropathies, and various syndromes. In most cases, the pathogenesis of such fiber type changes is unknown and the etiology is heterogeneous. Skeletal muscle mitochondrial respiratory chain analysis was performed in 10 children aged 3 weeks to 5 years with abnormalities in muscle fiber type, size, and proportion. Five children were classified as having definite, four as probable, and one as possible mitochondrial disease. Type 1 fiber predominance was the most common histological finding (six of 10). On light microscopy, four cases had subtle concomitants of a mitochondriopathy, including mildly increased glycogen, lipid, and/or succinate dehydrogenase staining, and one case had more prominent evidence of underlying mitochondrial disease with marked subsarcolemmal staining. Most cases (nine of 10) had abnormal mitochondrial morphology on electron microscopy. All were found to have mitochondrial electron transport chain (ETC) abnormalities and met diagnostic criteria for mitochondrial disease. We did not ascertain any patients who had isolated fiber type abnormalities and normal respiratory chain analysis during the period of study. We conclude that mitochondrial ETC disorders may represent an etiology of at least a subset of muscle fiber type abnormalities. To establish an etiologic diagnosis and to determine the frequency of such changes in mitochondrial disease, we suggest analysis of ETC function in individuals with fiber type changes in skeletal muscle, even in the absence of light histological features suggestive of mitochondrial disorders.

Prevalence and progression of mitochondrial diseases: a study of 50 patients

We report 50 patients with various clinical phenotypes of mitochondrial disease studied over the past 10 years in a large urban area (Madrid Health Area 5). The clinical phenotypes showed a large variety of abnormalities in molecular biology and biochemistry. The prevalence of mitochondrial diseases was found to be 5.7 per 100,000 in the population over 14 years of age. Clinical and electrophysiological assessment reveal signs of neuropathy in 10 patients. Electromyographic findings consistent with myopathy were obtained in 37 cases. Six patients died of medical complications. Disease phenotype influenced survival to some degree (P < 0.01). Age of onset and gender were not associated with differences in survival. Mitochondrial disease is thus far more common than expected and a common cause of chronic morbidity.

Current therapeutic strategies for patients with polyneuropathies secondary to inherited metabolic disorders

Supportive care, symptomatic treatment, and patient education should be provided for patients with inherited or acquired polyneuropathies. In addition, specific treatment is available for many of the acquired polyneuropathies. Genetic counseling is valuable for many patients with inherited polyneuropathies, but only rarely is specific treatment an option for these patients. However, specific treatments are available for many of the rare and devastating systemic disorders associated with polyneuropathies. Thus, clinicians must promptly diagnose these inherited disorders so that specific treatment may be initiated. The clinical features of these rare inherited disorders are emphasized.

Mitochondrial disease: a pulmonary and critical-care medicine perspective

The clinical spectrum of mitochondrial diseases has expanded dramatically in the last decade. Abnormalities of mitochondrial function are now thought to participate in a number of common adult diseases, ranging from exercise intolerance to aging. This review outlines the common presentations of mitochondrial disease in ICUs and in the outpatient setting and discusses current diagnostic and therapeutic options as they pertain to the pulmonary and critical-care physician.

A mitochondrial encephalo-myo-neuropathy with a nucleotide position 3271 (T-C) point mutation in the mitochondrial DNA

We report three members of a family, who exhibited a phenotype similar to 'myoclonus epilepsy with ragged-red fibers' but had a genotype usually associated with 'mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes'. The patients, a 48-year-old female, and her two sons, aged 21 and 19 respectively, presented with photo-reactive syncopal episodes, disturbances of gait and writing, dysarthria and finger tremor since the 3rd and 2nd decade of life, respectively, that were accompanied also by numbness and weakness of the extremities. Subsequently, cerebellar ataxia and myoclonus were also noted. Electromyography revealed both myogenic and neurogenic muscular changes, and nerve conduction studies demonstrated a sensory-motor neuropathy. Biopsy showed ragged-red fibers with strongly stained SDH-positive vessels in skeletal muscles, and a marked loss of myelinated fibers of the sural nerves. Mitochondrial (mt) DNA analyses of peripheral blood, muscles and nerves revealed that all members had a heteroplasmic np3271 (T-C) point mutation in the mitochondrial tRNA-Leu gene (UUR). This family is unique, in that all patients presented with a myoclonus epilepsy with ragged-red fibers-like phenotype and had a distinctive peripheral neuropathy, while the detected mtDNA 327l (T-C) mutation has been reported to date only in rare cases of mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes

Mitochondrial dysfunction and neuromuscular disease

Mitochondrial diseases are a heterogeneous group of disorders with widely varying clinical features, due to defects in mitochondrial function. Involvement of both muscle and nerve is common in mitochondrial disease. In some cases, this involvement is subclinical or a minor part of a multisystem disorder, but myopathy and neuropathy are a major, often presenting, feature of a number of mitochondrial syndromes. In addition, mitochondrial dysfunction may play a role in a number of classic neuromuscular diseases. This article reviews the role of mitochondrial dysfunction in neuromuscular disease and discusses a rational approach to diagnosis and treatment of patients presenting with a neuromuscular syndrome due to mitochondrial disease.

Electrophysiological study of neuromuscular system involvement in mitochondrial cytopathy

OBJECTIVES

To define the neuromuscular involvement in 'mitochondrial' patients with clinical evidence of a neuromuscular disorder, and to evaluate if the proposed electrophysiological protocol was suitable to reveal a subclinical neuropathy or myopathy in 'mitochondrial' patients with no clinical sign of a neuromuscular disturbance.

METHODS

Quantitative concentric needle electromyography (CNEMG), single fiber electromyography (SFEMG) and nerve conduction studies (NCS) were performed in 33 patients with mitochondrial cytopathies. Lastly, we studied 9 clinically unaffected relatives.

RESULTS

NCS were abnormal in 18% of patients, with CNEMG and SFEMG in 58% of cases, but there was not a complete overlapping of the positivity of the different techniques. No asymptomatic relatives showed abnormalities of the electrophysiological studies.

CONCLUSIONS

Electrophysiological findings did not correlate with any specific biochemical or genetic defect, but were consistent with clinical diagnosis in almost all of the patients with clinical signs of myopathy and/or neuropathy. Increase of both SFEMG jitter and fiber density was significantly tied to a neuropathic process. CNEMG and SFEMG were altered in about 30% of subjects without clinical signs of myopathy or neuropathy and were therefore able to reveal a subclinical involvement of neuromuscular system in some patients who had external ophthalmoplegia or retinitis only.

Impairment of central and peripheral myelin in mitochondrial diseases

Clinical or sub-clinical impairment of central and peripheral myelin is often part of the overlapping multisystem disorders associated with a variety of mitochondrial (mt)DNA abnormalities. Suboptimal energy metabolism of the oligodendrocytes and Schwann cells carrying mitochondrial defects may cause insufficient production of myelin. Further, edema, vascular and toxic factors may directly damage myelin. The recognition that certain mtDNA point mutations are associated with inflammatory demyelination of the central nervous system suggests that additional mechanisms besides degeneration need to be considered in the development of some forms of myelin damage.

Muscle-nerve involvement in autosomal dominant progressive external ophthalmoplegia with hypogonadism

Sixteen members of a family with a history of autosomal dominant progressive external ophthalmoplegia (adPEO) with hypogonadism were examined. The muscular involvement commenced cranially and descended in relation to increasing disease duration. The neuromuscular signs were PEO, dysarthria, dysphonia, limb muscle weakness with wasting, absence of Achilles tendon reflexes, and distal vibration sensory loss. The electromyogram (EMG) was myopathic in facial and proximal limb muscles. Neurogenic involvement was suspected in a few tibial anterior muscles. Neurography showed signs of axonal neuropathy correlated to clinical signs. F-responses were reduced in number or absent in peroneal nerves, and did not correlate to clinical signs or disease duration. Muscle biopsies in advanced cases had structural abnormalities of mitochondria, ragged-red fibers, and focal cytochrome c oxidase deficiency. A combination of muscle-nerve involvement with PEO, Achilles tendon areflexia, distal vibration sensory impairment, myopathic EMG, and abnormally low sural nerve responses seems to be typical of this type of mitochondrial disorder.

Increase of mitochondria in vasa nervorum of cases with mitochondrial myopathy, Kearns-Sayre syndrome, progressive external ophthalmoplegia and MELAS

Previous studies on patients with mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) revealed accumulations of mitochondria in endothelial cells, smooth muscle cells of pial arterioles, and small intracerebral arteries up to 250 microns in diameter; in pericytes of capillaries, endothelial cells, and smooth muscle cells of small blood vessels in skeletal muscle; and according to preliminary results also in endothelial and smooth muscle cells of capillaries and arterioles in sural nerves. These mitochondria do not show the prominent paracrystalline inclusions which are seen in striated muscle fibres and led to the identification of this group of disorders. To corroborate our preliminary findings in peripheral nerves, additional cases have been evaluated morphometrically by electron microscopy including cases in which mitochondrial DNA (mtDNA) mutations have been identified. In fact, an increase of the mean number and an enlargement of the mean cross-sectional area of mitochondria was noted in endothelial and smooth muscle cells of endoneurial and epineurial arterioles, and in endothelial cells and in pericytes of capillaries in sural nerves of the 20 cases with mitochondrial disorders studied. This increase was statistically significant compared to the control group. However, due to heteroplasmia, which is a common feature in mitochondrial disorders, and because of the limited number of measurable blood vessels and cases, no significant differences could be detected between the various types of mitochondrial diseases which were characterized by different point mutations or deletions of mtDNA. Our findings suggest that the mitochondria play a significant role in the pathogenesis not only of myopathic and encephalopathic symptoms, but also in the pathogenesis of peripheral neuropathy which appears to be regularly associated with mitochondrial disorders.

Laryngeal involvement in mitochondrial myopathy

A patient with a slowly progressive mitochondrial myopathy is presented. Mitochondrial myopathies are a diverse group of disorders both clinically and at the cellular level. In common with other neuromuscular disorders, bulbar symptoms may occur. However, though pharyngeal symptoms have been documented in all forms of the disorder, no previous account has described problems at the laryngeal level. We discuss the clinical findings and comment on the therapeutic options.

Mitochondrial myopathy with anemia, cardiomyopathy, and lactic acidosis: a distinct late onset mitochondrial disorder

A 40-year-old woman presented with profound muscle weakness resulting in failure to wean from a ventilator and persistent lactic acidosis after having recovered from a pneumonia complicated by adult respiratory distress syndrome, myocardial infarction, renal failure and shock. She had a 28 year history of chronic anemia and exercise intolerance. Anemia and thrombocytopenia persisted after admission. Nonobstructive hypertrophic cardiomyopathy was present. A stroke-like episode occurred. A mitochondrial myopathy with deficiencies in complexes IV and II was demonstrated, but no DNA defect has yet been found. This patient represents a distinct clinical presentation of a mitochondrial disorder characterized by late onset mitochondrial myopathy, chronic anemia, cardiomyopathy, and lactic acidosis.

Ophthalmoplegia, demyelinating neuropathy, leukoencephalopathy, myopathy, and gastrointestinal dysfunction with multiple deletions of mitochondrial DNA: a mitochondrial multisystem disorder in search of a name

This article describes a 37-year-old woman with progressive external ophthalmoplegia, peripheral neuropathy, and chronic intractable diarrhea. Laboratory studies disclosed lactic acidosis, ragged red fibers lacking cytochrome c oxidase, high-normal muscular mitochondrial enzymes, demyelinating neuropathy, leukoencephalopathy and multiple mitochondrial DNA deletions. This is the fourth patient described with this clinical syndrome, which represents a separate entity among multisystemic mitochondrial disorders. The patient described here is the first with this syndrome to have multiple mitochondrial DNA deletions.

Central and peripheral nervous system conduction in mitochondrial myopathy with chronic progressive external ophthalmoplegia

Involvement of the peripheral and central nervous systems in mitochondrial myopathy with chronic progressive external ophthalmoplegia (CPEO) has been demonstrated clinically and electrophysiologically. Systematic electrophysiological investigations of the peripheral and central nervous systems, particularly of cortico-spinal tract function, however, are not available. We studied peripheral and central nervous system involvement in 28 patients with histologically and biochemically proven mitochondrial CPEO by motor and sensory nerve conduction tests, by somatosensory, auditory and visual evoked potentials and, for the first time, by transcranial magnetic stimulation. Nervous system involvement could be demonstrated in 24 patients, affecting the peripheral and central nervous systems in 18 and 10 patients, respectively. Evidence of cortico-spinal tract involvement was found in 4 patients, which was clinically expected in only 2. Therefore, dysfunction of the cortico-spinal tract in mitochondrial CPEO may occur more frequently than so far assumed. Generally, electrophysiological tests serve as valuable supplements to clinical examination in patients with mitochondrial CPEO and may be especially helpful in therapeutic studies, i.e., coenzyme Q administration.

Neuropathy associated with mitochondrial disorders

Altered mitochondria within peripheral nerves were found in most cases of mitochondrial myopathy, in all cases of hereditary motor and sensory neuropathy with optic atrophy (HMSN VI) and in 25 cases out of a larger series of 280 unselected neuropathies studied by electron microscopy for diagnostic purposes. The mitochondrial changes differed from those seen in the corresponding skeletal muscle fibres. They comprised enlargements with an amorphous matrix and distorted cristae, hexagonal paracrystalline inclusions, sometimes longitudinally arranged in a zig-zag pattern, prominent cristae containing oblique striations and a variety of rare changes. Most mitochondrial abnormalities were found in Schwann cells. An occasional perineurial cell was also involved showing a unique paracrystalline inclusion. An increase of the number of mitochondria was noted in smooth muscle and endothelial cells of epineurial arterioles in three cases of mitochondrial encephalomyopathy (two cases with Kearns Sayre syndrome, and one with mitochondrial encephalomyopathy, lactic acidosis and stroke like episodes, i.e., "MELAS"). Neuropathy was present in all cases of mitochondrial myopathy as judged by morphometric analysis. Whether neuropathy is caused directly by mitochondrial dysfunction or by other pathogenetic mechanisms remains to be determined. Yet peripheral motor and sensory neurons with their peripheral axons are postmitotic, terminally differentiated cells which should be similarly prone to deleterious deletions of mitochondrial DNA as has been suggested as an etiologic factor for the predilection of mitochondrial diseases in muscle and brain.

Ultrastructural changes associated with reduced mitochondrial DNA and impaired mitochondrial function in the presence of 2'3'-dideoxycytidine

Incubation of Molt-4 cells in 4 microM 2'3'-dideoxycytidine did not produce a significant change in the mitochondrial ultrastructure after 4 days; however, by 12 days, the mitochondrial ultrastructure was distorted, with condensed cristae or vacuolization, or both. Concentration-dependent decreases in both cell growth (mean 50% inhibitory concentration, 4.70 +/- 0.5 microM) and mitochondrial DNA content (mean 50% inhibitory concentration, 0.46 +/- 0.06 microM) occurred after incubation with 2'3'-dideoxycytidine for 4 days.

Mitochondrial abnormalities in human sural nerves: fine structural evaluation of cases with mitochondrial myopathy, hereditary and non-hereditary neuropathies, and review of the literature

Fifteen cases of mitochondrial myopathy, three cases of hereditary motor and sensory neuropathy (HMSN) VI, and 280 cases of neuropathies of different etiologies were examined by electron microscopy for the presence of mitochondrial abnormalities in the sural nerve. Altered mitochondrial were found in most cases of mitochondrial myopathy, in all cases of HMSN VI, and in 25 cases out of the series of unselected neuropathies. The mitochondrial changes comprised enlargement with an amorphous matrix and distorted cristae, with hexagonal paracrystalline inclusions, and with prominent cristae containing oblique striations, and a variety of rare changes. Most mitochondrial abnormalities were found in Schwann cells. An increase of the number of mitochondria was noted in smooth muscle and endothelial cells of epineurial arterioles of two cases with mitochondrial encephalomyopathy. Neuropathy was present in all cases of mitochondrial myopathy according to morphometrical analysis. Whether neuropathy is caused directly by mitochondrial dysfunction or by other pathogenetic mechanisms remains to be determined.

Nerve-muscle involvement in a large family with mitochondrial cytopathy: electrophysiological studies

Thirteen patients with mitochondrial cytopathy were investigated. They represent different generations, ages, stages, and severities of the disease. All were assumed to have the same metabolic defect. The disease is a multisystem disorder with a metabolic defect located at complex 1 in the respiratory chain. Clinically, the disorder gives symptoms such as hearing loss, retinal pigmental degeneration, ataxia, cardiomyopathy, muscular fatiguability and neuropathy. The patients were investigated with nerve conduction studies, concentric needle EMG, SFEMG, and macro EMG examinations. Neurophysiologic studies revealed signs of myopathy in both the younger members and in those with slight muscular symptoms. In the more advanced stages, neuropathic changes of the axonal type were seen as well. Macro EMG was interpreted as indicating muscle fiber membrane abnormalities in the early stages. Single fiber EMG studies indicate that this metabolic defect does not disturb neuromuscular transmission.

AAEM minimonograph #34: polyneuropathy: classification by nerve conduction studies and electromyography

Electrodiagnostic evaluation of patients with suspected polyneuropathy is useful for detecting and documenting peripheral abnormalities, identifying the predominant pathophysiology, and determining the prognosis for certain disorders. The electrodiagnostic classification of polyneuropathy is associated with morphologic correlates and is based upon determining involvement of sensory and motor fibers and distinguishing between predominantly axon loss and demyelinating lesions. Accurate electrodiagnostic classification leads to a more focused and expedient identification of the etiology of polyneuropathy in clinical situations.

Familial mitochondrial myopathy associated with peripheral neuropathy: partial deficiencies of complex I and complex IV

Two brothers, 46 and 48 years old, presented with optic atrophy and blepharoptosis since childhood, and later developed muscle wasting and weakness of the extremities, and glove and stocking type sensory impairment. Biopsies of muscles and sural nerves clearly showed mitochondrial myopathy with many ragged-red fibers and peripheral neuropathy with onion-bulb formation. Biochemical studies of muscles disclosed partial deficiencies of complexes I and IV of the mitochondrial respiratory chain in both cases. Since the parents were first cousins, this mitochondrial disorder seemed to be transmitted as an autosomal recessive trait.

Pathological findings of the sural nerve in mitochondrial encephalomyopathy

We examined the muscle and peripheral nerve of a 55-year-old woman with familial mitochondrial encephalomyopathy. In the gastrocnemius muscle, many ragged red fibers and mitochondria containing paracrystalline inclusions in those fibers were observed by light and electron microscopy, respectively. Histopathological studies of the sural nerve revealed a marked decrease in the number of large myelinated fibers. Electron microscopic studies showed an accumulation of glycogen particles and mitochondria containing abnormal, structurally obscure cristae in the Schwann cell cytoplasm. These results suggest that the cause of loss of the large myelinated fibers may be some disturbance of metabolism in the Schwann cells due to mitochondrial dysfunction.

Peripheral neuropathy in mitochondrial disease

Clinical, electrophysiological, histological and biochemical studies of two patients with mitochondrial disease revealed a moderately advanced axonal neuropathy with mitochondrial paracrystalline inclusions in Schwann cells, fibroblasts and muscle fibers. In addition there was a myopathy, and the activity of muscle cytochrome c oxidase was diminished by more than 50%. There were electrophysiological signs of myopathy, neuropathy and failure of excitation-contraction coupling in both patients. The partial enzyme deficiency raises some questions as to its pathogenetic role in these neuromyopathies.

Peripheral neuropathy associated with mitochondrial myopathy

Twenty patients with mitochondrial myopathy were investigated for the presence of peripheral neuropathy. There were clinical features of a mild sensorimotor neuropathy in 5 patients (25%) and nerve conduction studies were abnormal in 10 patients (50%). Electrophysiological studies of the whole group showed significant impairment of motor and sensory conduction, compared with controls. Sural nerve biopsy and morphometric studies were performed on 4 patients with clinical neuropathy. There was a reduction in density of myelinated fibers and electron microscope features of axonal degeneration affecting myelinated and unmyelinated fibers. Abnormal mitochondria containing paracrystalline inclusions were seen in the Schwann cell cytoplasm of two nerves.

Mitochondrial myopathies. A clinico-pathological study of cases with and without extra-ocular muscle involvement

The clinical and pathological features of 28 patients with mitochondrial myopathy were reviewed. The cases were divided into a group with involvement of the extra-ocular muscles alone or with limb muscle involvement, and a group with a facioscapulohumeral syndrome or generalised weakness without extra-ocular muscle involvement. Cardiac and central nervous system manifestations occurred particularly in the first group which included six patients with multisystemic features and two with the complete Kearns-Sayre syndrome. Diabetes mellitus occurred in the second group only. Quantitative histology on limb muscle biopsies showed a higher proportion of fibres with abnormal mitochondrial aggregates in the second group. No one type of mitochondrial inclusion or other ultrastructural change was specific for either group of cases. The findings illustrate the clinical heterogeneity of cases of mitochondrial myopathy and the lack of specificity of any of the myopathological changes for different subgroups of patients.

Mitochondrial cytopathy. A multisystem disorder with ragged red fibres on muscle biopsy

Thirteen children with abnormal mitochondria in muscle tissue, and a progressive neurological disorder that affected the cerebrum, cerebellum, extrapyramidal system, vestibular system, retina, upper motor neuron, lower motor neuron, and musculature, are reported. Other signs and symptoms were short stature, diabetes mellitus, cardiopathy, hypoplastic anaemia, glomerulopathy, and renal tubular dysfunction. These symptoms may occur singly or in various combinations and the manifestation may differ even within the same family. The most common clinical picture was that of "ophthalmoplegia plus'. Occurrence in relatives varied from isolated symptoms to the complete syndrome with "ragged red fibres' and is not inconsistent with an autosomal dominant mode of inheritance with variable expressivity. Theories for the pathophysiological basis of this syndrome are discussed and the literature reviewed.