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

Toward a more personalized motor function rehabilitation in Myotonic dystrophy type 1: The role of neuroplasticity

Myotonic dystrophy type 1 (DM1) is the most prevalent adult muscular dystrophy, often accompanied by impairments in attention, memory, visuospatial and executive functions. Given that DM1 is a multi-system disorder, it requires a multi-disciplinary approach, including effective rehabilitation programs, focusing on the central nervous system neuroplasticity, in order to develop patient-tailored rehabilitative procedures for motor function recovery. Herein, we performed a transcranial magnetic stimulation (TMS) study aimed at investigating central motor conduction time, sensory-motor plasticity, and cortical excitability in 7 genetically defined DM1 patients. As compared to healthy individuals, DM1 patients showed a delayed central motor conduction time and an abnormal sensory-motor plasticity, with no alteration of cortical excitability. These findings may be useful to define patient-tailored motor rehabilitative programs.

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.

Motor excitability in myopathy

OBJECTIVE

To explore whether patients with myopathy present changes in motoneuronal excitability.

METHODS

Patients with well-defined myopathies were studied with single and paired pulse transcranial magnetic stimulations and electrical nerve stimulations to explore neuronal motor excitability. Motor-evoked potentials were recorded from the clinically unaffected first dorsal interosseous muscle (n=10) and the paretic deltoid muscle (n=8).

RESULTS

Compared to an age-matched healthy control group, myopathic patients showed a reduction of intracortical inhibition, enhancements of alpha-motoneuron excitability and increased amplitudes of motor-evoked potentials during target muscle contraction. These alterations were present in clinically affected and clinically unaffected muscles.

CONCLUSION

In myopathy, nervous system excitability may be altered, presenting as a motor disinhibition on cortical and subcortical levels.

Cochlear dysfunction in patients with mitochondrial myopathy

The present study investigates cochlear function in a group of 11 patients suffering from mitochondrial myopathy with normal or near normal audiometric pure tone thresholds, in most of whom diagnosis was histologically confirmed. A complete ENT, neurologic and audiological work-up, including transiently evoked otoacoustic emissions, was performed in all patients in order to estimate cochlear function. Compared to control subjects, most patients had absent otoacoustic emissions (OAE) in spite of normal hearing, indicating cochlear dysfunction. These findings suggest that subclinical involvement of the cochlea is quite common in patients with mitochondrial myopathy. Damage of the cochlea can be explained on the grounds of its increased metabolic rate, resulting in failure of the stria vascularis and the outer hair cells. Otoacoustic emissions might provide a useful tool in the clinical work-up and follow-up of these patients.

Functional involvement of central nervous system in mitochondrial disorders

Thirty-nine patients with mitochondrial diseases were studied with somatosensory and motor evoked potentials. Sixteen patients (41%) had clinical and 12 (31%) had neuroradiological evidence of central nervous system involvement. The overall incidence of electrophysiological abnormalities was 64%. Abnormal evoked potentials were also found in a significant percentage (33%) of patients with pure myopathic forms of mitochondrial diseases and in an asymptomatic carrier of MERRF mutation. Of the individual tests, somatosensory evoked potentials were abnormal in 49% of the patients and motor evoked potentials were abnormal in 46% of the patients. The outcome is that electrophysiological evidence of central nervous system involvement is present in a high percentage of patients with mitochondrial disorders, and that the threshold for central nervous system electrophysiological abnormalities is well below that for clinical and/or radiological manifestations.

Muscle fatigue, lactate, and pyruvate in mitochondrial myopathy with progressive external ophthalmoplegia

We studied muscle fatigue and serum lactate and pyruvate levels in 20 patients with mitochondrial myopathy with progressive external ophthalmoplegia (PEO). Fatigue was assessed in the adductor pollicis muscle (AP) using a low-intensity exercise protocol (20 min). Forces (TFs) and relaxation times of ulnar nerve evoked twitches, compound muscle action potentials (CMAPs), and maximal voluntary contractions (MVCs) were monitored. Serum lactate and pyruvate levels were independently measured at rest and after exercise on a bicycle (15 min, 30 W). Most patients showed abnormal fatigue of the AP with a reduction of TFs and MVCs and normal CMAPs. The reduced TFs were significantly correlated with lactate levels at rest (r= - 0.60, P<0.05) and less so with those after exercise (r=- 0.47,P<0.05). Pyruvate levels revealed a similar correlation although they were widely scattered. We conclude that abnormal fatigue in PEO is metabolic, is localized beyond the muscle fiber membrane, and involves the electrome-chanical coupling and the contractile apparatus. Serum lactate levels at rest are good predictors of fatigue in PEO.

[Chronic progressive external ophthalmoplegia with mitochondrial anomalies. Clinical, histological, biochemical and genetic analysis (9 cases)]

We report the clinical signs and histological findings in nine patients with mitochondrial ocular myopathies. There were four males and five females. Of age ranging from 47 to 82 years. A more often asymetrical ptosis was in all cases of chronic progressive external ophtalmoplegia (CPEO), but muscle weakness in limbs was not usual. The prognosis in this group was good, but ubidecarenone (150 mg/d) used for two cases, did not improve ophtalmoplegia. The serum creatine kinase was normal in eight of nine cases and electromyography showed myopathic changes in three cases. Histoenzymatic analysis of the muscle biopsy and biochemical studies of mitochondria isolated from the muscle sample demonstrated mitochondrial myopathy associated with partial deficiency of complexes I and/or IV of the electron transfer chain. One of seven patients studied had single deletion by Southern blot analysis, in a heteroplasmic state and another an A-->G transition at position 3243 within the mitochondrial tRNA leu (UUR) gene. Chronic progressive external ophtalmoplegia, without large deletion, may have abnormality in other coding regions of mt DNA such as tRNA, rRNA or protein genes.

Evoked potentials in mitochondrial disorders

Twenty-eight adult patients with mitochondrial disease were evaluated with muitimodal evoked potentials (EPs) to assess a possible CNS involvement The patients were classified into five groups: encephalopathy (two cases), progressive external ophthalmoplegia (PEO; four cases), pure myopatby (15 cases), cardiomyopathy (five cases) and asymptomatic relatives (two cases). EPs showed differences between encephalopathy (all EP modalities affected) and PEO groups (all patients with at least one EP altered), with lesser degrees of affection in pure myopathy and cardiomyopathy groups. The asymptomatics registered normal EPs. In view of these results, progressive CNS damage in mitochondriopathies, expressed by abnormal EPs, can be established as follows (from greatest to minor severity): encephalopathy, PEO, pure myopathy/cardiomyopathy and asymptomatic condition.