Electrophysiological studies of the visual system in myotonic dystrophy

The Kohs' blocks test as an important instrument to investigate the visuo-spatial impairments in myotonic dystrophy. Part I. Quantitative and qualitative analysis

This study presents the performance of 39 cases of myotonic dystrophy on Kohs' blocks test (21 females and 18 males, age range from 9 to 70 years). On this test, the patients have to reproduce figures from models previously showed to them. Some of the patients had some kind of professional activity, while others had never exerted a professional occupation. The patients denoted considerable difficulty to perform the test. Some cases constructed entirely different figures in comparison to the presented drawings, translating visuo-spatial and constructional disabilities. The performance was insufficient in 71.4% of the cases. These cases solved less than 50% of the test. The levels of analysis and synthesis were severely impaired. A total of 18 cases got less than 10 points, not reaching 20% of the test. The results showed the sensitivity of this test in detecting visuo-spatial impairment in myotonic dystrophy.

[Longitudinal psychological study in myotonic dystrophy]

The authors studied in two occasions a group of 12 patients with myotonic dystrophy in a mean interval of three years and a half between the examinations. The neuro-psychological battery included the following tests: Raven's progressive matrices (coloured and general scales), Wechsler children intelligence scale (WISC), Kohs' blocks and Piaget-Head. 50% of the patients had better scores on the second examination on RCPM, 81.89% on WISC-digit span, 63.67% on WISC-numbers, 44.44% on Piaget-Head 2 and 60% on Kohs' blocks. However, on Piaget-Head 1-3, the majority had worse results (87.56%) with statistical significative difference (p < 0.05). Though the scores from Kohs' blocks were better in 60% of the patients with p < 0.05, we have to consider that 60% had 0 point on first examination being their scores a little better on second one. This is enough to result on statistical significative difference, however, very low if compared to normal subjects.

Auditory abnormalities, including 'precocious presbyacusis', in myotonic dystrophy

Auditory function tests were performed on 13 patients with myotonic dystrophy (MD). Seven patients had a sensorineural high-frequency hearing loss (HFL) of 30-85 dB at 8 kHz in their pure-tone audiogram, which was in excess of that expected for their age and could be attributed to MD. Their hearing loss resembled 'precocious presbyacusis', i.e. if the patients had been considerably older (or 'functionally' older) than they really were, their HFL could (to some extent) have been attributed to presbyacusis alone. The HFL showed the phenomenon of (genetic) anticipation. Tympanograms and acoustic reflexes were normal. Brainstem auditory evoked potentials (BAEPs) showed a significant increase in the I-V interpeak interval (by 0.35-0.7 ms) and in the III-V interpeak interval (by 0.21-0.67 ms). There was no correlation between the BAEP and the audiometric findings. It should be noted that precocious presbyacusis may be linked to specific gene defects.

Electric and CO2 laser SEPs in a patient with asymptomatic syringomyelia

We recorded electrically stimulated somatosensory evoked potentials (electric SEPs) and pain-related SEPs following CO2 laser stimulation (CO2 laser SEPs) from a 17-year-old patient affected by myotonic dystrophy whose MRI disclosed a large syrinx extending from spinal level C2 to S3. Careful clinical and electromyographic examinations revealed no motor or sensory disturbances, apart from myotonia. The only abnormality noted in median and ulnar nerve short-latency electric SEPs (recorded with a non-cephalic reference electrode) was the absence of cervical component N13, the other SEP responses (N9, N10, N11, P14, N20) being normal. The cutaneous pain threshold and CO2 laser SEPs (both obtained by a CO2 laser beam applied to the back of the hand) were normal. Thus cervical component N13 appears to be highly sensitive to the effects of central cord lesions, even when these are asymptomatic.

Foveal photopigment kinetics--abnormality: an early sign in myotonic dystrophy?

Twelve subjects with minimal expression of the myotonic dystrophy (MyD) gene were investigated by retinal densitometry, a technique which has been used to study the properties of photopigments in the living eye and to detect photoreceptor abnormalities. Other investigations included slit-lamp examination, funduscopy, raleigh matches with the anomaloscope, tonometry, and neurological examination, including electroretinography (ERG) and pattern visual evoked potentials recording. Foveal densitometry demonstrated reduced values of the macular photopigment density difference with normal photopigment kinetics in early phases of the disease, even in asymptomatic individuals. The densitometric values correlated with decreased amplitudes of the photopic ERG a-wave. These findings may be explained by loss or dysfunction of the outer segments of foveal receptors. It is yet unknown whether or not these changes are secondary to other observed neuroretinal abnormalities in MyD. The most likely explanation might be an abnormality of the Na, Ca:K exchanger at the level of the outer segments of the photoreceptors whether or not in combination with a dysfunction of voltage generation systems, involving both photoreceptors and retinal pigment epithelium.

Multimodal evoked potentials in myotonic dystrophy (MyD)

By means of multimodal evoked potentials (EPs), we evaluated the central nervous system (CNS) involvement in 25 subjects suffering from myotonic dystrophy: brainstem auditory evoked potentials (BAEPs), middle-latency auditory evoked potentials (MLAEPs) and somatosensory evoked potentials (SEPs) from the upper limb were performed on all subjects, whereas only the 19 patients, whose clinical ocular abnormalities were slight, underwent pattern-electretinograms (PERGs) and pattern visual-evoked potentials (VEPs) in order to identify the site of lesion among visual pathways (retinal and/or retroretinal). PERGs were abnormal in 8/19 subjects, VEPs in 8/19 subjects (the two techniques were simultaneously abnormal in 8 eyes), BAEPs in 7/25 subjects, MLAEPs in 4/25 subjects (in one subject both BAEps and MLAEPs were abnormal) and SEPs were abnormal in 1/25 subjects. 13/25 of our subjects showed at least one EP that revealed a CNS involvement. The electrophysiological alterations were not correlated either with subject age or with disease duration. Multimodal EPs enabled us to demonstrate that CNS involvement in myotonic dystrophy is important and mainly affects the visual and auditory system.

Diagnostic value of ophthalmologic findings in myotonic dystrophy: Comparison with risks calculated by haplotype analysis of closely linked restriction fragment length polymorphisms

To determine diagnostic value of lens opacities in myotonic dystrophy (DM), we examined 98 at-risk members of 9 DM kindreds. Haplotype analysis of restriction fragment length polymorphisms (RFLPs) using ApoC2, CKMM, and pEFD4.2 supported the diagnosis of DM in 33 and excluded the diagnosis in 51 members. The sensitivities of bilateral iridescent lens opacities, posterior cortical lens opacities, orbicularis oculi weakness, low intraocular pressure, ptosis, and ocular myotonia were 46.7, 50.0, 60.6, 59.3, 51.5, and 3.0%, while their specificities were 100.0, 100.0, 98.0, 94.1, 96.1, and 100.0%, respectively. A peripheral pigmentary degeneration and central macular lesions of retina were not found on indirect fundoscopy. In 86.2% of DM patients, bilateral iridescent lens opacities, posterior cortical lens opacities, or both were present. Unilateral iridescent lens opacities occurred in only 3 of our DM patients, and 2 of non-DM relatives showed a few unilateral iridescent particles. Posterior cortical lens opacities in DM patients always affected both eyes in this series. We conclude that 1) bilateral iridescent lens opacities and posterior cortical lens opacities are highly specific for DM and useful for establishing clinical diagnosis of DM, 2) unilateral iridescent lens opacities are infrequent in DM and are seen in some non-DM members, and 3) ocular myotonia and clinical retinopathies are rare in DM.

Myotonic dystrophy: an electrophysiological study of cognitive deficits

Patients with Myotonic Dystrophy (MyD) frequently suffer from a dysfunction of the primary sensory pathways, as documented by abnormalities of short-latency evoked potentials. Impairment of intellectual functions has been less extensively investigated. Short-latency brainstem auditory evoked potentials (BAEPs) as well as long-latency auditory event-related potentials (ERPs) were recorded from 5 female and 6 male patients affected by MyD. A simple discrimination ("oddball") paradigm was used to record ERPs to tones from Fz, Cz, Pz. Both BAEPs and ERPs were significantly altered as compared to normals. BAEP abnormalities were detected in 9 patients and ERP components N2 and P3 were delayed or absent for all patients, who nonetheless correctly discriminated between tones. These data indicate that CNS dysfunction in MyD involves not only primary sensory systems but also neural mechanisms underlying cognitive events and ERP generation.

Multimodal evoked potentials, EEG and electroretinography in patients with dystrophia myotonica

EEG, somatosensory (SEP) and visual evoked potentials (VEP) and electroretinography (ERG) were recorded from 16 patients with clinical and electromyographically verified dystrophia myotonica. The results were compared to an age- and sex- matched control group and revealed statistically significant differences between the group mean values for almost all records. Furthermore, abnormal individual electrophysiological tests were relatively frequent in the patient group. No correlation was found between abnormalities in one test compared to abnormalities in the other tests. Furthermore, no correlation was found between the number of electrophysiological abnormalities and the frequency of the disease in the nearest family. The number of abnormal electrophysiological tests increased, however, with age of the patients and duration of the disease.