Unilateral pursuit-induced congenital nystagmus

Eye stabilization

This chapter summarizes how visual feedback could be used to stabilize the line of sight and optimize vision during attempted fixation of a stationary target. Quantitative features of oculomotor noise that causes drifts of the eye away from the target are analyzed. The sources of such noise, including the ripples in eye position due to muscle fiber twitches, and drifts of the eye away from the visual target due to vestibular imbalance, are examined. Evidence for a promptly responding stabilization system, distinct from optokinetic or pursuit eye movements is reviewed. Smooth eye movements that negate drifts of the eyes, which are discussed here, are distinct from microsaccades, which are discussed in chapter "Behavior of the saccadic system: Metrics of timing and accuracy" by Robinson.

Phenotypical characteristics of idiopathic infantile nystagmus with and without mutations in FRMD7

Idiopathic infantile nystagmus (IIN) consists of involuntary oscillations of the eyes. The familial form is most commonly X-linked. We recently found mutations in a novel gene FRMD7 (Xq26.2), which provided an opportunity to investigate a genetically defined and homogeneous group of patients with nystagmus. We compared clinical features and eye movement recordings of 90 subjects with mutation in the gene (FRMD7 group) to 48 subjects without mutations but with clinical IIN (non-FRMD7 group). Fifty-eight female obligate carriers of the mutation were also investigated. The median visual acuity (VA) was 0.2 logMAR (Snellen equivalent 6/9) in both groups and most patients had good stereopsis. The prevalence of strabismus was also similar (FRMD7: 7.8%, non-FRMD7: 10%). The presence of anomalous head posture (AHP) was significantly higher in the non-FRMD7 group (P < 0.0001). The amplitude of nystagmus was more strongly dependent on the direction of gaze in the FRMD7 group being lower at primary position (P < 0.0001), compared to non-FRMD7 group (P = 0.83). Pendular nystagmus waveforms were also more frequent in the FRMD7 group (P = 0.003). Fifty-three percent of the obligate female carriers of an FRMD7 mutation were clinically affected. The VA's in affected females were slightly better compared to affected males (P = 0.014). Subnormal optokinetic responses were found in a subgroup of obligate unaffected carriers, which may be interpreted as a sub-clinical manifestation. FRMD7 is a major cause of X-linked IIN. Most clinical and eye movement characteristics were similar in the FRMD7 group and non-FRMD7 group with most patients having good VA and stereopsis and low incidence of strabismus. Fewer patients in the FRMD7 group had AHPs, their amplitude of nystagmus being lower in primary position. Our findings are helpful in the clinical identification of IIN and genetic counselling of nystagmus patients.

Congenital abnormal eye movement without gaze nystagmus

Ten cases with congenital abnormal eye movements were reported. Eye movement abnormalities were confined to horizontal pursuit, OKN, and visual suppression. Horizontal saccade and VOR, and vertical eye movements were normal in most cases. Gaze nystagmus was not observed in any case. No serious lesion in the CNS concerning eye movements was detected. Although the cause of abnormal eye movements was not clear, congenital origin was postulated. Similar cases reported in the literature were discussed. These cases may be called "congenital nystagmus without gaze nystagmus."

Magnetic stimulation of the brain in generalized epilepsy: reversal of cortical hyperexcitability by anticonvulsants

Observations on experimental models suggest that diffuse cortical hyperexcitability is an important abnormality in the generalized epilepsies. We used the threshold for transcranial magnetic stimulation as an index of motor cortical excitability in 89 neurologically normal control subjects and 56 patients with idiopathic generalized epilepsy (20 untreated and 36 chronically treated with anticonvulsants). Magnetic stimulation was repeated in 10 patients after valproate monotherapy had been commenced and in 23 control subjects. The threshold intensity was significantly lower in the untreated patients (46 +/- 5% [mean +/- 95% confidence interval]) than in the control subjects (56 +/- 2%). Treated patients had significantly higher thresholds (64 +/- 4%) than did untreated patients and control subjects. A significant increase in threshold intensity (8 +/- 2%) occurred in patients retested after starting valproate; there was no significant change in retested control subjects (-1 +/- 2%). Threshold intensity was positively correlated with plasma valproate levels (rs = 0.37). The findings suggest that cortical excitability is increased in idiopathic generalized epilepsy and is reduced following anticonvulsant treatment. Transcranial magnetic stimulation is of use in examining the pathophysiology of generalized epilepsy. Furthermore, changes in threshold intensity in response to anticonvulsant treatment may prove useful in guiding therapy.

Transcranial magnetic stimulation (TMS) of the brain in patients with mesiotemporal epileptic foci

Transcranial magnetic stimulation (TMS) of the human brain is mainly used for the diagnosis of diseases with disturbed central motor conduction. Recent studies revealed controversial results concerning the possibility of a TMS-induced specific activation of epileptogenic foci in patients with localization-related epilepsies, which would make TMS an additional diagnostic tool for the presurgical localization of the primary epileptogenic zone. We applied TMS to 19 patients with complex-partial seizures and investigated its effects and safety. In 12 patients we performed TMS during scalp electroencephalogram (EEG) recordings. The remaining 7 patients with localization-related epilepsies of mesiobasal limbic seizure origin underwent EEG with additionally implanted foramen-ovale-electrodes (FOE). We did not notice any significant spike activation and even observed bilateral reduction of epileptic activity in some patients. On the contrary, hyperventilation induced a marked activation of the epileptic focus. Our findings support that TMS is safe since adverse effects did not occur. However, due to possible safety hazards, TMS in epileptic patients still requires cautious application until more data will be available.

Upbeat nystagmus: clinicopathological and pathophysiological considerations

We describe an electro-oculographic study of upbeat nystagmus in 4 patients, with neuropathological correlation in one. All patients had lesions in the pontine tegmentum. The electro-oculographic data may be explained by imbalanced vertical vestibular or smooth pursuit eye movement control. The nystagmus stopped or reversed direction during convergence or in supine head positions. We propose that changes in the intensity or direction of upbeat nystagmus that are induced by convergence or changes in head position, are caused by vertical imbalance in the otolithic-ocular reflex, when superimposed on an imbalanced vestibulo-ocular reflex (VOR). Imbalance of the otolithic-ocular reflex and the vertical VOR are caused by damage in the pontomedullary tegmentum.

Magnetic motor-evoked potentials in epilepsy: effects of the disease and of anticonvulsant medication

Magnetic motor-evoked potentials were recorded in 53 patients with medically intractable, mainly temporal lobe epilepsy and compared with potentials of 110 healthy volunteers. The motor-evoked potentials were reevaluated in 16 of the 53 patients after substantial reduction of antiepileptic drug doses. The objective was to assess the effect of epilepsy and of anticonvulsant medication on the central motor system. In subjects receiving antiepileptic treatment, cortical threshold intensities were markedly elevated and peripheral latencies were prolonged. Cortical threshold intensities and peripheral latencies decreased to approach control values after anticonvulsant medication was reduced but were increased in patients treated with 2 or 3 anticonvulsant agents instead of 1. Additionally, high levels of interictal epileptiform activity and a high frequency of seizures significantly decreased the central motor conduction time and, in part, threshold intensities. The central motor conduction time was further diminished after reduction of anticonvulsant treatment and increased when several drugs were administered. The duration of epilepsy, the location of the epileptic focus, and the type of the epileptic seizure did not affect motor-evoked potentials. Conclusively, central motor pathways are endogenously facilitated by epileptiform activity even if clinical signs of their involvement are absent. Anticonvulsant medication exerts major reversible effects on magnetic motor-evoked potentials.

AAEM minimonograph #35: Clinical experience with transcranial magnetic stimulation

We elicited motor evoked potentials (MEPs) using transcortical magnetic stimulation in 150 control subjects aged 14 to 85 years and 275 patients with a variety of diseases. There were no significant side effects. Cortex-to-target muscle latencies measured 20.2 +/- 1.6 ms (thenar), 14.2 +/- 1.7 ms (extensor digitorum communis), 9.4 +/- 1.7 ms (biceps), and 27.2 +/- 2.9 ms (tibialis anterior). Central motor delay between the cortex and the C-7 and L-5 measured 6.7 +/- 1.2 ms and 13.1 +/- 3.8 ms, respectively. Mean spinal cord motor conduction velocity measured 65.4 m/s. MEP amplitude expressed as a percentage of the maximum M wave was never less than 20% of the M wave. A value of less than 10% is considered abnormal. MEP latency increases linearly with age and central motor delay is longer in older subjects. Compound muscle action potentials and absolute MEP amplitudes decreased linearly with age. In multiple sclerosis (MS), MEP latency and central delay were often very prolonged. The MEP was more sensitive than the SEP in MS. In amyotrophic lateral sclerosis, MEP latencies were only modestly prolonged; the characteristic abnormality was reduced amplitude. When pseudobulbar features predominated MEPs were often absent. The MEP was of normal latency in Parkinson's disease, but age-related amplitude was often increased. MEP latency and amplitude were normal in Huntington's disease. Abnormal MEPs persisted several months after stroke despite good functional recovery. The MEP could be used to advantage to demonstrate proximal conduction slowing and block in demyelinating neuropathies. In plexopathy, ability to elicit an MEP several days after onset of paresis was good evidence of neuronal continuity in motor fibers.