Visual evoked potential importance in the complex mechanism of amblyopia

To compare the visual evoked potential (VEP) responses of amblyopic eyes with VEP responses of sound eyes in amblyopic children. A study of 65 amblyopic children with pattern-reversal VEPs elicited by checkerboard stimuli with large, medium and small checks. The children were classified into three groups: Group A, 22 children with anisometropic amblyopia; Group B, 16 children with exotropic strabismic amblyopia; and Group C, 27 children with esotropic strabismic amblyopia. Visual acuity (VA) was significantly worse in the amblyopic eye as compared to the sound eye. However, no statistically significant difference was found between the amblyopic and sound eye of amblyopic children in the three groups for VEP P1 amplitude and latencies for any check sizes. VEP is a very important tool in understanding the complex amblyopic mechanism. Although the sound eye has superior VA, the absence of differences in VEP P1 amplitudes and latencies demonstrate the functional abnormality of the eye considered ‘good’. More studies are necessary to explain why the sound eye in amblyopic children cannot be considered completely normal. Special attention should therefore be paid to amblyopic treatment, as patching can have a negative effect on the sound eye.

Binocular interactions and steady-state VEPs. A study in normal and defective binocular vision (Part II)

BACKGROUND

Recent evidence indicates that an index of binocular activity may be found in some properties of steady-state visual evoked potentials (VEPs), such as amplitude facilitation and phase shortening. We evaluated binocular interactions with steady-state VEPs in normal subjects as well as in patients with concomitant strabismus and defective binocular vision.

METHODS

Steady-state (8-Hz) VEPs to counterphased sinusoidal gratings (1.2 c/deg spatial frequency) of low contrast (3.2%) were recorded in 19 esotropic patients and in 18 age-matched controls. Patients had either anomalous retinal correspondence (ARC, n = 10) or suppression (n = 9) in casual seeing conditions (striated glasses). In all subjects, both binocular and monocular VEPs displayed a major component at twice the stimulation frequency (second harmonic), whose amplitude and phase were measured. A binocular interaction index was obtained by comparing binocular VEPs (BVEPs) with the sum (vectorial) of the two monocular VEPs (SMVEPs).

RESULTS

In normal subjects, BVEPs were larger in amplitude than SMVEPs (facilitation), and shortened in latency (phase). On average, both ARC and suppression patients displayed loss in amplitude facilitation and absence of phase shortening. However, 50% of ARC patients showed clear VEP facilitation. In both ARC and suppression patients, the amplitude ratio BVEP/SMVEP was negatively correlated with the amount of the angle of deviation.

CONCLUSION

These results suggest that losses in amplitude facilitation and phase shortening of binocular steady-state VEPs reflect abnormal binocular interactions associated with different forms of sensorial adaptation in concomitant strabismus.

Botulinum A toxin effects on rat jaw muscle spindles

Botulinum A toxin (Botox) is used for the treatment of many muscular dystonias. However, the relief of the sustained and abnormal postures induced by Botox administration is not fully explained. In this work the possibility was considered that Botox can produce a block not only at the alpha motor endings, but also at the gamma motor endings, consequently reducing the spindle inflow to the alpha motoneurons, which have a great role in maintaining the tonic myotatic reflex. Jaw muscle spindle discharge was recorded before and after Botox injection in the deep masseter muscle. The drug consistently reduced the spindle afferent discharge. Such an effect is suggested to be direct on gamma endings as: i) muscle tension was not modified by Botox during the recording time; ii) saline administration never changed the spindle discharge. The Botox effect on muscle spindles suggests that the relief from dystonias could be due not only to a partial motor paralysis, but also to a decrease of the reflex muscular tone.

Convergent strabismus fixus in high myopic patients

Patients with high myopia may develop a myopathy which frequently results in a sort of convergent strabismus fixus. Echographic and CT scan findings give evidence that a myopathic paralysis of the lateral rectus is supported by a slow pressure on this muscle squeezed between the lateral orbital wall and the enlarged eyeball.

Effect of botulinum toxin on extraocular muscle proprioception

Injections of botulinum toxin type A (BoTox) in one extraocular muscle (EOM) induce long lasting paretic lengthening of the muscle permitting realignment to occur in strabismus, while eye movements appear to be unaffected after the transitory period of induced paresis. It has been hypothesized a BoTox-induced change in the spindle discharge of EOMs to explain the effect in EOM length. In decerebrate lambs and goats, first order neurons of eye muscle spindles were identified in a cellular pool located in the medial dorsolateral portion of the semilunar ganglion. The belly of the muscle to which the recorded unit belonged was infiltrated with BoTox. A decrease in afferent discharge of the spindle and in its stretch sensitivity was observed. This effect began 10-15 minutes after the injection. There was no corresponding decrease in muscle tension during the first 45 minutes. This finding suggests that the block of release of acetylcholine at motor endings is earlier and more efficacious in gamma- than in alpha-motoneurons. As a result of the proprioceptive input reduction, an unbalance between the agonist and antagonist muscles should occur favouring the ocular realignment.

Simultaneous foveal and parafoveal electroretinograms in hereditary degeneration of the central retina

Simultaneous foveal and parafoveal electroretinograms (ERG) in response to two identical checks (6 degrees per side) alternating at constant mean luminance were recorded in 26 patients (52 eyes) affected by central hereditary chorioretinal diseases and in 14 age-matched normal subjects (14 eyes). Patients were divided into four groups according to clinical diagnoses: 1. Stargardt's disease; 2. cone dystrophy; 3. vitelliform degeneration; 4. pattern dystrophy. The amplitude and latency of the foveal ERG and the amplitude ratio between foveal and parafoveal ERG (F:PF ratio) were measured. The mean foveal ERG amplitude was significantly lower than the control mean in all patient groups. The foveal ERG latency showed a trend to a increase in all pathological groups. However, this difference was not statistically significant. The mean value of F:PF ratio was significantly reduced as compared with the control mean in Stargardt's disease and cone dystrophy only. In 46 of 52 affected eyes (88.5%) at least one of the electrophysiological parameters was abnormal. Our results suggest that the simultaneous foveal and parafoveal ERG recording may be a sensitive technique in hereditary degenerations of the central retina. This method may also contribute to a better understanding of cone degeneration pathophysiology.

Macular electroretinogram as a function of age of subjects

Macular and paramacular electroretinograms in response to two adjacent checks (6 deg/side), alternating at constant mean luminance, were recorded in 34 normal subjects ranging in age from 16 to 74 years. The macular electroretinogram declines progressively in amplitude with age (R = -0.42; P = 0.013). The amplitude ratio between macular and paramacular responses tends to be independent of age (R = -0.21; P = 0.22). Age-related changes in the macular electroretinogram shown in our study are consistent with previous anatomical and functional studies, which indicate a deterioration of photoreceptors beyond 20 years of age. These results suggest a possible use of this technique for future studies on macular degeneration.

Binocular interaction and steady-state visual evoked potentials. I. A study in normal subjects and in subjects with defective binocular vision

A correlate of binocular-neuron activity was found in some properties of visual evoked potentials (VEPs), such as facilitation (defined as a binocular response greater than the sum of the monocular responses) and changes in latency (shortening of binocular VEP latency as compared to that of monocular VEPs). Monocular and binocular steady-state VEPs in response to phase-alternating gratings of different contrast and both spatial and temporal frequency were recorded in three normal subjects. Fourier analysis of the responses was performed to isolate the component at the reversal frequency. Binocular VEPs showed facilitation in the low-contrast range (3%-10%). Facilitation was highest for gratings that had spatial frequency of 0.6-2 cycles/degree (c/d), alternating at 16 reversals per second. Phase shortening was found across a parameter range larger than that at which amplitude facilitation occurred. These results suggest that both amplitude facilitation and phase shortening in binocular VEPs may provide an objective measure of binocular visual function in clinical ophthalmology.

Simultaneously recorded macular and paramacular ERGs in diseases affecting the central retina

Macular and paramacular ERGs in response to two alternating checks (6 deg/side) obtained from a standard TV stimulator were recorded in 34 normal subjects and in 54 patients affected by different macular diseases. Macular ERG amplitude and the amplitude ratio between macular and paramacular ERGs (M/P ratio) were measured. In normals, the intersubjects' variability of macular ERG was comparable to that reported using different stimulation/recording techniques. Macular ERG amplitude (but not the M/P ratio) was significantly affected by age. In patients, the mean amplitude of macular ERG and the mean M/P ratio were significantly lower than normal values. At least one of these two parameters was abnormal in 78.5% of eyes with maculopathy. Both macular ERG amplitude and M/P ratio were significantly correlated with visual acuity. The present results indicate that simultaneous macular and paramacular ERGs can be easily and reliably recorded in clinical routine to evaluate macular function. This technique appears to be particularly sensitive in hereditary macular diseases in which photoreceptors are primarily involved.

Simultaneous macular and paramacular ERGs recorded by standard techniques

A technique for the simultaneous recording of two small-field electroretinograms (ERGs), macular and paramacular, employing standard apparatuses for stimulation and analysis is described. The stimuli consisted of two adjacent checks (6 deg/side) obtained by masking the display of a commercially available TV pattern stimulator. The checks were square-wave-modulated in counterphase at 3.12 Hz. The subjects fixated the center of one of two checks. In one stimulus cycle, two ERGs could be distinguished: one from the macular area and the other from the adjacent area. The macular ERG is about twice as large as the paramacular. ERGs recorded by this technique can be considered focal since (i) they show a sharp fall-off in amplitude when the stimulus is displaced from the fovea, and (ii) they are no longer recordable when the stimulus is centered on the optic disk or on a large macular scar.

Surgical correction of convergent strabismus: its relationship to prism compensation

Esotropic patients whose angle of strabismus has been corrected by prisms frequently increase their angle deviation to compensate for the prismatic correction. This sensorio-motorial reaction to prism correction has been given the name of anomalous movements (a.m.). Quantification of a.m. has been made according to the amount of prisms that an esotropic patient is capable of compensating for (progressive prism compensation test--p.p. test). Some esodeviation does not compensate for any prisms at all since a.m. have not yet developed. Other cases compensate for as much as 40 or 60 prism diopters and more of over-correction of the angle deviation and they therefore have powerful a.m. The interference of these innervational forces acting on the medial recti to corrective surgery has been studied in 126 operated esotropic patients. A significant decrease from the expected surgical result (p less than 0.001) has been found in patients having powerful a.m., as can be judged by the p.p. test. It is believed that a.m. are an important drawback contributing to vitiate any formula on the amount of muscle surgery to be performed in patients having no possibilities of restoring normal binocular vision. Practical advice on how to eliminate this drawback and theoretical reasoning on the significance of a.m. are offered.

Active blockage and rest position nystagmus: electromyographic demonstration of two types of ocular induced head-turn

A differentiation of two types of head-turns due to nystagmus, by means of electromyography (EMG) is demonstrated in this paper. The first type is represented by patients who actively block the nystagmus, by means of an increase of discharge of the extraocular muscles who are synergistic and responsible for the head-turn. This block has the same features of the block of nystagmus in convergence and usually exceeds 10-15 degrees from the primary position. The second type is made out of patients whose head-turn is explained with the null-position of Kestenbaum. Here the nystagmus simply disappears in the position of head-turn, which usually is of no more than 10-15 degrees. Both types of patients show the same electronystagmographic features in the position of head-turn. This differentiation is useful from the clinical standpoint. In fact, only the first type of head-turn may require, besides a classical Anderson or Kestenbaum procedure, also a posterior fixation suture according to Cüppers. This operation would be useless in the second type of head-turn. Clinical signs useful for differentiating these two types of head-turn are presented as well.

Objective evaluation of sensorial and sensorimotorial status in esotropia: their importance in surgical prognosis

Appropriate use of base-out prisms may be useful objective test for detecting persistence of normal binocular vision (4-dioptre prism test). By prolonged observation of prismatic correction of an esotropic patient one may infer the presence of an anomalous sensorial status. This can be done when the prismatic correction is compensated for by an increase of the angle of esotropia (prism adaptation test). The increase in the angle of esotropia induced by base-out prisms, here called anomalous movements, is probably related to a type of anomalous movement fusional in nature. When anomalous movements are present, it is important to realise how powerfully they have developed. This may be inferred by determining what amount of prism overcorrection of the esotropic angle the patient is capable of compensating for (progressive prism compensation test). This has important implications for surgery. It has been statistically demonstrated that esotropia with strong anomalous movements tends to respond less effectively to surgery than esotropia without or with weak anomalous movements.

Plagiocephaly causing superior oblique deficiency and ocular torticollis. A new clinical entity

Unilateral coronal suture stenosis provokes a shortening of the orbital roof of one side. It is argued that the effectiveness of the superior oblique muscle is reduced because of reduced length of its unreflected part and because of the increased angle between the reflected part of the tendon and ocular axis. This condition causes a clinical picture of superior oblique palsy, with ocular torticollis. We report four cases of this newly described clinical entity with various degrees of involvement of ocular motility.

Pattern reversal visually evoked potentials in general anesthesia

Various authors have tried to determine visual acuity both electrophysiologically (Sokol, 1976 and 1978; Marg et al., 1976; Maffei and Fiorentini, 1977) and with psychophysical methods (Teller et al., 1974; Banks and Salapatek, 1976; Atkinson et al., 1976) in awake children. Our experiments try to verify the possibility of assessing visual acuity in children under Ethrane anesthesia. We conclude that under Ethrane anesthesia VEPs produced by phase-alternating square-wave gratings do not allow a fine assessment of visual acuity. Gross indications of visual acuity can however be obtained.

[Areas of anomalous binocular single vision in small angle concomitant esotropia: evaluation by means of a modification of the v. Graefe technique (author's transl)]

A red filter in front of the fixing eye according to the v. Graefe technique was used in order to detect binocular visual field in patients with anomalous retinal correspondence and small angle esotropia. A striated glass was also placed in front of the deviated eye, in order to check whether single perception was related to suppression or to binocular vision. It has been demonstrated that these patients prevalently enjoy an anomalous binocular vision supported by an anomalous retinal correspondence. The suppression scotoma described by the v. Graefe perimetric technique may therefore in certain cases be in reality areas of single anomalous binocular vision. It has also been seen anomalous retinal correspondence is more deeply rooted at the center than at the periphery of the binocular visual field.

Relative photometric measurements of retinal circulation (dromofluorograms): a televison technique

Television fluoroangiography is used in order to obtain fluorescein dilution curves (dromofluorograms [From the Greek word: epsilon rho omicron mu omicron, which means flowing. We coined this word because dilution curves do not afford a true measurement of the blood circulation, or simply of a dilution, but of the fluorescein transport in the circulation!] from single retinal vessels. An optical fiber conveys the light from a single retinal artery, vein, or any other retinal area on the video screen to a photomultiplier. After low-pass electronic filtration, the dilution curves are recorded on a strip-chart recorder. The measurements considered are all related to time: the time interval between initial instants of the arterial and venous curve; the time interval between maximum instants; the arterial and venous curve rising times. It is seen that (1) the method is sensitive enough to enable variations in the quantities in pathologic cases to be detected; (2) owing to the great variability from part to part of the vessels, it is important to specify where the retinal dromofluorogram has been recorded.

Sensorial anomalies in strabismus. (suppression, anomalous correspondence, amblyopia)

Some concepts regarding suppression, anomalous correspondence and amblyopia are revised according to the sensorial findings obtainable from esotropic patients directly in casual seeing (with the aid of the striated glasses test) and by grading a sensorial dissociating effect (with the aid of a bar of optical filters). The following points are emphasized: 1. Suppression appears to be minimal in small angle strabismus where diplopia seems mainly to be avoided by an anomalous correspondence mechanism. On the contrary, suppression is the prevalent mechanism in large angle strabismus. 2. The anomalous correspondence mechanism may lead to a weak type of anomalous binocular vision which is easily interrupted by light optical filters or by dissociating tests. 3. The subjective space of patients with anomalous binocular vision resembles that of normal binocular vision in some aspects. 4. The development of amblyopia is interpreted in the light of these new concepts on suppression and anomalous binocular vision. 5. Postoperatively, anomalous correspondence rapidly re-adapts to the smaller angle deviation and may normalize if the deviation is completely eliminated. This is evident only in casual seeing; for a certain time, dissociating tests reveal the preoperative correspondence status. This behaviour of correspondence in casual seeing has led to attempts at normalizing anomalous correspondence by prism therapy. Newly observed sensotio-motorial obstacles, however, have been found to frequently hamper treatment in casual seeing.