Simultaneous pattern visual evoked potential and pattern electroretinogram in strabismic and anisometropic amblyopia

BACKGROUND

Amblyopia is a relatively common condition in which visual acuity through an eye is subnormal despite no overt pathology. Pattern visual evoked potential (PVEP) can detect any defect from optic nerve to occipital cortex and pattern electroretinogram (PERG) can detect retinal defects specially the ganglion layers. This study was performed to evaluate the cortical and retinal activity in strabismic and anisometropic amblyopia.

METHODS

PVEP and PERG were recorded simultaneously in 40 amblyopes (20 strabismics and 20 anisometropics) and 20 normal control subjects. Normal subjects were age and sex matched with patients.

RESULTS

The P(100) latency in PVEP was increased in both groups of patients but the P(100) amplitude was reduced only in anisometropic group. In PERG, the amplitude of P(50) was reduced in all patients with no significant change in latency.

CONCLUSION

Beside reduced PVEP responses in strabismic and anisometropic amblyopia, the activity of retina reduced too. It is likely that retinal impulses can affect the development of visual system.

Abnormal Cambridge low-contrast grating sensitivity results associated with diabetic retinopathy as a potential screening tool

Contrast sensitivity is proposed as a potential screening tool for the early detection of diabetic retinopathy. A cross-sectional study was performed in a tertiary referral university eye centre. A total of 80 diabetes patients were recruited and tests were performed on 154 eyes. Contrast sensitivity was checked using Cambridge low-contrast grating. Abnormal contrast sensitivity was observed in 27.1% of eyes with diabetic retinopathy, compared with 9.0% in unaffected eyes, a statistically significant difference. Cambridge low-contrast grating is a potential screening tool for early detection of diabetic retinopathy by non-ophthalmologists.

The effects of blockade of retinal cell action potentials on ocular growth, emmetropization and form deprivation myopia in young chicks

To investigate the influence of brain mediated functions on control of ocular growth, young chicks were treated monocularly with intravitreally injected tetrodotoxin (TTX) to block retinal ganglion cell action potentials. TTX injections (0.7 micrograms in 7 microliters) were given on day 6 after hatching in both binocularly open and monocularly deprived chicks. Injections were repeated every 48 hr for a period of 8 days (TTX-open; TTX-MD). Control groups of animals received intravitreally injected phosphate buffered saline (PBS-open; PBS-MD) to one eye on the same schedule. There was a minimum of eight animals in each group. Recovery from form-deprivation myopia during blockade of retinal cell action potentials was also investigated. Results demonstrate that blockade of retinal cell action potentials by TTX produces reduced growth of the anterior segment of the eye and crystalline lens in both binocularly open and MD chicks. Blockade of retinal cell action potentials does not prevent form-deprivation induced vitreous chamber elongation and myopia. Form deprived myopic eyes were found to emmetropize despite blockade of retinal ganglion cell action potentials giving further evidence for local ocular control of emmetropization. Blockade of retinal ganglion cell action potentials did not prevent changes in choroidal thickness in eyes developing axial myopia or eyes recovering from induced myopia.

Experimental myopia in a diurnal mammal (Sciurus carolinensis) with no accommodative ability

1. We examined the functional morphology of the intra-ocular muscles of the grey squirrel using pharmacological and histological methods. Using sympathomimetic (phenylephrine) and parasympathomimetic (carbachol) agents, administered by transcorneal iontophoresis, the response of the iris sphincter and dilator muscles and the ciliary muscle were recorded. Measurement techniques included both streak retinoscopy and coincidence optometry for measurement of ocular refraction and high resolution ultrasonography to monitor changes in the intra-ocular component dimensions. 2. The grey squirrel was found not to possess a functional accommodative system. No change in ocular refraction or intra-ocular dimensions could be induced with 40% carbachol. Marked changes in pupil diameter occurred with topical application of both phenylephrine (dilation) and carbachol (constriction). Histological findings were in agreement with pharmacological findings in showing well developed iris sphincter and dilator muscles but only a poorly developed ciliary muscle. 3. Calculation of the depth of focus of the grey squirrel eye reveals that this could be sufficient to account for the behavioural observations of near viewing habits. 4. We then determined whether we could induce axial elongation of the vitreous chamber and a consequent myopia by monocular deprivation (MD) of pattern vision. 5. Monocular deprivation of pattern vision produced a significant experimental myopia due to axial elongation of the vitreous chamber in the deprived eye. 6. The results demonstrate that a functional accommodative system is not necessary to induce experimental myopia in the grey squirrel eye.

Atropine reduces experimental myopia and eye enlargement via a nonaccommodative mechanism

PURPOSE

To determine whether the muscarinic antagonist atropine effectively reduces or prevents experimentally induced myopia via a nonaccommodative mechanism.

METHODS

Chicks were monocularly deprived (MD) of pattern vision by placement of a translucent occluder over the left eye. In two of the three MD groups, chicks received a series of intravitreal injections of atropine (n = 8) or saline vehicle (n = 8) with MD. Control groups (n = 8) of chicks were employed to assess the effects of MD, intravitreal injections, and drug effects.

RESULTS

In sham-injected or saline-injected MD chicks, 8 days of MD produced -18.5 D and -20.9 D of experimental myopia, respectively. In atropine-injected MD chicks, 8 days of MD produced only -2.8 D of experimental myopia. This significant reduction in experimentally induced myopia in atropine-injected MD chicks was associated with a marked reduction in the relative axial elongation of the deprived eye (0.21 mm) when compared to saline-injected or sham-injected MD chicks (1.04 mm and 1.00 mm). This reduction in axial length in atropine-injected MD chicks was predominantly the result of a reduction in vitreous chamber elongation, although a reduction in anterior segment depth also was observed. Mean equatorial diameter was significantly reduced in atropine-injected MD chicks compared to saline-injected and sham-injected MD chicks, although to a lesser extent. Control experiments demonstrated that intravitreally injected atropine did not reduce carbachol-induced accommodation or light-induced pupil constriction in the skeletal intraocular muscles of the chick eye.

CONCLUSIONS

These findings demonstrate that chronic administration of the muscarinic antagonist atropine prevents experimentally induced myopia in chick via a nonaccommodative mechanism.