ELECTRO-RETINOGRAPHY / ELECTRO-RETINOGRAM

 

INTRODUCTION:

The electroretinogram (ERG) represents a record of the action potential produced by the retina on stimulation by light of adequate intensity.

 

The test is useful as it provides a fast and non-invasive assessment of the electrophysiology of the retina.

The electroretinography recording is usually made by using an active electrode placed on the cornea or on the skin, just below the lower eyelid and a reference electrode positioned on the forehead of the subject.
 

 

COMPONENTS OF ERG:

The potential between the electrodes is amplified and displayed in the form of a graph. The normal ERG is biphasic. It consists of the following waves:

a-wave: It is the initial fast negative deflection produced by the photoreceptors, with a peak (implicit time) of 100 msec.

b-wave: It is the next slower positive deflection, having a higher amplitude. It is generated from fluxes of potassium ions, within and surrounding Muller cells. However, it is directly dependent on photoreceptor function and is a reflection of photoreceptor integrity.

The amplitude of b-wave is measured from the trough of the a-wave to the peak of b-wave. It increases with dark adaptation, as well as, increased light stimulus. 

 

The b-wave is subdivided into two sub-components:

b-1 represents rod and cone activity

b-2 represents cone activity alone

Rod and cone activity can be singled out by special techniques.

The normal ERG consists of 5 recordings. The first 3 are elicited after 30 minutes of dark adaptation (scotopic), the last 2 after 10 minutes of moderately bright diffuse illumination (photopic). In children it is difficult to dark adapt for 30 minutes, so dim light (mesopic) can be used.

SCOTOPIC ERG	Rod	Elicited with very dim flash of white light or a blue light, resulting in a large b-wave and a small or non-recordable a-wave
	Combined Rod and Cone responses	Elicited with a very bright white flash resulting in a prominent a-wave and a b-wave
	Oscillatory potentials	Elicited by using a bright flash and changing the recording parameters. The oscillatory wavelets occur on the ascending limb of the b-wave and are generated by cells within the inner plexiform layer as a consequence of inhibitory feedback loops involving primarily amacrine cells.

 

PHOTOPIC ERG	Cone responses	They are elicited with a single bright flash, resulting in an a-wave and a b-wave with small oscillations
	Cone flicker	Used to isolate cones by using a flickering light stimulus at a frequency of 30Hz to which rods cannot respond. The amplitude and implicit time of the cone b-wave are measured by this element. Cone responses can be elicited up to 50Hz in normal eyes, beyond which individual responses are no longer recordable (critical flicker fusion).

 

TYPES OF ERG:

The ERG evoked by a flash of light (flash ERG) is affected more by outer retinal elements and is not typically abnormal in glaucoma. Flash ERG is the summed electrical activity of different groups of retinal cells, in response to a flash of light. The primary waveforms in the flash ERG are the a- and b-waves. These arise predominantly from the outer and middle retinal layers respectively. However, the flash ERG is rather a gross method to diagnose or monitor glaucoma, since the condition is characterized by selective loss of retinal ganglion cells (RGCs).

ERG related studies have demonstrated no difference in a-wave, b-wave, and implicit time between POAG and normal subjects. Some studies have demonstrated that these parameters may be affected later in the disease, suggesting involvement of outer or middle retinal layers as the disease progresses.

However, other studies have shown the photopic negative response (PhNR) in primary open glaucoma glaucoma (POAG) could be affected. This is a slow negative potential that follows the b-wave in response to a photopic stimulus and originates from the inner retina.

PhNR amplitudes are found to be consistently smaller in POAG patients and correlate with the mean deviation determined by static perimetry, even with mild visual field sensitivity losses.

Another study has shown that PhNR amplitudes correlate with decrease in function and morphology of retinal neurons in eyes with OAG, indicating that inner retinal function apparently declines proportionately with neural loss in glaucomatous eyes.

The amplitude of the PhNR has been shown to decrease with an increase in visual field defects and is significantly correlated with the retinal nerve fiber layer thickness (RNFLT) and the rim area of the optic disc as well as the cup:disc ratio.

ERGs can also be evoked by providing a stimulus in the form of alternate gratings (e.g. checkerboard stimuli) at a constant mean luminance. This is called Pattern-ERG (PERG). It represents a more focal response from a specific area of the retina, being stimulated. PERG is a direct and objective measure of RGC and optic nerve functions. 

 

A number of studies have reported PERG to have reduced amplitudes in glaucomatous individuals. PERG is found to be abnormal in early glaucoma and those ocular hypertensives who are prone to convert to POAG. This could prove useful in discriminating between those patients with ocular hypertension who will develop visual field loss and those who will not. These abnormal results have been recorded even one year prior to the development of visual field changes suggestive of conversion.

A study has demonstrated that PERG amplitude correlates with RNFL thickness in early glaucoma but not ocular hypertension.

Decreased amplitude and increased peak latency reportedly correlate with increasing age, paralleling the estimated normal loss of ganglion cells. Reduction in PERG is also directly related to histologically defined optic nerve damage in a monkey model.

Multifocal ERG (mfERG) permits simultaneous recording of multiple spatially localized ERGs. mfERG permits topographic display of retinal function and allows for much more detailed assessment of retinal function at specific areas affected by disease.

 

It consists of the same components as a standard ERG. Preliminary studies suggest that it does not appear to correlate well with glaucomatous damage and may be able to detect abnormalities before automated achromatic visual fields can show changes.

MfERG has shown utility in assessing inner retinal function by identification of the optic nerve head component (ONHC), which is of inner retinal origin and is thought to arise in the vicinity of the optic nerve head. This component shows most relevance to the detection of early glaucoma because it has been shown that its propagation time correlates with the length of the ganglion nerve fibers and thus seems to be dependent on the nerve fiber layer.

Studies have demonstrated significant changes in the mfPERG of glaucoma patients, with components of the mfPERG significantly decreased, most distinctly in the central ring, and decreasing further with a progression of glaucoma stage.

Though abnormalities can be readily detected in mfERG recordings from glaucomatous eyes, the advantage of topographic analysis offered by the technique has not proven important for glaucoma diagnosis.

 

CONCLUSION:

While electrophysiological studies such as ERG showed early promise, but the cumbersome instrumentation, lack of understanding and utility of the procedures made this method go into relative obscurity in clinical practice. With newer instruments and studies there is renewing interest in these modalities.

GONIOTOMY

 

INTRODUCTION:

Goniotomy (Gk. gonio—angle and tomein—to cut) is a procedure to open the angle of the anterior chamber by an ab interno technique. The operation was popularized by Otto Barkan in 1938, following development of the gonioscope. The procedure did not do well in adults as it led to scarring of the angle and subsequent failure of the surgery. Barkan utilized it for congenital glaucoma, where it proved relatively successful, compared to the bleak prognosis conventionally associated with this group of diseases.

The steps of the procedure have largely remained unchanged, a testament to the success of the technique.

Barkan surmised that an abnormal membrane (Barkan membrane) obstructed the angle. And goniotomy opens a route for aqueous humor to egress out of the eye through the Schlemm’s canal by removing the tissue obstructing the angle. However, it is now believed that the incision is not through a membrane, but rather through the inner portion of the trabecular meshwork. This presumably relieves the compressive traction of the anterior uvea on the meshwork and eliminates any resistance imposed by incompletely developed inner meshwork. In any case, successful goniotomy does appear, however, to reduce the IOP by improving facility of aqueous outflow.

In cases of corneal opacification, endoscopic visualization has been utilized to open up the angle.

Recently, goniotomy techniques in adults have gained traction with the development of gonioscopy assisted transluminal trabeculotomy and the Kahook Dual Blade.  

https://ourgsc.blogspot.com/search?q=kahook 

INDICATIONS:

Goniotomy is most successful in the treatment of primary congenital glaucoma presenting between 3 and 12 months of age, but it may also be used in other primary developmental and secondary glaucomas, although with reduced success  rates.

Examples of these other primary glaucomas include juvenile open-angle glaucoma and early-onset glaucomas associated with Sturge-Weber syndrome, neurofibromatosis, and Lowe syndrome. Several secondary glaucomas may respond favorably to goniotomy in some cases, including glaucoma complicating chronic anterior uveitis and selected cases of aphakic glaucoma presenting early after congenital cataract surgery.

PRE-OPERATIVE PREPARATION:

Pharmacological therapy is given for a few days to reduce IOP, this may help in clearing the cornea. Medications include oral acetazolamide, topical dorzolamide, apraclonidine 0.5% and beta-blockers. Just prior to surgery the pupil is miosed to prevent inadvertent injury to the lens. This can be done by topical pilocarpine or intra-cameral miochol (Acetylcholine chloride 1:100).  Apraclonidine 0.5%, may be applied to the eye just before surgery and may help decrease intraoperative bleeding.

Topical hypertonic saline (Sodium chloride 5%) helps in clearing the corneal edema and improves visualization.

STEPS OF SURGERY:

1. A low profile wire speculum is preferred to avoid pressure on the globe.
2. The goniotomy lens (Barkan, Lister, Swan-Jacob, Hill or Khaw) is placed on a mound of viscoelastic on the cornea. The microscope is tilted approximately 45-degrees from the vertical. And the child’s head turned towards the surgical side. The patient's head is tilted 30-degrees away from the surgeon to have a proper view of the angle.
3. The globe is stabilized using locking forceps on the insertion of Tenon’s capsule.
4. The eye is entered through peripheral clear cornea 1 mm from the limbus, using a 25-gauge, 1.5-inch needle (goniotomy needle or knife). The knife/needle is kept parallel to the iris. Keeping the knife/needle above the iris, the anterior trabecular meshwork is entered.
5. An effective trabecular meshwork incision is made by keeping the incision superficial and into anterior trabecular meshwork, passing first in one direction, then the other; the assistant rotates the globe while the needle is not engaged in the meshwork.
6. Care should be taken to incise only the anterior third of the trabecular meshwork, just posterior to the Schwalbe’s line.
7. A circumferential incision is made for about 4 to 5 clock-hours.
8. A deeper cleft, with exposure of whiter tissue may be noted in the wake of the incision, with a widening of the angle, and a posterior movement of peripheral iris in some cases.
9. Remove the needle carefully over the iris and have the assistant “relax” any pull on locking forceps at this time.
10. The entry site should be quickly compressed with a cotton bud to minimize chamber collapse.
11. The anterior chamber is refilled with balanced salt and filtered air bubble.
12. The wound is closed with an absorbable 10-0 suture.

POST-OPEARTIVE TREATMENT:

Postoperative treatment includes the use of topical antibiotic, steroid, and miotic agents. (Miotics are often omitted, however, in cases of uveitic glaucoma.) The baby's head should be kept elevated (a car seat works well for this), and the eye should be shielded for 1 to 2 nights, until any hyphema has settled.

COMPLICATIONS:

Mild to moderate hyphemas commonly occur after goniotomy, but they almost always clear rapidly without sequelae over several days.

Other complications after goniotomy are rare and include iridodialysis, cyclodialysis, appearance of small peripheral anterior synechiae in the incised angle, damage to the crystalline lens, and retinal detachment in eyes with high myopia.

Occasionally, a significant amount of blood will reflux into the anterior chamber during the first 72 hours after surgery; when this becomes complicated by an elevated IOP, washout of the blood should be considered.

RESULTS:

The success of goniotomy in controlling glaucoma varies with the cause of the glaucoma. The best results—70% to more than 90% success after one to two procedures—are achieved in infants with primary congenital glaucoma presenting between 3 months and 1 year of age.

Success rates with goniotomy (and angle surgery in general) are much lower for cases of primary congenital-infantile glaucoma presenting at birth or after 12 months of age (success in these groups is usually about 30% to 50%).

Both immaturity of the juxtacanalicular meshwork and/or underdevelopment of Schlemm’s canal or collector channels could explain the failure of goniotomy.

CONCLUSION:

Goniotomy can prove an effective procedure for control of IOP in selected cases. The appeal of this conjunctiva sparing procedure makes it a worthwhile approach in the management of pediatric glaucoma, and is becoming increasingly popular for adult glaucomas.