Electroretinogram and Visual Evoked Potentials

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Questions and Answers

What role does the muscular structure referred to as Müller's muscle play in eyelid function?

It closes the eyelid completely.

It provides direct insertion into the inferior palpebral furrow.

It causes significant ptosis when innervated by cranial nerve III.

It elevates the upper eyelid by widening the palpebral fissure. (correct)

Which of the following conditions is characterized by a lack of sympathetic innervation leading to mild upper lid ptosis?

CN III palsy

Horner's syndrome (correct)

Ptosis due to aging

Thyroid eye disease

What is the primary function of the inferior tarsal muscle in relation to the lower eyelid?

It causes retraction of the palpebral fissure.

It is responsible for closing the eyelid.

It acts as a minor retractor for the lower eyelid. (correct)

It elevates the lower eyelid significantly.

What anatomical feature is created by the insertion of the levator aponeurosis into the eyelid's skin?

Superior palpebral furrow Signup and view all the answers

What cranial nerve is responsible for the closure of the eye?

CN VII Signup and view all the answers

What does a missing or reduced B-wave in an electroretinogram (ERG) indicate?

An electronegative ERG Signup and view all the answers

Which statement about Retinitis Pigmentosa (RP) is accurate?

Scotopic (rod) ERG abnormalities are present in early RP. Signup and view all the answers

What is the primary purpose of conducting a Visual Evoked Potential (VEP) test?

To measure the electrical response of the visual cortex to stimuli Signup and view all the answers

Which type of ERG is utilized to assess multiple locations within the retina for diagnosing retinal diseases?

Multifocal ERGs Signup and view all the answers

What is the typical contribution ratio of rods to cones concerning B-wave amplitude under dark-adapted conditions?

13:1 Signup and view all the answers

Which statement accurately describes the composition of aqueous humor compared to plasma?

Aqueous humor has a higher concentration of ascorbate than plasma. Signup and view all the answers

What is the main reason for the higher lactate concentration in aqueous humor compared to plasma?

Anaerobic glycolysis in the lens and cornea. Signup and view all the answers

Which part of the blood aqueous barrier is NOT involved in regulating the composition of aqueous humor?

Ciliary stromal capillaries Signup and view all the answers

Which ion concentration is lower in aqueous humor than in plasma?

Bicarbonate ions Signup and view all the answers

What occurs as a result of a breakdown in the blood aqueous barrier?

Development of uveitis. Signup and view all the answers

What does the Arden ratio indicate regarding the health of the RPE?

It is a ratio derived from the comparison of light rise to dark trough. Signup and view all the answers

In the electroretinogram (ERG) testing process, which condition must the patient meet before testing?

The patient should be maximally dilated and dark adapted for a certain period. Signup and view all the answers

Which of the following diseases may the EOG be helpful in diagnosing?

Best disease Signup and view all the answers

What components are included in the ERG response?

Responses from photoreceptor cells, and bipolar cells Signup and view all the answers

What are the designated functions of the blue and red flashes during ERG testing?

Red flash for isolating cone function; blue flash for rod function. Signup and view all the answers

What electrical potential pattern is primarily measured by the EOG?

The difference in electrical charge between the front and back of the eye. Signup and view all the answers

How long does the electrical potential reach its lowest level during the dark adaptation process in the EOG?

After about 8 minutes. Signup and view all the answers

Which option correctly describes the ERG's ability in terms of differentiating retinal diseases?

It is not clinically useful in distinguishing RPE disease but analyzes outer retinal layers. Signup and view all the answers

Which of the following conditions can lead to obstruction of the trabecular meshwork?

Proliferative diabetic retinopathy Signup and view all the answers

What is the effect of carbonic anhydrase inhibitors on aqueous humor production?

Disrupt bicarbonate ion gradients Signup and view all the answers

Which condition is associated with damage to the trabecular meshwork due to chronic inflammation?

Fuchs' heterochromic iridocyclitis Signup and view all the answers

What type of glaucoma may result from trauma causing the separation of the iris from the iris root?

Angle recession glaucoma Signup and view all the answers

Which of the following factors can lead to a significant increase in intraocular pressure (IOP)?

Uveitic inflammation Signup and view all the answers

What causes pigment dispersion glaucoma?

Pigment release from the iris Signup and view all the answers

Hyphema can impede aqueous outflow primarily by which mechanism?

Accumulation of blood in the anterior chamber Signup and view all the answers

Which condition involves accumulation of material that damages the trabecular meshwork primarily due to aging?

Pseudoexfoliative glaucoma Signup and view all the answers

What is the primary function of the Na+/K+ ATPase pump in aqueous humor formation?

To pump Na+ out of non-pigmented ciliary epithelium into the posterior chamber Signup and view all the answers

Which process accounts for the majority of aqueous humor formation?

Active secretion of charged substances Signup and view all the answers

How does bicarbonate influence aqueous humor production?

It enhances the flux of Cl⁻ and Na⁺ into the posterior chamber Signup and view all the answers

What role does diffusion play in the formation of aqueous humor?

It plays a minimal role compared to other mechanisms Signup and view all the answers

What effect do alterations in blood pressure have on aqueous humor formation?

They have little to no effect on production Signup and view all the answers

What is the total volume of aqueous humor in the eye?

250 µL Signup and view all the answers

How often is the entire volume of aqueous humor replaced?

Every 100 minutes Signup and view all the answers

What effect does blood pressure have on intraocular pressure?

It has no consistent effect on IOP Signup and view all the answers

Which of the following conditions can lead to an increase in episcleral venous pressure?

Wearing a necktie Signup and view all the answers

What role does aqueous humor play in the eye aside from maintaining pressure and shape?

It removes metabolic waste Signup and view all the answers

What condition describes the loss of eyelashes?

Madarosis Signup and view all the answers

Which layer of the eyelid contains no fat?

Skin Layer Signup and view all the answers

What is the primary muscle found in the Orbicularis Layer of the eyelid?

Orbicularis oculi Signup and view all the answers

What refers to the abnormally increased distance between the medial canthi of the eyelids?

Telecanthus Signup and view all the answers

Which condition results from a turning inward of eyelashes?

Trichiasis Signup and view all the answers

What is the primary pathway for the majority of aqueous outflow in the eye?

Corneoscleral outflow Signup and view all the answers

How does elevated intraocular pressure affect corneoscleral outflow?

It may cause Schlemm's canal to collapse, hindering outflow Signup and view all the answers

What is the correct relationship between aqueous production and drainage in a healthy eye?

Production equals drainage Signup and view all the answers

Which of the following statements about uveoscleral outflow is accurate?

It drains through the ciliary stroma into surrounding vessels Signup and view all the answers

What does the variable $F_{out}$ represent in the provided summary equation?

Aqueous outflow Signup and view all the answers

What is the primary function of the orbital septum in the eyelids?

To act as a barrier and prevent orbital fat from influencing eyelid margins Signup and view all the answers

Where does the superior palpebral elevator muscle originate?

From the lesser wing of the sphenoid at the orbital apex Signup and view all the answers

What anatomical feature does the levator aponeurosis attach to within the eyelid?

The tarsal plate anteriorly Signup and view all the answers

What role does Whitnall's ligament play in eyelid function?

It acts as a pivot point for the superior palpebral elevator muscle Signup and view all the answers

Which of the following best describes the condition of preseptal cellulitis?

An infection that is located anterior to the orbital septum Signup and view all the answers

Which component of the upper lid's submuscular areolar layer is important for eyelid structure?

The levator aponeurosis Signup and view all the answers

How does the orbital septum relate to the lacrimal sac?

It does not provide protection to the lacrimal sac from infections Signup and view all the answers

What structures run through the upper lid's submuscular areolar layer?

The levator muscle and main lacrimal gland Signup and view all the answers

Study Notes

Electroretinogram (ERG)

ERG measures the electrical activity of the retina in response to light stimuli.
A-wave represents photoreceptor activity.
B-wave represents bipolar and Muller cell activity.
C-wave represents RPE cell activity.
Rods contribute significantly to the B-wave amplitude in dark-adapted conditions.
Cones contribute a smaller portion to the B-wave amplitude in dark-adapted conditions.
Pattern ERGs use complex stimuli to target ganglion cells.
Multifocal ERGs record responses from various retinal locations to identify disease areas.
Retinitis Pigmentosa (RP) is characterized by vessel attenuation, bone-spicule pigmentation, and waxy optic disc pallor.
In early RP, only the scotopic (rod) ERG is abnormal; in late stages, the ERG is completely extinguished due to rod and cone dysfunction.

Visual Evoked Potentials (VEP)

VEP measures the electrical brain response (latency) to visual stimuli.
Wires are placed on the scalp over the primary visual cortex (occipital lobe).
The patient views an alternating checkerboard pattern.
Abrupt pattern changes in the checkerboard stimulus elicit a large positive wave (90-110 milliseconds) after stimulus onset.
Waves peaking later than 110 msec are considered abnormal.

Eyelid

The superior palpebral furrow is formed by the levator aponeurosis insertion into the upper eyelid's skin.
The inferior palpebral furrow is formed by the inferior rectus muscle's indirect attachment to the lower eyelid's skin.

Muscle of Müller

Müller's muscle (superior tarsal muscle) is smooth muscle innervated by the sympathetic nervous system.
It originates on the levator and extends into the tarsal plate.
It slightly widens the palpebral fissure (minor retractor for upper eyelid elevation).
Inferior tarsal muscle (analogous to Müller's muscle) originates from the inferior rectus muscle's fascial sheath and extends into the lower eyelid's tarsal plate.
It's also innervated by the sympathetic nervous system and provides minor lower eyelid retraction.

Conditions Affecting Eyelids

Horner's syndrome causes mild upper lid ptosis and reversed lower lid ptosis due to lack of sympathetic innervation.
CN III palsy leads to significant upper lid ptosis due to lack of motor innervation to the levator.
Thyroid eye disease can cause retraction of the Müller muscle, creating a "stare" appearance.

Gross Electrical Potentials

Normal interpalpebral fissure distance in adults is 10-12 mm.
Cranial nerve III (CN III) opens the eye.
Cranial nerve VII (CN VII) closes the eye.

Electrooculogram (EOG)

EOG measures the electrical potential difference between the front and back of the eye.
EOG analyzes the health of the RPE by examining differences in electrical potentials generated during eye movements under dark-adapted and light-adapted conditions.
Electrodes are attached near the inner and outer canthus of the eye.
The patient performs a series of right and left eye movements, and the electrical potential is recorded for approximately 30 minutes.
The electrical potential is lowest after about 8 minutes of dark adaptation (dark trough) and highest after about 10 minutes of light adaptation (light rise).
The Arden ratio (light peak/dark trough) provides an indication of RPE health.
A normal Arden ratio is greater than 1.8.
An Arden ratio of 1.65-1.80 is considered subnormal.
An Arden ratio less than 1.65 is considered very abnormal.

Electroretinogram (ERG)

ERG records graded potentials produced within the retina in response to light.
It represents the activity of the outer retinal layers (photoreceptors and bipolar cells) but does not include the ganglion cell layer.
The patient is dilated and dark adapted for about 45 minutes before the ERG is performed.
The retina is then stimulated with various rates, wavelengths, and intensities of light.
Testing is done under dark-adapted and light-adapted conditions to isolate cone and rod function.
Rod function is isolated using a blue flash with a slow flicker in a dim background.
Cone function is isolated using a red flash with a fast flicker in a bright background.
The ERG response has three waves.

Aqueous

Active transport of sodium, chloride, and bicarbonate ions creates a gradient for water movement and aqueous humor production.
Oral cardiac glycosides (affecting the Na+/K+ ATPase pump) and carbonic anhydrase inhibitors can disrupt this process.

Factors Influencing Rate of Flow

Covering of the trabecular meshwork (TM) can impede aqueous outflow and may lead to glaucoma.
- Diabetes: Proliferative diabetic retinopathy can lead to neovascularization and fibrous tissue in the angle, causing obstruction of the TM.
- Uveitis: Inflammatory cells can clog the TM, and synechiae can cause angle closure.
- Hyphema: Blunt trauma leading to bleeding in the anterior chamber can impede outflow.
Injury to the TM can also impede aqueous outflow.
- Fuchs' heterochromic iridocyclitis: Chronic inflammation can permanently damage the TM.
- Glaucomatocyclitic crisis: Acute inflammation of the TM leading to a rapid increase in IOP.
- Angle recession glaucoma: Trauma to the iris can cause separation of the iris from the root, damaging the TM.
Occlusions of the TM can impair aqueous outflow.
- Pseudoexfoliative Glaucoma: Aging epithelial cells release material that accumulates within the angle, damaging the TM.
- Pigment Dispersion Glaucoma: Pigment released from the iris accumulates within the angle, damaging the TM.

Aqueous Humor Composition

Aqueous humor has less protein but more amino acids than plasma.
The protein concentration is less than 1% of plasma, minimizing light scattering.
Aqueous humor has high amounts of ascorbate (vitamin C), 20 times higher than in plasma.
Aqueous humor contains more lactate than plasma due to anaerobic glycolysis in the lens and cornea.
Aqueous humor has less bicarbonate ions than plasma and is slightly more acidic (pH = 7.2).

Blood Aqueous Barrier

Aqueous humor formation includes substances passing through the ciliary stroma and then through both ciliary epithelial layers before entering the posterior chamber.
The ciliary stromal capillaries are fenestrated, allowing substances to diffuse out of the vessels.
Tight junctions in the NPCE lining the posterior chamber regulate the substances that form aqueous humor.
The blood aqueous barrier consists of tight junctions in the iris vessels, endothelium of Schlemm's canal, and non-pigmented ciliary epithelium.
Uveitis results from a breakdown in the blood aqueous barrier.

Formation of Aqueous Humor

Aqueous humor is produced and secreted by the non-pigmented ciliary epithelium of the ciliary processes.
Formation involves diffusion, ultrafiltration, and active secretion.

Diffusion

Passive movement of ions across membranes based on ion size and solubility.
Small lipid-soluble substances easily diffuse from the fenestrated capillaries into the ciliary stroma.
Plays a minor role in aqueous humor production.

Ultrafiltration

Passive flow of blood plasma from the capillaries into the ciliary stroma due to increased hydrostatic pressure.
Most substances require active secretion for aqueous humor formation.
Aqueous humor is approximately 5 mOsm hypertonic to the blood.

Active Secretion

Active transport of large, water-soluble, charged substances against an electrochemical gradient.
Requires Adenosine Triphosphate (ATP).
Accounts for 80-90% of total aqueous humor formation.
Alterations in blood pressure have minimal effect on aqueous humor formation.

Key Parts of Active Secretion

Na+/K+ ATPase pump: pumps Na+ out of the NPCE into the posterior chamber, creating a gradient.
Carbonic Anhydrase: catalyzes the reaction that yields bicarbonate: CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻, contributing to increased aqueous production by increasing Cl⁻ and Na⁺ flux into the posterior chamber.

Eyelid Layers

Skin Layer: thin with fine hairs, sweat glands, and sebaceous glands; thinnest skin layer in the body, containing no fat.
Subcutaneous Areolar Layer: thin layer of loose connective tissue between the outer skin and the orbicularis muscle.
Orbicularis Layer: contains the palpebral portion of the orbicularis oculi muscle.

Eyelid Additional Information

Telecanthus: abnormally increased distance between the medial canthi of the eyelids.
Poliosis: whitening of the eyelashes.
Madarosis: loss of eyelashes.
Trichiasis: turning inward of eyelashes, often secondary to entropion.
Phthiriasis palpebrarum: infection of the eyelashes caused by Phthirus pubis.

Submuscular Areolar Layer

Contains the levator aponeurosis and a portion of the lacrimal gland.
The peripheral and marginal arcades are also present in this layer.

Orbital Septum

Acts as a barrier within the upper and lower eyelids.
Prevents orbital fat from falling onto the eyelid margins and helps to contain infections to the anterior portion of the eyelid.
The orbital septum is continuous with the periorbita and periosteum of the skull, attaching medially to the posterior lacrimal crest.
The superior orbital septum acts as an insertion point for the levator aponeurosis.
Preseptal cellulitis is an infection anterior to the orbital septum, while orbital cellulitis occurs posterior to the orbital septum.

Posterior Muscular System

Contains the superior levator palpebrae muscle, and superior (Muller's) and inferior tarsal muscles.
Superior Palpebral Elevator Muscle: Arises from the lesser wing of the sphenoid at the orbital apex, acting as the main muscle to retract the upper eyelid.
Whitnall's ligament acts as a pivot point that allows the levator muscle to move from anterior-posterior to superior-inferior, enabling its function.
Course: Shortly after reaching Whitnall's ligament, the levator muscle forms a fan-shaped tendon (levator aponeurosis) within the eyelid, extending anteriorly through the orbital septum to attach to the skin and the anterior surface of the tarsal plate.
Attachments: The lateral horn of the levator aponeurosis runs across the lacrimal gland, attaching to Whitnall's ligament. The medial horn attaches to the medial palpebral ligament.

Aqueous Outflow

Aqueous outflow rate is approximately 2.5 µL/min.
Total aqueous humor volume is 250 µL, replaced every 100 minutes.
Increased episcleral venous pressure (EVP) increases intraocular pressure (IOP).
Wearing a necktie can compress the external jugular vein, increasing EVP and IOP.
A 5 mmHg increase in EVP results in a 1:1 increase in IOP.
Sturge-Weber syndrome and arteriovenous fistulas can also increase EVP and IOP.

Factors Influencing IOP

Body position: IOP is highest when supine (lying on the back).
Corneal thickness: thicker corneas lead to artificially high IOP readings, while thinner corneas result in artificially low readings.
Blood pressure: no consistent effect on IOP.
Prolonged exercise: can decrease IOP.
Blinking/Eye strain: can increase IOP.
Caffeine: can sometimes cause a temporary rise in IOP.

Aqueous Functions

Maintains eye shape and pressure.
Provides a transparent, colorless refractive index, enhancing vision.
Nourishes eye tissues (cornea, lens, anterior vitreous, TM).
Removes waste products.

Aqueous Volume, Osmolarity, Viscosity

Volume: 250 µL, completely replenished every two hours.
Osmolarity: slightly higher than plasma osmolarity.
Viscosity: 1.025-1.040 compared to water.

Factors Controlling Aqueous Outflow

The amount of aqueous entering the posterior chamber must equal the amount leaving the anterior chamber to maintain the pressure gradient.
Aqueous leaves the anterior chamber through two routes: corneoscleral outflow and uveoscleral outflow.

Corneoscleral Outflow

Drains 2.25 µL/min (80% of aqueous outflow).
Aqueous flows from the anterior chamber across the trabecular meshwork into Schlemm's canal.
The episcleral veins drain aqueous from Schlemm's canal.
The rate of drainage is pressure dependent, increasing with higher intraocular pressure (IOP).
In cases of acutely elevated IOP, Schlemm's canal may collapse, preventing aqueous entry into the venous system.

Uveoscleral Outflow

Drains 0.25 µL/min (20% of aqueous outflow).
Aqueous drains through the ciliary stroma into surrounding venous vessels.
The rate of aqueous outflow is independent of IOP.

Summary Equation for Aqueous Outflow

$F_{out} = Corneoscleral (IOP - EVP) + Uveoscleral$
$F_{out}$: Aqueous outflow
$EVP$: Episcleral venous pressure
The total amount of aqueous drainage is the sum of outflow through the corneoscleral meshwork (pressure dependent) and the uveoscleral meshwork (pressure independent).

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Description

This quiz covers the concepts of Electroretinogram (ERG) and Visual Evoked Potentials (VEP), including their measurement, components, and clinical significance. Learn about the A, B, and C waves of the ERG and how VEP measures brain responses to visual stimuli. Test your knowledge on conditions like Retinitis Pigmentosa and their impact on ERG readings.