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?

    <p>Superior palpebral furrow</p> Signup and view all the answers

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

    <p>CN VII</p> Signup and view all the answers

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

    <p>An electronegative ERG</p> Signup and view all the answers

    Which statement about Retinitis Pigmentosa (RP) is accurate?

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

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

    <p>To measure the electrical response of the visual cortex to stimuli</p> Signup and view all the answers

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

    <p>Multifocal ERGs</p> Signup and view all the answers

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

    <p>13:1</p> Signup and view all the answers

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

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

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

    <p>Anaerobic glycolysis in the lens and cornea.</p> Signup and view all the answers

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

    <p>Ciliary stromal capillaries</p> Signup and view all the answers

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

    <p>Bicarbonate ions</p> Signup and view all the answers

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

    <p>Development of uveitis.</p> Signup and view all the answers

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

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

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

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

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

    <p>Best disease</p> Signup and view all the answers

    What components are included in the ERG response?

    <p>Responses from photoreceptor cells, and bipolar cells</p> Signup and view all the answers

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

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

    What electrical potential pattern is primarily measured by the EOG?

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

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

    <p>After about 8 minutes.</p> Signup and view all the answers

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

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

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

    <p>Proliferative diabetic retinopathy</p> Signup and view all the answers

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

    <p>Disrupt bicarbonate ion gradients</p> Signup and view all the answers

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

    <p>Fuchs' heterochromic iridocyclitis</p> Signup and view all the answers

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

    <p>Angle recession glaucoma</p> Signup and view all the answers

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

    <p>Uveitic inflammation</p> Signup and view all the answers

    What causes pigment dispersion glaucoma?

    <p>Pigment release from the iris</p> Signup and view all the answers

    Hyphema can impede aqueous outflow primarily by which mechanism?

    <p>Accumulation of blood in the anterior chamber</p> Signup and view all the answers

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

    <p>Pseudoexfoliative glaucoma</p> Signup and view all the answers

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

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

    Which process accounts for the majority of aqueous humor formation?

    <p>Active secretion of charged substances</p> Signup and view all the answers

    How does bicarbonate influence aqueous humor production?

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

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

    <p>It plays a minimal role compared to other mechanisms</p> Signup and view all the answers

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

    <p>They have little to no effect on production</p> Signup and view all the answers

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

    <p>250 µL</p> Signup and view all the answers

    How often is the entire volume of aqueous humor replaced?

    <p>Every 100 minutes</p> Signup and view all the answers

    What effect does blood pressure have on intraocular pressure?

    <p>It has no consistent effect on IOP</p> Signup and view all the answers

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

    <p>Wearing a necktie</p> Signup and view all the answers

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

    <p>It removes metabolic waste</p> Signup and view all the answers

    What condition describes the loss of eyelashes?

    <p>Madarosis</p> Signup and view all the answers

    Which layer of the eyelid contains no fat?

    <p>Skin Layer</p> Signup and view all the answers

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

    <p>Orbicularis oculi</p> Signup and view all the answers

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

    <p>Telecanthus</p> Signup and view all the answers

    Which condition results from a turning inward of eyelashes?

    <p>Trichiasis</p> Signup and view all the answers

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

    <p>Corneoscleral outflow</p> Signup and view all the answers

    How does elevated intraocular pressure affect corneoscleral outflow?

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

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

    <p>Production equals drainage</p> Signup and view all the answers

    Which of the following statements about uveoscleral outflow is accurate?

    <p>It drains through the ciliary stroma into surrounding vessels</p> Signup and view all the answers

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

    <p>Aqueous outflow</p> Signup and view all the answers

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

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

    Where does the superior palpebral elevator muscle originate?

    <p>From the lesser wing of the sphenoid at the orbital apex</p> Signup and view all the answers

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

    <p>The tarsal plate anteriorly</p> Signup and view all the answers

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

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

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

    <p>An infection that is located anterior to the orbital septum</p> Signup and view all the answers

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

    <p>The levator aponeurosis</p> Signup and view all the answers

    How does the orbital septum relate to the lacrimal sac?

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

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

    <p>The levator muscle and main lacrimal gland</p> 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.

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