Lesson 20 Notes: Senses - Pathophysiology and Alterations

Fundamental Mechanisms of Vision

  • Refraction and Light Focus

    • Vision requires light rays to be focused precisely on the retina.

    • Light rays begin to diverge as they travel further from an object; therefore, refraction (bending of light) is necessary to refocus them on the retinal focal point.

    • The Cornea: Provides the majority of the refraction for the eye.

    • The Lens: Provides the remaining refraction necessary for fine-tuned focus.

  • Distance Vision (Emmetropia)

    • Emmetropia refers to the state where the eye is relaxed and focused on distant objects.

    • During distance vision, the lens flattens because the suspensory ligaments stretch.

  • Near Vision (Accommodation)

    • Accommodation is the process of changing the shape of the lens from flattened to rounded to focus on near objects.

    • Pupillary Constriction: The pupil constricts to cover the edges of the lens, which helps focus the light.

    • Convergence: The eyeballs move medially as needed to maintain focus on a closing object.

LASIK: Laser-Assisted In Situ Keratomileusis

  • Purpose and Mechanism

    • LASIK is a surgical procedure that reshapes the cornea to adjust the amount of refraction.

    • The goal is to ensure light is focused on the focal point of the retina at all distances.

  • Conditions Treated by LASIK

    • Hyperopia: Often called farsightedness; near objects appear blurry.

    • Myopia: Often called nearsightedness; distant objects appear blurry.

    • Astigmatism: Occurs when the curvature of the lens or cornea is irregular. This results in light rays being refracted unevenly, causing blurred vision at all distances.

  • The Surgical Procedure

    • Pre-operative: Numbing drops are applied to the eyes. A suction ring is used to hold the eye in its fixed position.

    • Intra-operative: The patient must stare at a light. There is a distinct odor present as the laser removes corneal tissue.

    • Post-operative: The corneal flap heals on its own without stitches. Eye drops are used to maintain comfort.

    • Potential Side Effects: Patients may experience halos, dry eyes, reduced night vision, or regression (return of previous vision issues).

Cataracts

  • Definition and Incidence

    • A cataract is an irreversible cloudy lens.

    • It represents one of the most frequent causes of blindness globally.

  • Etiology (Causes)

    • Trauma to the eye.

    • Exposure to UV radiation.

    • Diseases such as Diabetes.

    • Aging: The most common cause of cataract development.

  • Pathophysiology of Aging Cataracts

    • With age, the lens becomes thicker and significantly less flexible.

    • This physical change causes eye tissue to break down, which eventually clouds the lenses.

  • Treatment

    • The entire clouded lens is surgically replaced with a clear artificial lens known as an Intraocular Lens (IOL).

    • Standard Implant (Monofocal Lens): Typically corrects for distance vision only.

    • Premium Implants (Accommodative or Multifocal Lens): Corrects for both distance and near vision.

    • Minor adjustments may still require glasses or contact lenses post-surgery.

Anatomy and Physiology of Extrinsic Eye Muscles

  • Rectus Muscles

    • Medial Rectus: Moves the eye medially (toward the nose).

    • Lateral Rectus: Moves the eye laterally (toward the temporal bone).

    • Superior Rectus: Elevates the eye superiorly and turns it medially (diagonal move toward the nose).

    • Inferior Rectus: Depresses the eye inferiorly and turns it medially (diagonal move toward the maxilla).

  • Oblique Muscles (Intrinsic/Extrinsic Action)

    • Note: Oblique muscles are attached at an angle and move the eye in the opposite direction of their name.

    • Superior Oblique: Rotates the eye inferiorly and laterally.

      • Origin: Posterior orbit.

      • Insertion: Superolateral eyeball.

    • Inferior Oblique: Rotates the eye superiorly and laterally.

      • Origin: Anterior maxilla.

      • Insertion: Inferior and lateral rectus muscles.

Strabismus and Diplopia

  • Clinical Definition and Pathophysiology

    • Injury to one or more cranial nerves (CN) that innervate the extrinsic eye muscles prevents the eyes from moving in sync.

    • Relevant Cranial Nerves:

      • Oculomotor (CN III)

      • Trochlear (CN IV)

      • Abducens (CN VI)

    • Symptoms include blurred vision or double vision (Diplopia).

  • Strabismus

    • This is the term for diplopia that is present from birth (often called "lazy eye").

  • Treatment Options

    • Patching: Covering the "good eye" with a patch to force the weaker eye to strengthen its muscles.

    • Surgery: Procedures involve removing a part of the affected muscle or repositioning and reattaching muscles to correct alignment.

Olfactory Alterations: Anosmia and Hyposmia

  • Definitions and General Impacts

    • Anosmia: A complete lack of olfaction (sense of smell).

    • Hyposmia: A reduced sensitivity to odors.

    • Risks: Individuals may be unable to detect smoke, gas leaks, or the odor of spoiled food, potentially leading to malnutrition.

  • Health Causes (Often Temporary)

    • Nasal polyps (generally non-tender).

    • Deviated or fractured nasal septum.

    • Swollen turbinates.

    • Swollen respiratory epithelium.

    • Olfaction usually returns once the physical airflow to the olfactory epithelium is restored.

  • Permanent Anosmia and Aging

    • Head Injury: Damage to neural pathways at any point disrupts olfaction. The olfactory nerve is specifically vulnerable to fractures of the cribriform plate.

    • Neural Degeneration: Conditions like Alzheimer’s disease and Parkinson’s disease cause smelling loss.

    • Aging: Hyposmia develops naturally as fewer olfactory neurons are replaced by basal cells; this is especially noticeable after the age of 6060.

  • Treatments (Specialist Interventions)

    • Oral medicines to reduce inflammation.

    • Antibiotics for infections.

    • Allergy management: Allergy testing and regimes using antihistamines or corticosteroids.

    • Nasal surgery.

Auditory Alterations: Tinnitus

  • Definition

    • The sensation of hearing noises (ringing, buzzing, whistling, or hissing) in the absence of an actual external sound.

  • Pathophysiology (Primary Cause)

    • Damage to the stereocilia of cochlear hair cells causes them to become bent or misshapen.

    • These damaged hair cells depolarize enough to spontaneously release neurotransmitters onto the cochlear nerve endings.

  • Secondary Causes

    • Fusion of the middle ear ossicles.

    • Inflammation or irritation of the tympanic membrane.

    • Medications: Large doses of aspirin are known to cause tinnitus.

    • Damage to central nervous system (CNS) neural pathways.

Types of Hearing Loss

  • Conduction Hearing Loss

    • Pathology: Problems in the outer or middle ear prevent sound waves from reaching the inner ear.

    • Causes: Excessive ear wax buildup, middle ear infection, perforated tympanic membrane, or fusion of auditory ossicles.

    • Bone Conduction: In these cases, vibrations can still reach the fluid of the inner ear through the temporal bone.

  • Sensorineural Hearing Loss

    • Neural Hearing Loss: Occurs when signals fail to travel through the cochlear branch of the vestibulocochlear nerve or CNS pathways.

      • Causes: Strokes or tumors such as Acoustic Neuroma (a benign tumor outside the brainstem at the junction of the medulla and pons).

    • Sensory Hearing Loss: Action potentials cannot be generated in the cochlea, typically due to hair cell dysfunction.

      • Causes: Exposure to loud sounds, aging (loss of stereocilia in the proximal cochlea).

      • Pharmacological Cause: Aminoglycoside antibiotics (like gentamicin) act as potassium channel blockers, preventing hair cells from depolarizing.

  • Hearing Loss Treatments

    • Hearing aids.

    • Cochlear implants.