Basic Science Flashcards

A patient with Duane's Retraction Syndrome Type I would most likely exhibit which primary sign during an ocular motility test?

Answer: Limited abduction (outward movement)

Duane's Retraction Syndrome (DRS) is a congenital disorder of eye movement. Type I, the most common form, is characterized by a marked limitation or absence of abduction (the eye's ability to move outward, toward the ear). This is due to the abnormal development of the 6th cranial nerve, which controls the lateral rectus muscle responsible for abduction.

A patient experiences a severe chemical burn from an alkali substance like sodium hydroxide. The deep and rapid penetration of the chemical into the ocular tissue is primarily due to which chemical process?

Answer: Saponification

Alkali substances are lipophilic and cause saponification, which is the breakdown of fatty acids in cell membranes. This process disrupts the cellular structure, allowing the chemical to penetrate deeply and rapidly into the cornea and anterior chamber, causing extensive damage. In contrast, acids typically cause protein coagulation, which creates a barrier that limits deeper penetration.

Which of the following conditions is defined by a splitting of the retinal layers, particularly the neurosensory retina, rather than a separation of the retina from the underlying retinal pigment epithelium (RPE)?

Answer: Retinoschisis

Retinoschisis is a condition where the retina itself splits into two layers. This is distinct from a retinal detachment, where the neurosensory retina separates from the retinal pigment epithelium (RPE). While both can cause vision loss, the underlying pathophysiology is different. Retinoschisis often creates an absolute visual field defect because the split interrupts the neural pathway.

A patient who has had cataract surgery on only one eye complains of headaches and spatial distortion, stating that objects look larger to them through the operated eye. This binocular vision problem, caused by a difference in perceived image size between the two eyes, is known as:

Answer: Aniseikonia

Aniseikonia is the term for a difference in the perceived size of images between the two eyes. It is a known complication following unilateral cataract surgery or in cases of significant anisometropia, leading to symptoms like headaches, eye strain, and distorted spatial perception. Anisometropia is a difference in refractive error, which can cause aniseikonia, but aniseikonia specifically refers to the image size difference.

A patient with advanced keratoconus experiences significant visual distortion, including monocular diplopia and ghosting of images. This is primarily a result of:

Answer: Higher-order aberrations from an irregular corneal surface

Keratoconus is characterized by the thinning and bulging of the cornea into a cone-like shape. This irregular corneal surface induces significant higher-order aberrations (like coma and trefoil) in addition to regular astigmatism. These complex optical errors scatter and distort light as it enters the eye, preventing it from focusing to a single point on the retina and causing symptoms like ghosting, halos, and monocular diplopia.

Using Prentice's Rule, calculate the amount of vertical prism induced if a patient looks 8 mm below the optical center of a lens with a power of -5.00 D in the 90-degree meridian.

Answer: 4.0 Δ Base Up

Prentice's Rule states: Prism (Δ) = Power (D) x Decentration (cm). First, convert the decentration from millimeters to centimeters: 8 mm = 0.8 cm. Then, apply the formula: Δ = 5.00 D * 0.8 cm = 4.0 Δ. For a minus lens, the base of the induced prism is in the same direction as the decentration. Since the patient is looking down (below the OC), the prism induced is Base Down. However, to correct this, a Base Up prism is required. The question asks for the induced prism, which for a minus lens is Base Up when looking below the optical center. A minus lens is like two prisms apex-to-apex. Looking down through a minus lens induces a base-up effect.