Contact Lens Optics — MCQs
How is dioptric power related to focal length?
Which of the following is an example of an incorrect correction for astigmatism?
Rigid gas permeable (RGP) lenses are made from which of the following combinations of materials?
Interstitial keratitis is associated with all of the following except:
Jack in box scotoma is seen after correction of Aphakia by?
Keratometer is used to assess:
Pseudopapilledema with tigroid fundus appearance is seen in?
Slit lamp examination helps in examination of?
A young girl with a previous history of repeated pain over the medial canthus and chronic use of decongestants now presents with intense chills, rigors, and diplopia on lateral gaze. Examination shows an optic disc that is congested. The most likely diagnosis would be:
Which drug does NOT cause optic neuropathy?
Contact Lens Optics Indian Medical PG Practice Questions and MCQs
Question 1: How is dioptric power related to focal length?
- A. Directly to square of focal length
- B. Inversely to focal length (Correct Answer)
- C. Directly to focal length
- D. Inversely to square of focal length
Explanation: ***Inversely to focal length*** - Dioptric power, measured in **diopters**, is defined as the **reciprocal of the focal length** when the focal length is expressed in meters. - This inverse relationship means that a shorter focal length corresponds to a higher dioptric power, indicating stronger light-bending ability. *Directly to square of focal length* - The relationship between dioptric power and focal length is **linear** (inverse), not squared. - There is no direct proportional relationship with the square of the focal length in optical power calculations. *Directly to focal length* - Dioptric power is **inversely proportional** to focal length, not directly proportional. - As focal length increases, the power of the lens to converge or diverge light decreases. *Inversely to square of focal length* - Dioptric power is inversely proportional to the **focal length itself**, not its square. - The square of the focal length is not typically used in defining the dioptric power of a lens.
Question 2: Which of the following is an example of an incorrect correction for astigmatism?
- A. -3 cyl 180
- B. +2 cyl 180
- C. -1.25 cyl 90
- D. -2 spherical 180 (Correct Answer)
Explanation: ***-2 spherical 180*** - This option describes a **spherical lens** correction, which is used for **myopia** (nearsightedness) or **hyperopia** (farsightedness), not astigmatism. - Astigmatism specifically requires a **cylindrical lens** to correct the uneven curvature of the cornea or lens. - A spherical lens has the same power in all meridians and **cannot correct the different refractive powers** in different meridians that characterize astigmatism. *-1.25 cyl 90* - This is a valid correction for astigmatism, indicating a **cylindrical power** of -1.25 diopters at an **axis of 90 degrees**. - The "cyl" notation signifies a cylindrical lens, which is necessary to correct for astigmatism. *-3 cyl 180* - This represents a valid cylindrical correction for astigmatism, with a **cylindrical power** of -3 diopters at an **axis of 180 degrees**. - The presence of "cyl" and an axis indicates a correction specifically designed for astigmatism. *+2 cyl 180* - This is also a valid cylindrical correction for astigmatism, with a **cylindrical power** of +2 diopters at an **axis of 180 degrees**. - Positive cylindrical powers are used for **hyperopic astigmatism**, while negative ones are for **myopic astigmatism**.
Question 3: Rigid gas permeable (RGP) lenses are made from which of the following combinations of materials?
- A. Hydroxymethylmethacrylate
- B. Cellulose acetate Butyrate
- C. Polymethylmethacrylate
- D. Copolymer of PMMA, Silicon containing monomer & cellulose acetyl butyrate (Correct Answer)
Explanation: ***Copolymer of PMMA, Silicon containing monomer & cellulose acetyl butyrate*** - **Rigid gas permeable (RGP) lenses** are designed to be permeable to oxygen, which is achieved through the incorporation of **silicon-containing monomers**. - The combination of **PMMA** (for rigidity), **silicon** (for oxygen permeability), and **cellulose acetyl butyrate** (for improved wettability and flexibility) provides the desired mechanical and optical properties. *Polymethylmethacrylate* - **PMMA** was the primary material for the earliest **hard contact lenses** but offered virtually no oxygen permeability. - This lack of oxygen permeability led to significant corneal hypoxia issues and limited wear time. *Hydroxymethylmethacrylate* - **Hydroxymethylmethacrylate (HEMA)** is a key material in **hydrogel soft contact lenses**, known for its ability to absorb water. - HEMA is not used in RGP lenses because it would make the lens soft and flexible, contrary to the "rigid" characteristic. *Cellulose acetate Butyrate* - **Cellulose acetate butyrate (CAB)** was an early material used for **gas permeable lenses**, offering some oxygen permeability. - While it was an improvement over PMMA, it did not achieve the high level of oxygen permeability seen with newer silicon-containing materials.
Question 4: Interstitial keratitis is associated with all of the following except:
- A. Syphilis
- B. Acanthamoeba (Correct Answer)
- C. Chlamydia Trachomatis
- D. Herpes Zoster Virus (HZV)
Explanation: ***Acanthamoeba*** - **Acanthamoeba keratitis** is a **suppurative keratitis** characterized by a painful, ring-shaped infiltrate with epithelial ulceration, typically associated with contact lens use and contaminated water exposure. - It causes **ulcerative stromal inflammation**, not the **non-ulcerative deep stromal inflammation** that characterizes classic interstitial keratitis. - **This is NOT a cause of interstitial keratitis.** *Syphilis* - **Congenital syphilis** is the **CLASSIC cause** of bilateral **interstitial keratitis**, often presenting in late childhood with "salmon patch" appearance, photophobia, lacrimation, and eventual ghost vessels. - The inflammation is **non-ulcerative and chronic**, affecting the **deep corneal stroma** with preservation of epithelium. - This is the most important association with interstitial keratitis to remember. *Chlamydia Trachomatis* - **Chlamydia trachomatis** causes **trachoma**, a chronic keratoconjunctivitis leading to **superficial keratitis with pannus formation** (superficial vascularization from the limbus). - The corneal involvement in trachoma is **superficial**, not the deep stromal inflammation seen in classic interstitial keratitis. - While listed in some references, **Chlamydia is NOT a standard cause of interstitial keratitis** in major ophthalmology textbooks. - **Note:** This option is potentially debatable, but Acanthamoeba is the more definitively incorrect answer. *Herpes Zoster Virus (HZV)* - **Herpes zoster ophthalmicus** can lead to **interstitial keratitis** and **disciform keratitis** (immune-mediated stromal inflammation with disc-shaped corneal edema). - Similarly, **HSV (Herpes Simplex Virus)** causes stromal keratitis, a form of interstitial keratitis. - The corneal involvement includes **deep stromal inflammation, scarring**, and potential neurotrophic changes leading to vision impairment.
Question 5: Jack in box scotoma is seen after correction of Aphakia by?
- A. IOL
- B. Spectacles (Correct Answer)
- C. Contact lens
- D. None of the options
Explanation: ***Spectacles*** - **Jack-in-the-box scotoma** describes a visual phenomenon where objects appear to jump into and out of the field of vision. This occurs due to the **peripheral scotoma** and **ring scotoma** created by high-plus aphakic spectacle lenses. - Aphakic spectacles cause significant **magnification of the central visual field** (about 25-30%) and a corresponding minification/displacement of the peripheral field, leading to areas where objects are transiently obscured or reappear. *IOL* - An **intraocular lens (IOL)** replaces the natural lens within the eye, providing a much more stable and centered optical correction. - IOLs generally do not cause significant magnification changes or the peripheral scotoma associated with aphakic spectacles. *Contact lens* - **Contact lenses** sit directly on the cornea, offering a visual correction that is much closer to the nodal point of the eye than spectacles. - This placement results in less peripheral distortion and magnification compared to spectacles, making jack-in-the-box scotoma unlikely. *None of the options* - As **aphakic spectacles** are known to cause jack-in-the-box scotoma, this option is incorrect.
Question 6: Keratometer is used to assess:
- A. Curvature of lens
- B. Curvature of cornea (Correct Answer)
- C. Thickness of cornea
- D. Diameter of cornea
Explanation: ***Curvature of cornea*** - A **keratometer** (or ophthalmometer) is specifically designed to measure the **radius of curvature** of the **anterior surface of the cornea**. - This measurement is crucial for fitting **contact lenses**, diagnosing **astigmatism**, and planning **refractive surgeries**. *Curvature of lens* - The curvature of the **crystalline lens** inside the eye is not directly measured by a keratometer. - Lens curvature changes with **accommodation** and is assessed more indirectly through an **autorefractor** or during cataract surgery planning with specific formulas. *Thickness of cornea* - The **thickness of the cornea** is measured using a **pachymeter**, not a keratometer. - **Pachymetry** is important for diagnosing conditions like **glaucoma** and evaluating suitability for **refractive surgery**. *Diameter of cornea* - The **diameter of the cornea** (from limbus to limbus) is typically measured using a **ruler or calipers**, not a keratometer. - This measurement is relevant for contact lens fitting and surgical planning.
Question 7: Pseudopapilledema with tigroid fundus appearance is seen in?
- A. Astigmatism
- B. Presbyopia
- C. Hypermetropia
- D. Myopia (Correct Answer)
Explanation: ***Myopia*** - **Pseudopapilledema** with a **tigroid fundus** (tessellated or salt-and-pepper appearance) is characteristically observed in high myopia due to the oblique entry of the **optic nerve** into the globe and thinning of the choroid and retinal pigment epithelium. - The pseudopapilledema is caused by the crowding of axons and glial tissue within the optic disc, giving a raised appearance, and is distinct from true papilledema which involves **optic disc edema** due to increased **intracranial pressure**. - The tigroid fundus results from the visibility of underlying **choroidal vessels** through the attenuated retinal pigment epithelium in the stretched, elongated myopic eye. *Hypermetropia* - **Hypermetropia** (farsightedness) typically presents with a small, compact optic disc, but does not exhibit the specific findings of **pseudopapilledema** or tigroid fundus. - This condition is characterized by the eye being too short or the lens having insufficient power, causing light to focus behind the retina. *Astigmatism* - **Astigmatism** is characterized by an **irregularly shaped cornea** or lens, leading to blurred vision at all distances. - While it can cause some distortion, it is not associated with the specific optic disc appearance of **pseudopapilledema** or the fundus changes seen in high myopia. *Presbyopia* - **Presbyopia** is an age-related condition where the eye's natural lens loses its flexibility, making it difficult to focus on **near objects**. - It affects the **accommodative ability** of the eye and does not manifest with any characteristic changes in the optic disc morphology such as **pseudopapilledema** or retinal/choroidal changes.
Question 8: Slit lamp examination helps in examination of?
- A. Anterior 2/3rd of choroid
- B. Anterior 1/3rd of choroid
- C. Posterior 1/3rd of choroid
- D. Posterior capsule (Correct Answer)
Explanation: ***Posterior capsule*** - A slit lamp examination is essential for visualizing the **posterior capsule** of the lens, particularly after cataract surgery to detect **posterior capsular opacification (PCO)**. - The high magnification and illumination allow for detailed assessment of its integrity and clarity. *Anterior 2/3rd of choroid* - The choroid is a vascular layer posterior to the retina and lies beneath the retinal pigment epithelium. It is not directly visible via slit lamp and requires more advanced imaging like **fundus camera** or **optical coherence tomography (OCT)** for detailed examination. - The anterior structures visualized by slit lamp are primarily the cornea, iris, lens, and anterior vitreous, not the choroid. *Anterior 1/3rd of choroid* - As with other portions of the choroid, the anterior 1/3rd cannot be directly visualized with a slit lamp due to its posterior location. - Evaluation of the choroid often involves specialized techniques that can penetrate the retina. *Posterior 1/3rd of choroid* - The posterior 1/3rd of the choroid, similar to other parts of the choroid, is not accessible for direct examination using a slit lamp. - Imaging modalities like **OCT** and **fluorescein angiography** are used to assess the posterior choroid.
Question 9: A young girl with a previous history of repeated pain over the medial canthus and chronic use of decongestants now presents with intense chills, rigors, and diplopia on lateral gaze. Examination shows an optic disc that is congested. The most likely diagnosis would be:
- A. Orbital Apex Syndrome
- B. Orbital Cellulitis
- C. Ethmoidal Sinusitis
- D. Cavernous Sinus Thrombosis (Correct Answer)
Explanation: ***Cavernous Sinus Thrombosis*** - The combination of **chills**, **rigors**, **diplopia on lateral gaze** (due to abducens nerve palsy), and a **congested optic disc** points strongly to cavernous sinus thrombosis [1]. - A history of recurrent **medial canthus pain** and **decongestant use** suggests underlying sinusitis, which is a common predisposing factor for this thrombotic event [1]. Rigors specifically represent a rapid rise in body temperature often associated with bacterial infection [2]. *Ethmoidal Sinusitis* - While ethmoidal sinusitis can spread to the orbit, it typically presents with **localized pain**, **tenderness**, and **periorbital swelling**, rather than systemic symptoms like chills, rigors, and diplopia indicating cranial nerve involvement. - A **congested optic disc** is more indicative of increased intracranial pressure or orbital congestion, which is a more severe complication than isolated ethmoidal sinusitis [3]. *Orbital Cellulitis* - **Orbital cellulitis** presents with **proptosis**, **ophthalmoplegia**, **eyelid swelling**, and **erythema**, often with fever. While it can cause diplopia, the intense systemic symptoms (rigors) and a congested optic disc are more suggestive of an intracranial rather than purely orbital process. - It does not typically cause the prominent abducens nerve palsy or the systemic severity seen in cavernous sinus thrombosis without direct spread. *Orbital Apex Syndrome* - **Orbital apex syndrome** involves cranial nerves II, III, IV, V1, and VI, leading to **vision loss**, **ophthalmoplegia**, and **facial numbness**. While it includes diplopia and can affect the optic nerve (leading to congestion), the intense systemic symptoms of **chills** and **rigors** (suggesting widespread infection/sepsis) are less characteristic of orbital apex syndrome itself and more indicative of a direct thrombotic or septic process like cavernous sinus thrombosis.
Question 10: Which drug does NOT cause optic neuropathy?
- A. Chloramphenicol
- B. Penicillin (Correct Answer)
- C. Ethambutol
- D. INH
Explanation: ***Penicillin*** - Penicillin is a widely used antibiotic that is **not associated with optic neuropathy** - Its primary side effects are **allergic reactions and hypersensitivity** - Visual disturbances or optic nerve damage are **not characteristic** of penicillin therapy *Chloramphenicol* - Known to cause **dose-dependent and duration-dependent optic neuropathy**, especially with prolonged use - Can lead to visual impairment, including reduced visual acuity and color vision defects - May be **irreversible** in some cases *Ethambutol* - **Most notorious** antitubercular drug for causing optic neuritis - Causes **dose-dependent bilateral visual loss** and **red-green color blindness** - Requires regular visual monitoring during therapy - Potentially **irreversible** optic nerve damage *INH (Isoniazid)* - Can cause optic neuropathy, though **less frequently** than ethambutol - Usually associated with **high doses** or prolonged therapy - Risk increases in slow acetylators and those with nutritional deficiencies
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