A 32-year-old man comes to the physician after a day of blurred vision in his right eye. He has no pain, ocular discharge, or gritty sensation. On examination, visual acuity is slightly reduced in the right eye. Fluorescein examination shows a large geographic corneal staining defect. Which of the following nerve dysfunctions is most likely responsible for this patient's impaired corneal sensation?

Question

A 32-year-old man comes to the physician after a day of blurred vision in his right eye.  He has no pain, ocular discharge, or gritty sensation.  On examination, visual acuity is slightly reduced in the right eye.  Fluorescein examination shows a large geographic corneal staining defect.  Which of the following nerve dysfunctions is most likely responsible for this patient's impaired corneal sensation?

Quizplus · Accepted Answer

11ec25c8_d2d4_efde_bc81_939133b8dbd3_MD0003_00 This patient's presentation suggests a corneal abrasion. Common causes of corneal abrasion include trauma (eg, fingernail, paws), foreign body (eg, paper, wood, glass) lodging under the lid, and contact lens use leading to a corneal epithelial defect. Abrasions can also occur without obvious corneal trauma. Patients typically develop severe eye pain and can further damage the surface by rubbing or washing the eye repeatedly. They ultimately can have photophobia with reluctance to open the eye and a foreign body sensation. Visual acuity may be normal or reduced. Fluorescein examination is notable for a corneal staining defect. Indications for ophthalmology referral include ulceration, pus, drop in visual acuity (the potential cause of blurry vision in this patient), or lack of healing within 3-4 days. Although the patient has a corneal abrasion, he does not complain of eye pain. This indicates a likely cranial nerve (CN) V lesion. CN V (trigeminal nerve) contains sensory fibers in its 3 branches; the V3 (mandibular) branch also contains motor fibers. The V1 (ophthalmic) branch carries sensory fibers to the scalp, forehead, upper eyelid, conjunctiva, cornea, nose, and frontal sinuses. Damage to the V1 branch (specifically to the nasociliary nerve mediating the afferent limb of the corneal reflex) causes corneal anesthesia. Common causes of trigeminal nerve dysfunction include tumor, trauma, or prior herpes zoster infection. (Choice A) The facial nerve (CN VII) is responsible for facial movement, taste in the anterior 2/3 of the tongue, lacrimation (via the greater petrosal/vidian nerves), salivation (via chorda tympani), and eyelid closure. The facial nerve also carries sensory fibers supplying sensation to parts of the external ear and the nasopharynx. (Choice B) The oculomotor nerve (CN III) is responsible for the majority of eye movement (adduction via the medial rectus, elevation via the superior rectus and inferior oblique, and depression via the inferior rectus), eyelid opening, and pupillary constriction. The oculomotor nerve is a pure motor nerve carrying general somatic efferent and general visceral efferent fibers. (Choice C) The optic nerve (CN II) is responsible for carrying visual information to the brain as well as mediating the afferent limb of the pupillary light reflex pathway. Damage to this nerve can cause monocular blindness and an afferent pupillary defect. However, corneal sensation would be spared. (Choice E) The vagus nerve (CN X) is responsible for swallowing, palate elevation, phonation, and parasympathetic outflow to the thoracoabdominal viscera, including monitoring of the aortic arch baro- and chemoreceptors. The vagus nerve is also responsible for taste in the epiglottis and sensation from parts of the external ear; it also mediates the afferent limb of the cough reflex and the efferent limb of the gag reflex. Educational objective: Common causes of corneal abrasion include trauma, foreign body lodging under the lid, and contact lens use leading to a corneal epithelial defect. Abrasions can also occur without obvious corneal trauma. Patients typically develop severe eye pain, photophobia with reluctance to open the eye, and a sensation of a foreign body in the eye. Absence of eye pain suggests trigeminal nerve dysfunction as the ophthalmic branch (V1) of the trigeminal nerve (cranial nerve V) controls corneal sensation. References: Evaluation and management of corneal abrasions. (http://www.ncbi.nlm.nih.gov/pubmed/23317075) Corneal pain activates a trigemino-parabrachial pathway in rats. (http://www.ncbi.nlm.nih.gov/pubmed/24418463)

A 32-Year-Old Man Comes to the Physician After a Day