II - Optic Nerve Flashcards
What are the main parts of an CN II?
The optic nerve is divided into four main parts, all of which play a role in the relay of visual signals to the brain.
* These parts include:
1. The intraocular portion
2. The intraorbital portion
3. The intracanalicular portion
4. The intracranial portion
Outline the pathway of CN II
- Origin: The optic nerve begins at the back of the eye, where it is connected to the retina through retinal ganglion cells.
- Exit from the eye: It exits the eye through a structure called the optic foramen, along with other optic vessels.
- Optic Chiasm: Upon exiting the eye, the optic nerve fibers enter the optic chiasm, where fibers from the opposite sides of each eye cross over to the opposite hemisphere of the brain.
- Optic Tract: After crossing at the optic chiasm, the nerve fibers continue as the optic tract. The optic tract then travels through the brain, specifically passing through the thalamus.
- Optic Radiation: Next, the nerve fibers form a structure known as the optic radiation. This pathway extends from the thalamus and continues until it reaches the visual cortex in the occipital lobe of the brain.
- Visual Cortex: Finally, the optic radiation delivers visual signals to the visual cortex, which is responsible for interpreting and processing these signals into the images that we perceive.
What is the blood supply of CN II?
- Arterial Circle of Willis
- Carotid arterial system
- Vertebral arterial system
What is the venous drainage of CN II?
Optic nerve head
Central retinal vein Orbital part
Peripheral pial plexus
Central retinal vein Intracranial part
Pial plexus which ends in anterior cerebral & basal vein
Explain the causes of CN II lesions
Causes of Optic Nerve Lesions:
Optic Atrophy: This refers to the degeneration or loss of the optic nerve fibers. It can be caused by various conditions such as glaucoma, ischemic optic neuropathy, or multiple sclerosis.
Optic Neuropathy: This is a broad term indicating damage or disease of the optic nerve. It can be due to various factors including nutritional deficiencies, toxins, or infections.
Acute Optic Neuritis: This is inflammation of the optic nerve, often associated with demyelinating conditions like multiple sclerosis.
Traumatic Avulsion of Optic Nerve: This is a severe injury where the optic nerve is torn or detached from its connection to the eye due to trauma.
What are the features of CN II lesions?
Complete Blindness in Affected Eye: This means loss of vision (blindness) in the eye where the optic nerve lesion has occurred.
Loss of Direct and Consensual Light Reflexes: The affected eye will not respond to light (direct response) and neither will the other eye (consensual response). Normally, shining a light into one eye causes both pupils to constrict (direct response) and also the other pupil (consensual response). With an optic nerve lesion, these responses are absent.
Preserved Near Reflex: The near reflex, which involves the eyes converging and accommodating when focusing on near objects, remains intact. This means that despite the optic nerve lesion affecting distant vision and light responses, the ability to focus on near objects is retained.
What are the features of lesions through the optimal part of CN II?
Ipsilateral Blindness: Vision loss in the eye on the same side as the optic nerve problem.
Contralateral Hemianopia: Loss of vision in half of the visual field on the opposite side of the optic nerve issue. For example, if the right optic nerve is affected, the left side of both eyes’ visual fields will be affected.
Loss of Light Reflexes: When light is shined into the affected eye:
The affected eye doesn’t respond by constricting its pupil (direct light reflex).
The other eye (opposite side) also doesn’t respond by constricting its pupil (concensual light reflex).
Intact Near Reflex: Ability of the eyes to focus and adjust when looking at close objects remains normal.
Describe the causes of the lesions through the optic chiasm
Suprasellar Aneurysm: An aneurysm located above the sella turcica (where the pituitary gland sits) can compress the optic chiasm.
Tumors of Pituitary Gland: Benign or malignant growths within the pituitary gland can extend upward and compress the optic chiasm.
Craniopharyngioma: A type of brain tumor that commonly arises near the pituitary gland and can affect the optic chiasm.
Suprasellar Meningioma & Glioma of 3rd Ventricle: Tumors originating from the meninges (membranes covering the brain) or gliomas (tumors arising from glial cells) located near the third ventricle can impact the optic chiasm.
Third Ventricular Dilatation due to Obstructive Hydrocephalus: Enlargement of the third ventricle due to increased pressure within the brain can cause compression of the optic chiasm.
Chronic Chiasmal Arachnoiditis: Inflammation of the arachnoid membrane around the optic chiasm, often due to chronic conditions like infections or autoimmune disorders
What are the features of central optic nerve chiasm lesions?
Bitemporal Hemianopia: Loss of vision on the outer (temporal) halves of the visual fields in both eyes.
Bitemporal Hemianopic Paralysis of Pupillary Reflex: This refers to a specific type of visual reflex abnormality where the pupillary light reflex (constriction of pupils in response to light) is impaired
Explain the causes of lesions to the optic tract
Syphilitic Meningitis/Gumma: Infections like syphilis can affect the meninges (membranes surrounding the brain) and lead to optic tract involvement.
Tuberculosis: Tuberculosis can cause inflammation and damage to the optic tract.
Tumors of Optic Thalamus: Tumors located in or near the thalamus can compress or disrupt the optic tract fibers.
Aneurysm of Superior Cerebellar or Posterior Cerebral Arteries: Aneurysms (abnormal ballooning of blood vessels) in these arteries can press on the optic tract and cause lesions
What are the features of optic tract lesions?
Incongruous Homonymous Hemianopia: This refers to loss of vision in half of the visual field on the same side in both eyes, but the patterns of visual loss differ between the eyes.
Partial Descending Optic Atrophy: Damage to the optic nerve fibers leads to partial degeneration of these nerve fibers, resulting in optic atrophy
How do you examine the optic nerve?
The examination of the optic nerve involves several tests and procedures to assess different aspects of vision and the health of the optic nerve itself. Here’s a breakdown of each component:
Distance and Near Vision (Uncorrected Visual Acuity/Best Corrected Visual Acuity):
This evaluates how well a person can see at various distances, both with and without corrective lenses (glasses or contact lenses).
Colour Vision:
Assessing the ability to distinguish different colors can provide information about the function of the optic nerve and retina.
Field of Vision - Central/Peripheral:
Testing the visual field helps identify any abnormalities or areas of vision loss. This can be done using various methods, such as confrontation testing or automated perimetry.
Pupillary Reaction:
Checking how the pupils respond to light can indicate the integrity of the optic nerve and visual pathways.
Intraocular Pressure:
Measuring intraocular pressure is important for detecting conditions like glaucoma, which can affect the optic nerve.
Ophthalmoscopy:
This involves examining the optic nerve head and retina using a specialized instrument called an ophthalmoscope to detect abnormalities like optic nerve swelling, pallor, or cupping.
Slit Lamp Biomicroscopy:
This detailed examination of the eye’s anterior segment (front portion) can reveal any abnormalities affecting the optic nerve or surrounding structures.
Fluorescein Angiography:
This imaging technique uses a special dye to evaluate blood flow in the retina and optic nerve head, helpful in diagnosing vascular disorders.
X-Ray (Skull/PNS), Optic Foramen/Canal and Orbit:
X-rays can provide structural imaging of the skull, paranasal sinuses, optic foramen/canal, and orbit to identify bony abnormalities affecting the optic nerve.
Ultrasonography:
Ultrasound imaging can assess the anatomy of the eye and orbit, helping to detect masses or structural changes impacting the optic nerve.
CT Scan and MRI:
These imaging techniques provide detailed pictures of the brain, orbits, and optic nerves to identify lesions, tumors, or other abnormalities affecting the optic nerve.
