Cranial Nerve II - The Optic Nerve

Overview

Anatomy

Structure

• Carries approximately 1.2 million afferent nerve fibers, each originating from a retinal ganglion cell
• The fibers are divided into ~600 bundles by fibrous septae derived from the pia mater
• Most fibers synapse at the lateral geniculate body (LGB); a smaller proportion reach the pretectal nuclei in the midbrain (for pupillary light reflex)
• Nearly one-third of all fibers subserve the central 5° of the visual field

Four Anatomical Segments

• Kanski's Clinical Ophthalmology, 10th ed.

Surrounding Layers

• Innermost: delicate, vascular pia mater
• Middle: arachnoid mater (subarachnoid space contains CSF, continuous with cerebral CSF)
• Outermost: dura mater (fuses with the sclera anteriorly and the periosteum of the optic canal)

Blood Supply

• Ophthalmic artery (from internal carotid artery) runs inferior to the optic nerve in the optic canal
• Central retinal artery enters the optic nerve 8-12 mm behind the globe, supplying the inner retina
• Optic nerve head receives blood from the circle of Zinn-Haller (fed by posterior ciliary arteries, choroidal vessels, and the pial arterial network)

Course and Optic Canal

The Visual Pathway

1. Retina - ganglion cell axons form the optic nerve
2. Optic chiasm - nasal retinal fibers (carrying temporal visual field) cross to the opposite side; temporal retinal fibers (carrying nasal visual field) remain ipsilateral
3. Optic tract - carries visual information from the contralateral visual hemifield of both eyes
4. Lateral geniculate nucleus (LGN) - primary thalamic relay
5. Optic radiation - axons project to the primary visual cortex (V1) in the occipital lobe
6. Primary visual cortex (Brodmann area 17, calcarine cortex)

• Eric Kandel, Principles of Neural Science, 6th ed.

Visual Field Defects by Lesion Location

Papilledema and Intracranial Pressure

Signs of Optic Nerve Dysfunction

• Reduced visual acuity (non-specific but common)
• Relative afferent pupillary defect (RAPD) - a key objective sign
• Dyschromatopsia - impaired color vision, classically red-green; easily tested with Ishihara plates
• Diminished light brightness sensitivity (may persist after acuity recovers, e.g., post-optic neuritis)
• Diminished contrast sensitivity
• Visual field defects - central scotomas, centrocecal scotomas, arcuate/nerve fiber bundle defects, altitudinal defects

Visual Evoked Potential (VEP)

• Assess optic neuropathy (especially demyelination)
• Monitor visual function in infants
• Investigate functional (non-physiological) visual loss

Clinical Disorders of CN II

1. Optic Neuritis

• Demyelinating inflammation of the optic nerve
• Presents with painful, subacute monocular vision loss + RAPD + dyschromatopsia

• 50% of patients eventually develop multiple sclerosis

• Retrobulbar neuritis: disc looks normal initially but central/paracentral scotoma present
• Optic neuritis from MS is not steroid-responsive; treat underlying MS
• Also seen in: SLE, vasculitis, sarcoidosis (may be steroid-responsive), neuromyelitis optica (NMO) (with anti-AQP4 antibodies + transverse myelitis)

2. Ischemic Optic Neuropathy

• Non-arteritic (NAION): small vessel disease; associated with hypertension (50%) and diabetes (25%); causes altitudinal visual field defect
• Arteritic (AAION): usually giant cell arteritis (GCA); consider in patients >50 years; treat with oral glucocorticoids

3. Compressive Optic Neuropathy

• Tumors or space-occupying lesions between orbit and chiasm cause gradual central vision loss or sector field defect
• Chronic compression leads to optic atrophy
• Pituitary adenomas can compress the intracranial segment

4. Toxic/Nutritional Optic Neuropathy

• Ethambutol (TB treatment): dose/duration-dependent; bilateral painless blurring, red-green dyschromatopsia; dose should not exceed 15 mg/kg/day; baseline and 6-monthly screening recommended
• Isoniazid: rare toxic optic neuropathy, risk higher when combined with ethambutol
• Methanol poisoning, tobacco amblyopia
• B-vitamin deficiency (especially thiamine)

5. Leber Hereditary Optic Neuropathy (LHON)

• Mitochondrial inheritance
• Bilateral central/cecocentric scotomas
• Young males predominantly affected
• Goldman-Cecil Medicine; Kanski's Clinical Ophthalmology, 10th ed.

Summary Table

MRC Grading of Muscle Strength - Focus on Antigravity Function

The MRC Scale (Medical Research Council Scale)

• Adams and Victor's Principles of Neurology, 12th ed.; Localization in Clinical Neurology, 8e; Bailey and Love's Surgery, 28th ed.

The Antigravity Threshold - Grade 3

• At Grade 2 and below: the limb must be supported horizontally (gravity eliminated) to observe active movement
• At Grade 3: the patient can lift the limb against gravity, e.g., raising the arm or leg off the bed
• At Grade 4 and above: movement is also possible against examiner's resistance

Modified MRC Grading (with +/- Subdivisions)

"Further gradations may be added, specified as 4+ for barely detectable weakness and 4- for easily detected weakness, 3+ and 3-, and so on." - Adams and Victor's Principles of Neurology, 12th ed.

What Are Antigravity Muscles?

In the Lower Motor Neuron context:

• Lower limbs: Extensors (quadriceps, hip extensors, plantar flexors/gastrocnemius) - maintain the leg extended against gravity during standing and walking
• Upper limbs: Flexors (biceps, wrist and finger flexors) - resist gravity when the arm is held up

Physiological control:

• Controlled by the vestibulospinal tract and medial reticulospinal tract, which facilitate alpha and gamma motor neurons of antigravity muscles
• The vestibular nuclei transmit strong excitatory signals to antigravity muscles via the lateral and medial vestibulospinal tracts
• These muscles have a particularly high density of muscle spindles and are strongly driven by the gamma efferent system
• Guyton & Hall Textbook of Medical Physiology; Localization in Clinical Neurology, 8e

In UMN (spasticity) pattern:

• Upper limb: Flexors (biceps, wrist flexors) - patient develops flexed arm posture
• Lower limb: Extensors (quadriceps, calf muscles) - patient develops extended leg posture

"Spasticity predominates in antigravity muscles (flexors of the upper extremities and extensors of the lower extremities)." - Localization in Clinical Neurology, 8e

Practical Testing Tips

1. Grade 2 testing: Lay the limb horizontally on a flat surface (eliminating gravity), then ask for movement - this is gravity-eliminated position
2. Grade 3 testing: Have the patient lift the limb vertically against gravity (e.g., straight leg raise, arm elevation)
3. Grade 4 testing: Apply progressive manual resistance to the movement
4. For trunk and girdle muscles (large, powerful): Manual resistance alone may miss mild weakness - have the patient perform functional tasks: squatting, rising from a chair, walking on toes/heels
5. For hand muscles: Use a dynamometer for quantification

• Adams and Victor's Principles of Neurology, 12th ed.

Decerebrate Rigidity and Antigravity Muscles

Summary: MRC Grade vs. Antigravity

Grade 0 ——— No contraction
Grade 1 ——— Trace contraction only
Grade 2 ——— Movement ONLY with gravity eliminated
           ← ANTIGRAVITY THRESHOLD →
Grade 3 ——— Full movement AGAINST gravity (no resistance)
Grade 4 ——— Against gravity + resistance (submaximal)
Grade 5 ——— Normal (full) power

Sternocleidomastoid (SCM) Muscle - Anatomy, Testing, and Grading Systems

Anatomy & Function Recap

• Unilateral: Rotates the head to the contralateral side + tilts/laterally flexes toward the ipsilateral shoulder
• Bilateral: Flexes the neck (draws the chin toward the chest)
• Localization in Clinical Neurology, 8e

How to Test the SCM

Rotation Test (Unilateral SCM)

• Patient turns their head to one side against the examiner's resistance
• The contralateral SCM contracts and should be visible and palpable
• e.g., to test the right SCM: instruct the patient to rotate the head to the left against resistance

Flexion Test (Bilateral SCM)

• Patient in supine; lifts the head off the bed (chin tuck/head raise)
• Both SCMs contract simultaneously
• Assessed for bilateral symmetry and ability to hold against resistance

Lateral Flexion Test

• Patient tilts the head toward one shoulder against resistance
• The ipsilateral SCM contracts
• Localization in Clinical Neurology, 8e

Grading Systems Beyond the MRC

1. MRC Scale (Medical Research Council)

2. Kendall & McCreary Scale (0-100% / Percentage Scale)

• The patient must selectively recruit the SCM while the examiner palpates it
• Resistance is applied along the precise vector of the muscle's fiber direction
• This approach is more anatomically specific but demands greater examiner skill

3. Daniels & Worthingham Scale (Named/Functional Grades)

• RehabMeasures Database - Manual Muscle Test; Pinnacle Healthcare Consulting - Muscle Testing

Key Differences Between the Three Systems

4. IMACS 10-Point Scale (Myositis-Specific)

• Rheumatology, 2-Volume Set (Elsevier, 2022)

5. Dynamometry

SCM-Specific Clinical Correlations

"Bilateral sternocleidomastoid paresis causes weakness of neck flexion, with the head tending to fall backward when the patient attempts to stand erect." - Localization in Clinical Neurology, 8e

Summary