Paralytic Strabismus

Definition and Key Concepts

• Paralytic (incomitant): The angle of deviation varies with gaze direction - it is greatest in the direction of action of the weak muscle.
• Nonparalytic (comitant): The misalignment is the same in all directions of gaze; usually has no discrete neurologic cause.

• A phoria is a latent misalignment overcome by fusion.
• A tropia is a manifest misalignment that cannot be overcome by fusion.
• Prefixes: eso- (inward), exo- (outward), hyper- (upward), hypo- (downward).

"Paralytic strabismus creates an incomitant misalignment, which is of greatest magnitude with gaze in the direction of action of the weak muscle(s) and less magnitude with gaze in the opposite direction."

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

Extraocular Muscles and Their Innervation

Clinical Features of Individual Nerve Palsies

Third Nerve (Oculomotor) Palsy

• Complete palsy produces:
  • Ptosis (levator palpebrae weakness)
  • Eye positioned "down and out" (unopposed CN IV + VI action)
  • Diplopia - false image projected upward and medially
  • Dilated, non-reactive pupil (cycloplegia + iridoplegia) due to parasympathetic fiber involvement
• Key distinction - pupil-sparing vs. pupil-involving:
  • Pupil spared → microvasculopathic (e.g., diabetic) - pupilloconstrictor fibers lie superficially and are preserved by ischemia affecting central nerve fascicles
  • Pupil involved → compressive lesion (e.g., posterior communicating artery aneurysm) - compresses the superficial parasympathetic fibers early

Fourth Nerve (Trochlear) Palsy

• Most common cause of isolated symptomatic vertical diplopia
• Superior oblique weakness → poor downward movement in adduction
• Patient complains of difficulty reading or going downstairs
• Bielschowsky sign: compensatory head tilt to the opposite shoulder to reduce diplopia; diplopia is worsened by ipsilateral head tilt
• Bilateral trochlear palsy (e.g., after head trauma) → alternating hyperdeviation depending on gaze direction

Sixth Nerve (Abducens) Palsy

• Paralysis of lateral rectus → medial deviation of the affected eye (esotropia)
• Horizontal diplopia maximal on gaze toward the side of the palsy and in the distance
• With incomplete palsy: turning the head toward the paretic side can overcome the diplopia

Diplopia Fields Diagram (Maddox Rod Test)

Analysis of Diplopia - Two Rules

1. The direction of maximal image separation = the action of the paretic muscle.
   • Maximal horizontal separation on right gaze → right lateral rectus OR left medial rectus weakness.
   • Mainly vertical separation → paresis in a vertically acting muscle.
2. The image projected farther from center belongs to the eye with the paretic muscle.
   • The false image from the non-fixing paretic eye is displaced in the direction of the weak muscle's action.
   • A red glass is placed over one eye (usually the right) and the patient identifies which image (red vs. white) is peripheral in each gaze position.

Causes by Location

CN III Palsy

CN IV Palsy

CN VI Palsy

Note: CN VI palsy is a false localizing sign in raised intracranial pressure, as the nerve is stretched over the petrous ridge by downward displacement of the brainstem.

Important Differentials to Consider

• Myasthenia gravis - fatiguable ophthalmoplegia, variable ptosis, pupil always spared; can mimic any cranial nerve palsy
• Thyroid eye disease - restrictive (not paralytic) due to fibrotic EOM enlargement; diplopia typically worse on upward/outward gaze
• Internuclear ophthalmoplegia (INO) - ipsilateral adduction deficit with contralateral abducting nystagmus; from MLF lesion (MS, brainstem stroke)
• Duane syndrome - congenital; retraction of globe on adduction with limited abduction
• Orbital fracture ("blow-out") - restrictive, not neurogenic

Management Principles

• Treat the underlying cause (e.g., steroids for Tolosa-Hunt, antibiotics for cavernous sinus thrombosis, neurosurgical referral for aneurysm)
• Prism glasses for symptomatic diplopia relief
• Occlusion (patch) to eliminate diplopia acutely
• Botulinum toxin injection to the antagonist muscle during recovery phase (especially CN VI palsy)
• Surgery (recession/resection of EOMs) after 6-12 months if spontaneous recovery has not occurred and deviation is stable