Differential Diagnosis in Neurology

5.6. Cranial Nerve VI

Anatomy

• Composed of 6000–7000 fibers
• Along the ventral pons as it is crossed by AICA and the internal auditory arteries
• Enters the cavernous sinus beneath the petroclinoid ligament of Grubar; may be involved within the inferior petrosal sinus as it courses through Dorello's canal
• Next to the carotid artery in the cavernous sinus

Congenital Defects

• Unilateral or bilateral absence of VIth nerve nuclei in association with abnormalities of CN III; other cranial nerves and hypoplasia or aplasia of brainstem nuclei
• Innervation of the lateral rectus muscles by III in the absence of VIth nerve nuclei
• Duane's syndrome:
  • Congenital deficiency of abduction
  • Globe retracts on adduction; occasionally globe moves upwards
  • More common in females than males
  • Either unilateral or bilateral
  • Left eye more often affected than the right
  • Anomalous pattern of innervation of the IIIrd and VIth nerves
• Moebius syndrome:
  • Facial diplegia usually with bilateral abducens palsy
  • Convergence strabismus (50% of patients)
  • Rarely ptosis and paralysis of III
  • No internal ophthalmoplegia
  • Rare: paralysis and atrophy of the tongue; hearing loss, clubfoot and other skeletal defects
• Congenital horizontal gaze palsy
• No convergent strabismus
• No facial diplegia
• Substituted convergence and cross fixation used to accomplish lateral gaze
• Vertical eye movements spared

Trauma

• Head trauma injuries VI; a few patients suffer combined IV and VI injury < III and VI injury
• VI injuries are commonly due to:
  • Long course along the basiocciput
  • Dural attachment over the clivus
  • 90° bend under the petroclinoid ligament
• Fracture through the temporal bone or posterior clinoid injures it directly
• Fracture of the orbital wall
• Transient injury of VI after maxillary sinus surgery

Neoplasms

• Primary tumors:
  • Pontine glioma
  • Meningioma
  • Pinealoma
  • Hemangioendothelioma
  • Hemangioma
  • Craniopharyngioma
  • Acoustic neurinoma (late)
  • Chordoma
  • Pituitary adenoma (extension into cavernous sinus)
  • Neuroma
  • Malignant pituitary tumor (rare)
• Secondary tumors:
  • Carcinomatosis of the meninges
  • Lymphoma
  • Leukemia
  • Nasopharyngeal cancer:
    • Squamous cell carcinoma
    • Lymphoepithelioma
    • Lymphosarcoma
  • Pilocystic adenoma
  • Breast, thyroid, lung metastases

Vascular Lesions

• Penetrating pontine branch lesion: Ramón's syndrome: ipsilateral VIth nerve and contralateral hemiparesis
• Millard Gubler syndrome:
  • Ipsilateral VI, VII nerve palsy
  • Contralateral hemiparesis
  • Ipsilateral Vth and Horner's (rare)
  • Contralateral spinothalamic loss of sensation
  • Most often hemorrhage rather than ischemic lesions
• Infarction of the inferolateral trunk of the external carotid artery:
  • Superior division (associated infarction of III, IV, VI in the S.O.F.)
  • Embolic or diabetic cause
• Aneurysm of AICA
• Cavernous sinus aneurysm (frequently affects VIth nerve first); then III, IV, VI involved
• Gradenigo's syndrome:
  • Short circumferential artery from the basilar
  • Facial numbness; masseter, pterygoid, and temporalis weakness
  • Contralateral loss of pain and temperature below the clavicle
  • Contralateral hemiparesis

Infections

• Rare in Eastern and Western equine encephalitis, St. Louis, Bulbar poliomyelitis HZ, measles, influenza, pertussis and EBV
• Bacterial meningitis:
  • III and VI equally affected
  • Transient; recovery with successful treatment
• Syphilitic meningitis (stage II) VI nerve affected at entry zone
• Tuberculous meningitis
• Lyme's disease
• Cryptococcosis
• Coccidiomycosis
• Mucormycosis
• Aspergillosis
• HIV
• Gradenigo's syndrome (mastoiditis with spread through a pneumatized petrous bone to the petrous apex)

Collagen Vascular Disease/Arteritis

• SLE (III and VI most commonly affected)
• Periarteritis nodosa
• Wegener's granulomatosis
• Mixed collagen vascular disease; (overlap of SLE, Scleroderma, polymyositis)
• Hypereosinophilic syndrome
• Tolosa Hunt syndrome (in association with III, IV, VI)

Systemic Disease

• Wernicke–Korsakoff's disease
• Diabetes (III more common than VI)
• Idiopathic pachymeningitis (in conjunction with other cranial nerves)
• Pseudotumor cerebri
• Thyroid ophthalmopathy
• Sarcoidosis
• Cranial arteritis (usually muscle involvement more frequent than nerve)

Immune Mediated

• AIDP
• CIDP
• Miller Fisher variant of GBS (GD1b or GQ1b epitopes)
• Multiple sclerosis (VI more frequent than III)
• ADEM (acute disseminated encephalomyelitis)
• Post immunization

Miscellaneous Entities

• Trauma
• Superior orbital fissure syndrome (tumor)
• Cavernous sinus syndrome
• Post LP (not uncommon)