Enhancing Cranial Nerve(s)

Anne G. Osborn, MD, FACR

DIFFERENTIAL DIAGNOSIS

Common

Metastases
Neurofibromatosis Type 2
Neurofibromatosis Type 1
- Plexiform Neurofibroma
- Optic Nerve Glioma
Multiple Sclerosis
- Optic Neuritis

Less Common

Viral, Post-Viral Neuritis
- Bell Palsy
- Herpes Zoster
- ADEM
Lyme Disease
Lymphoma
Neurosarcoid
Opportunistic Infection, AIDS
Leukemia

Rare but Important

Ischemia
- Diabetes
- Arteriolosclerosis (Microvascular Disease)
Langerhans Cell Histiocytosis
Chronic Inflammatory Demyelinating Polyneuropathy (CIDP)

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Enhancement of cisternal, cavernous sinus CN segments always abnormal
Which cranial nerve(s) affected?
- Optic nerve: MS, NF1 (optic glioma), viral/post-viral
- CN3, 6: Often ischemia (diabetes, arteriolosclerosis)
- CN7: Bell palsy, Herpes zoster (Ramsay Hunt)
- CN8: Schwannoma (sporadic or NF2 associated), metastasis
If multiple nerves involved, consider
- Metastases, lymphoma, leukemia
- NF2
- Lyme disease
- CIDP (especially if nerves massively enlarged)
History important
- Optic neuritis (majority have or develop MS)
- Known neoplasm
- Flu-like illness (ADEM, viral neuritis)

Helpful Clues for Common Diagnoses

Metastases
- Most common: CSF spread
  - Involves pia, CNs, may extend along perivascular spaces
  - Multiple thickened nerves > solitary involvement
  - Fundus of CPA/IAC most common site
- Less common: Perineural tumor extension from extracranial primary
  - Extension into cisternal CN uncommon
  - Squamous cell, adenoid cystic carcinoma (CN5, 7 involvement most common)
Neurofibromatosis Type 2
- Multiple schwannomas
- Bilateral acoustic schwannomas diagnostic
- Acoustic schwannoma plus schwannoma of one other CN highly suggestive
- Schwannoma of “small” CN (e.g., CN3, 4) should raise consideration of NF2
Neurofibromatosis Type 1
- Plexiform Neurofibroma
  - Intracranial involvement less common than scalp, orbit, face (e.g., parotid gland)
  - Plexiform neurofibromas of CN3 or CN5 may extend intracranially, involve cavernous sinus
- Optic Nerve Glioma
  - Most are typical pilocytic astrocytomas (PAs)
  - 15-20% of NF1 patients develop PA (most commonly in optic pathway)
  - Up to 1/3 of patients with optic pathway PA have NF1
  - Enhancement varies from none to striking
  - May be uni- or bilateral, extend to/from orbit, involve nerves/chiasm/hypothalamus
Multiple Sclerosis
- Optic nerve (ON) most commonly affected
- 50-60% of patients with optic neuritis ultimately develop MS
- Imaging
  - Mildly enlarged, enhancing ON
  - 40% extend to intracanalicular, prechiasmatic/chiasmatic segments
  - Other CNs (e.g., trigeminal nerve) less commonly affected
- Non-MS associated optic neuropathy
  - Infectious (viral)
  - Anterior ischemic optic neuropathy (AION)

Helpful Clues for Less Common Diagnoses

Viral, Post-Viral Neuritis
- Bell Palsy
  - Enhancement of intratemporal facial nerve
  - “Tuft” of enhancement in IAC less common
- Herpes Zoster
  - Ramsay Hunt syndrome (Herpes zoster oticus) = vesicular rash of pinna, involvement of CN7, 8 in IAC, cochlea
  - Other CNs (e.g., 5) less common
- ADEM
  - Rare manifestation of post-viral demyelination
  - Affected nerve minimally enlarged, enhances transiently
Lyme Disease
- Most common = MS-like lesions in patient with skin rash, flu-like illness following deer tick bite
- Can involve multiple CNs (CN7 most common)
Lymphoma, Leukemia
- Diffuse pial tumor spread → multiple CNs
Neurosarcoid
- Most common intracranial involvement = optic nerve/chiasm/hypothalamus
- Other CNs rare
Opportunistic Infection, AIDS
- Tuberculous meningitis, CMV neuritis (retina, optic nerve)

Helpful Clues for Rare Diagnoses

Ischemia
- Diabetes, microvascular disease
  - CN3, 6 most commonly affected
  - Optic nerve (anterior ischemic optic neuropathy) less common
- Transient enhancement, then atrophy
Langerhans Cell Histiocytosis
- Usually children
- Optic nerve/chiasm/hypothalamus/infundibular stalk most common
  - Infiltrated, thickened structures enhance strongly, uniformly
- Disseminated intracranial LCH rare
  - Sulcal/cisternal enhancement
  - Multiple enhancing CNs
Chronic Inflammatory Demyelinating Polyneuropathy (CIDP)
- Typical setting = chronic MS
- Serial demyelination, remyelination → “onion bulb” thickening of affected nerves
- Massive enlargement, enhancement of spinal, cranial nerves (spinal > > CNs)