Nerve sheath tumors represent 25-30% of all intradural spinal tumors, arising from Schwann cells (schwannomas, 70-80%) or nerve sheath fibroblasts (neurofibromas, 20-30%). These benign tumors present with radicular pain and progressive myelopathy. Schwannomas are typically solitary, encapsulated, and amenable to complete resection with nerve preservation, while neurofibromas are more infiltrative and associated with neurofibromatosis type 1.

Key Recognition Features:

• Radicular pain as initial symptom (80-90% of cases)
• Intradural extramedullary mass on MRI with contrast enhancement
• CSF protein elevation
• Dumbbell configuration in 15-30% (intraspinal plus paraspinal extension)

Critical Diagnostic Pathway:

1. MRI whole spine with gadolinium (gold standard imaging)
2. Screen for neurofibromatosis (café-au-lait spots, family history)
3. Assess dumbbell extension with CT for bony anatomy
4. Pre-operative neurophysiology if motor nerve root involvement

High-Yield Exam Points:

• Schwannomas: encapsulated, eccentric to nerve root, complete resection without nerve sacrifice in 95%
• Neurofibromas: infiltrative, involve nerve fascicles, nerve sacrifice often required, NF1 association
• Dumbbell tumors require combined posterior laminectomy plus lateral approach (sometimes staged)
• Intraoperative neuromonitoring for motor root identification
• Recurrence rate less than 5% for schwannomas after gross total resection

Spinal Nerve Root Anatomy

Understanding nerve root anatomy is critical for surgical planning and minimizing post-operative deficits:

Nerve Root Organization:

• Dorsal (sensory) root: enters posterolateral cord
• Ventral (motor) root: exits anterolateral cord
• Dorsal root ganglion: sensory cell bodies, common schwannoma origin site
• Fusion of roots forms mixed spinal nerve in foramen

Schwann Cell Distribution:

• Schwann cells myelinate peripheral nerves
• Present on dorsal and ventral roots distal to root entry/exit zone
• Most schwannomas arise from dorsal (sensory) roots (90%)
• Motor root schwannomas rare but clinically significant (10%)

Anatomical Compartments:

• Intradural: within dural sac, most common location (65%)
• Foraminal: within neural foramen (20%)
• Dumbbell: both intradural and foraminal/paraspinal (15%)
• Paraspinal: entirely extraforaminal (rare, 5%)

Regional Nerve Root Function:

Cervical Roots:

• C1-C4: neck and upper shoulder, sacrifice tolerable
• C5: shoulder abduction (deltoid) - preserve if possible
• C6: elbow flexion (biceps), wrist extension - preserve
• C7: elbow extension (triceps), wrist flexion - preserve
• C8: hand intrinsics, finger flexion - preserve
• T1: hand intrinsics - preserve

Thoracic Roots:

• T2-T12: intercostal muscles and dermatomal sensation
• Generally sacrificable with minimal morbidity
• Bilateral sacrifice can cause truncal instability
• Unilateral sacrifice causes numbness only

Lumbar and Sacral Roots:

• L2-L4: hip flexion, knee extension, thigh sensation - preserve
• L5: foot dorsiflexion, great toe extension - critical for gait, preserve
• S1: foot plantarflexion, ankle reflex - critical for gait, preserve
• S2-S5: bladder, bowel, sexual function - preserve if possible

Tumor Pathology

Schwannoma Characteristics:

• Origin: Schwann cells, typically from dorsal root ganglion
• WHO Grade: I (benign)
• Macroscopic: well-encapsulated, yellow-tan, eccentric to nerve
• Microscopic: Antoni A (compact spindle cells) and Antoni B (loose myxoid) areas
• Verocay bodies: palisading nuclei in Antoni A regions
• S100 immunostaining: diffusely positive
• Growth pattern: slow-growing, displaces nerve fascicles

Neurofibroma Characteristics:

• Origin: nerve sheath fibroblasts and Schwann cells
• WHO Grade: I (benign), but potential for malignant transformation (5-10% in NF1)
• Macroscopic: fusiform enlargement of nerve, not encapsulated
• Microscopic: mix of Schwann cells, fibroblasts, perineural cells
• S100 immunostaining: focally positive
• Growth pattern: infiltrative within nerve fascicles
• Association: neurofibromatosis type 1 (NF1) in 40-50% of spinal neurofibromas

Malignant Peripheral Nerve Sheath Tumor (MPNST):

• Rare (less than 5% of nerve sheath tumors)
• WHO Grade: III-IV
• High association with NF1 (50-70% of cases)
• Rapid growth, pain, neurological deficit
• Poor prognosis even with aggressive treatment
• Wide en bloc resection required

Pathophysiology of Clinical Presentation

Radicular Pain Mechanism:

• Tumor compression and traction on nerve root
• Inflammation and chemical mediators
• Worse with Valsalva, coughing, bending
• Often nocturnal (recumbent position increases intraspinal pressure)

Myelopathy Development:

• Progressive cord compression from ventral or lateral tumor
• Vascular compromise of spinal cord
• Late manifestation (tumor often large by this stage)
• Indicates urgent surgical intervention

Natural History:

• Schwannomas: slow growth (1-2 mm per year), may remain asymptomatic for years
• Neurofibromas: variable growth, faster in NF1 patients
• Malignant transformation: rare in schwannomas (less than 1%), higher in NF1 neurofibromas (5-10%)

Symptom Progression

Early Stage (Radicular Pain Predominant):

• Unilateral dermatomal pain in 80-90% of patients
• Lancinating or burning character
• Worse with movement, Valsalva, recumbency
• Partial relief with specific positions
• May precede imaging findings by months to years

Intermediate Stage (Motor and Sensory Deficits):

• Dermatomal paresthesias and numbness
• Progressive motor weakness in myotomal distribution
• Atrophy of affected muscles
• Reflex asymmetry or loss
• Gait disturbance if lower limb involvement

Late Stage (Myelopathy):

• Bilateral weakness below tumor level
• Spasticity and hyperreflexia
• Sensory level
• Bladder and bowel dysfunction
• Indicates significant cord compression requiring urgent surgery

Physical Examination

Inspection:

• Café-au-lait spots (6 or more, greater than 5 mm prepubertal, greater than 15 mm postpubertal): NF1
• Axillary or inguinal freckling: NF1
• Subcutaneous neurofibromas: NF1
• Lisch nodules (iris hamartomas): NF1
• Skeletal abnormalities: scoliosis (NF1-associated dystrophic curve)

Motor Examination:

• Myotomal weakness pattern
• Muscle atrophy (chronic denervation)
• Fasciculations (lower motor neuron)
• Pyramidal signs if myelopathy (hyperreflexia, spasticity, Babinski)

Sensory Examination:

• Dermatomal sensory loss (pinprick and light touch)
• Posterior column dysfunction (vibration, proprioception) if cord compression
• Sensory level if myelopathy present

Special Tests:

• Straight leg raise: positive if lumbar or sacral root involvement
• Spurling test: positive if cervical root compression
• Hoffman and Babinski signs: if myelopathy present

Neurofibromatosis Screening

Additional NF1 Features:

• Optic pathway glioma
• Distinctive osseous lesion: sphenoid dysplasia, long bone cortical thinning
• Learning disabilities and attention deficit
• Increased risk of malignancy: MPNST, pheochromocytoma, breast cancer

NF1 and Spinal Tumors:

• Multiple nerve sheath tumors (schwannomas and neurofibromas)
• Paraspinal plexiform neurofibromas
• Dystrophic scoliosis (short segment, severe angulation)
• Dural ectasia
• 5-10% risk of malignant transformation (MPNST)

MRI Imaging (Gold Standard)

Standard Protocol:

• Whole spine sagittal and axial sequences
• T1-weighted pre- and post-gadolinium
• T2-weighted for cord signal
• STIR sequences for edema and bone involvement
• Thin cuts (3 mm) through tumor

Schwannoma MRI Characteristics:

• Location: intradural extramedullary (65%), foraminal (20%), dumbbell (15%)
• Shape: ovoid or round, well-defined margins
• T1 signal: iso to hypointense relative to cord
• T2 signal: hyperintense (may have hypointense areas if fibrosis)
• Enhancement: intense homogeneous enhancement (small tumors) or heterogeneous (large tumors with cystic degeneration)
• Target sign: central low T2 signal with peripheral high signal (specific but uncommon)
• CSF cap: crescent of CSF around tumor (extramedullary location)

Neurofibroma MRI Characteristics:

• Location: often foraminal or paraspinal
• Shape: fusiform nerve enlargement
• T1 signal: isointense
• T2 signal: very hyperintense (myxoid matrix)
• Enhancement: heterogeneous enhancement
• Central low signal on T2: target sign (more common than schwannoma)
• Often multiple tumors if NF1

Dumbbell Tumor Features:

• Intraspinal component: intradural extramedullary mass
• Foraminal component: widened neural foramen (best seen on axial CT)
• Paraspinal component: soft tissue mass lateral to spine
• Hourglass configuration at foramen
• May cause bony erosion and facet destruction

CT Imaging

Indications:

• Assessment of bony anatomy in dumbbell tumors
• Surgical planning for foraminal enlargement
• Evaluation of facet joint involvement
• Detection of bony erosion or scalloping

CT Findings:

• Widened neural foramen
• Facet erosion or destruction
• Vertebral body scalloping
• Paraspinal soft tissue mass
• Calcification rare (if present, consider meningioma)

Differential Diagnosis Imaging

Laboratory and Additional Tests

Pre-Operative Workup:

• Full blood count, renal and liver function
• Coagulation profile
• Group and screen (blood products rarely needed)
• ECG and anesthesia assessment

CSF Analysis (rarely needed):

• Elevated protein (60-80% of cases)
• Normal glucose and cell count
• Froin syndrome: very high protein (greater than 1000 mg/dL), xanthochromia, coagulation
• Indicates complete CSF block from large tumor

Neurophysiology:

• EMG and nerve conduction studies: localize root involvement
• Pre-operative motor and sensory nerve conduction baselines
• Intraoperative monitoring: SSEP for sensory roots, triggered EMG for motor roots

Surgical Indications

Absolute Indications:

• Progressive neurological deficit
• Myelopathy (cord compression)
• Intractable radicular pain affecting quality of life
• Suspicion of malignancy (rapid growth, NF1 patient)

Relative Indications:

• Asymptomatic tumor with radiological progression
• Large tumor (greater than 2 cm) even if minimally symptomatic
• Dumbbell tumor with foraminal widening
• Patient preference after discussion of natural history

Observation Criteria:

• Small asymptomatic tumor (less than 1 cm)
• Elderly or medically unfit patient
• Multiple tumors in NF1 (prioritize symptomatic lesions)
• Serial MRI every 6-12 months to assess growth

Surgical Approaches and Techniques

Nerve Root Preservation Strategies

Pre-Operative Planning:

• Review MRI to determine root of origin
• Predict motor versus sensory based on location
• Discuss potential for root sacrifice and expected deficit

Intraoperative Root Identification:

• Triggered EMG stimulation of proximal root
• Low threshold response (less than 0.5 mA) indicates motor root
• High threshold or no response indicates sensory root
• Stimulate distal root after tumor resection to confirm function

Nerve Preservation Techniques:

• Schwannoma: eccentric nerve displacement, fascicles preserved on capsule
• Sharp dissection along capsule preserving fascicles
• If motor root origin, attempt intracapsular debulking to preserve root
• Accept subtotal resection if nerve preservation critical

Root Sacrifice Decision Algorithm:

• Sensory root (C1-C4, T2-T12, S3-S5): safe to sacrifice for GTR
• Motor root (C5-T1, L2-S2): attempt preservation
• If motor root completely non-functional (no EMG, severe atrophy), sacrifice acceptable
• Bilateral motor root sacrifice never acceptable

Complications and Management

Early Complications:

CSF Leak:

• Incidence: 2-5% of cases
• Presents with wound drainage or positional headache
• Conservative management: bed rest, acetazolamide, pressure dressing
• Surgical repair if persistent beyond 5-7 days or large volume leak

Epidural Hematoma:

• Incidence: 1-2%
• Presents with acute neurological deterioration, severe pain
• Urgent MRI confirms diagnosis
• Emergency surgical evacuation if cord compression

Infection:

• Superficial wound infection: 2-3%, treated with antibiotics and local wound care
• Deep infection or meningitis: less than 1%, requires surgical washout and IV antibiotics

New Neurological Deficit:

• Expected deficit from planned root sacrifice
• Unexpected deficit: motor weakness, suggests nerve injury during dissection
• Management: high-dose steroids, close observation, rehabilitation

Late Complications:

Spinal Instability:

• Risk factors: bilateral facetectomy greater than 50%, multilevel laminectomy, pre-existing scoliosis
• Presents with mechanical back pain, progressive deformity
• Prevention: prophylactic fusion if high-risk facet resection
• Management: posterior instrumented fusion if symptomatic

Tumor Recurrence:

• Schwannoma: less than 5% after GTR, higher if subtotal resection
• Neurofibroma: 10-15% recurrence, higher in NF1 patients
• MPNST: high recurrence even after GTR, poor prognosis
• Surveillance: MRI at 3 months, 1 year, then every 2 years for schwannoma; annually for neurofibroma and NF1

Chronic Pain:

• Persistent or new radicular pain: 5-10% of patients
• Mechanisms: nerve injury, epidural scarring, arachnoiditis
• Management: multimodal analgesia, neuropathic pain medications, pain clinic referral

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This comprehensive topic provides Gold Standard coverage of nerve sheath tumors for FRACS examination preparation, emphasizing surgical decision-making, technique, and nerve preservation strategies.