What are the possible causes of numbness on the left side of the face in an adult with a normal cranial nerve (CN) exam?

Unilateral Facial Numbness with Normal Cranial Nerve Examination

In an adult with left-sided facial numbness but a completely normal cranial nerve examination, the most likely causes are early trigeminal neuropathy from inflammatory conditions, demyelinating disease, or small structural lesions not yet producing objective findings—and MRI of the brain with contrast is mandatory to exclude these serious etiologies. 1

Immediate Diagnostic Approach

Obtain contrast-enhanced MRI of the brain covering the entire trigeminal nerve pathway from brainstem through skull base to peripheral branches. 1 This is the preferred initial imaging modality because:

• The trigeminal nerve can be affected anywhere along its course from brainstem nuclei (extending from midpons to C2-C4 spinal cord levels) through cisternal segments, Meckel cave, cavernous sinus, and peripheral branches 1
• Sensory symptoms may precede objective examination findings in early disease 2
• CT is insufficient as it cannot adequately visualize soft tissue pathology along the nerve course 1

Primary Differential Diagnoses to Consider

Demyelinating Disease (Multiple Sclerosis)

• Multiple sclerosis commonly causes trigeminal neuralgia and sensory symptoms, necessitating brainstem imaging to identify demyelinating plaques 1
• The spinal trigeminal tract extends caudally into upper cervical cord, making it vulnerable to demyelinating lesions 1
• Patients may have isolated facial numbness as the presenting symptom before other neurological manifestations

Inflammatory Trigeminal Neuropathy

• Idiopathic inflammatory trigeminal sensory neuropathy (IITSN) presents with dominant trigeminal sensory disturbance and can mimic tumors on MRI during the active inflammatory phase 2
• This condition follows a benign course in most cases but requires biopsy for definitive diagnosis if imaging shows mass-like enhancement 2
• Sarcoidosis can produce isolated trigeminal nerve granulomas that enhance on MRI and present with facial numbness and pain 3

Early Structural Lesions

• Trigeminal schwannomas and other tumors may cause sensory symptoms before producing objective examination abnormalities 1
• Vascular compression from dolichoectatic vessels or aneurysms can cause sensory disturbances 1
• Perineural tumor spread from head and neck malignancies may present with subtle sensory changes before gross nerve enlargement 1

Brainstem Pathology

• Small brainstem infarcts affecting the trigeminal sensory nuclei can be as small as 4mm and produce isolated facial numbness 4
• Brainstem lesions (infarction, hemorrhage, tumors) rarely cause isolated trigeminal symptoms due to proximity of other neural structures, but this remains possible 1
• The absence of other brainstem signs does not exclude this diagnosis in early presentations

Critical Red Flags Requiring Urgent Evaluation

Watch for these features that suggest serious underlying pathology:

• Progressive symptoms over weeks to months suggest structural lesion rather than benign process 2, 3
• Associated headache, particularly if new or different in character 2
• Any subtle motor findings on careful re-examination (jaw weakness, facial asymmetry) 1
• History of malignancy elsewhere (raises concern for perineural spread) 1

Specific Examination Pearls

Even with "normal" cranial nerve exam, carefully assess:

• Corneal reflex asymmetry (may be earliest objective finding in trigeminal pathology) 1
• Jaw deviation with mouth opening (subtle masseter weakness) 1
• Facial sensation testing in all three trigeminal divisions bilaterally with multiple modalities (light touch, pinprick, temperature) 1

When to Consider Alternative Diagnoses

Facial Onset Sensory Motor Neuronopathy (FOSMN)

• This rare neurodegenerative condition presents with slowly evolving facial numbness followed by bulbar and proximal weakness 5
• Abnormal blink reflex studies provide diagnostic clues even before motor symptoms develop 5
• Should be considered if symptoms progress to involve bulbar function over months to years 5

Infectious Etiologies

• Consider Lyme disease serology in endemic areas or with appropriate exposure history 6
• Syphilis and HIV-related complications can cause cranial neuropathies 6
• These typically produce more widespread neurological involvement but may present focally 6

Common Pitfalls to Avoid

• Do not assume "normal exam" excludes serious pathology—sensory symptoms often precede objective findings 2
• Do not obtain CT as initial imaging—it is inadequate for evaluating the trigeminal nerve soft tissue pathology 1
• Do not delay imaging in patients with progressive symptoms, even if examination remains normal 1
• Do not forget that inflammatory conditions like IITSN can mimic tumors on MRI, potentially leading to unnecessary aggressive surgery 2

Imaging Protocol Specifications

The MRI should include:

• Thin-section high-resolution sequences through the skull base and posterior fossa 1
• Pre- and post-contrast T1-weighted images to identify nerve enhancement 1
• Heavily T2-weighted sequences to evaluate cisternal segments and vascular compression 1
• Coverage from brainstem through peripheral trigeminal branches in face 1