Numb Chin Syndrome

• Numbness/decreased sensation of the chin and lower lip
• Easily underemphasised by clinicians and patients
• Caused by a wide range of differentials - some associated with significant morbidity and mortality

Commonly affected area in numb chin syndrome

Clinical Features

• Paraesthesia, dysaesthesia or anaesthesia to chin and lower lip (mental/inferior alveolar nerve region)
  • Malignancy tends to be associated with numbness (pain occurs in <10% of cases associated with malignancy)
• Patients with skull based metastasis can also present with other cranial nerve abnormalities
• Patients with mandibular tumours may also present with swelling, loose teeth etc

Aetiology and Pathogenesis

Pathophysiology

• Can occur from a lesion anywhere along the course of the trigeminal nerve (including as proximal as trigeminal ganglion and pons)
• Lesions can be divided into:
  1. Peripheral lesions - involving mandible, direct nerve infiltration , inflammation or compression
  2. Central lesions - involving the base of skull, leptomeninges, or brainstem
• Malignancy associated cases can occur through the following mechanisms:
  1. Direct infiltration of the nerve
     • Head and neck primary or metastasis from distant site to mandible
  2. Leptomeningeal seeding
     • May have no obvious mass in the mandible or skull base
     • These cases are thought to be due to leptomeningeal seeding
  3. Cervical lymphadenopathy
     • Nodes in the deeper upper cervical region can exert pressure on the inferior alveolar nerve just before it enters the mandibular foramen
  4. Paraneoplastic Phenomenon
     • Antibodies may be directed against unknown antigens in the nervous system

Causes

Dental and Traumatic Injury

• Most common cause
• Dental (odontogenic cysts, abscesses, trauma)
• Facial trauma
• Iatrogenic injury (extraction of teeth, local anaesthetic blocks, endodontic treatment, orthographic surgery, dental implants, MRONJ)

Malignancy

• Primary tumours
  • Squamous cell carcinoma
  • Melanoma (of lower lip)
  • Lymphoma
  • Myeloma
• Metastasis
  • Mandible is the most common site for metastatic lesions to present in the oral and maxillofacial complex
  • 1-3 percent of oral and maxillofacial tumours are metastatic lesions
  • Common primary sites:
    1. Breast cancer (40%)
    2. Lymphoma (21%)
    3. Prostate cancer (7%)
    4. Leukemia (5%)

Infection

• Syphilis
• Lyme's disease
• Herpes

Drugs

• Mefloquine (malaria)
• Allopurinol (gout)
• Interferon-alpha (leukaemia, melanoma, lymphoma, Hep B and Hep C)

Inflammatory/Autoimmune

• Numb chin syndrome can be a rare presentation for the following:
  • Multiple Sclerosis
  • Giant cell arteritis
  • Post-hepatitis B vaccination
  • Systemic Lupus Erythematosus
  • Sjögren syndrome
  • Scleroderma
  • Rheumatoid arthritis
  • Mixed connective tissue disease
  • Dermatomyositis
  • Sarcoidosis
  • Diabetic polyneuropathy (usually subclinical)

Sickle Cell Disease

• Vaso-occlusive disease → pain and numbness of chin

Investigations

• Rule out dental, traumatic or iatrogenic causes
• Following investigations may be helpful in evaluating new onset cases without obvious causes

Imaging

• Orthopantogram (OPG)
  • Investigate for dental or traumatic cause
  • Will also demonstrate obvious boney pathology and lytic lesions
• Computed tomography (CT)
  • For further delineation of boney pathology/trauma
• Magnetic resonance imaging (MRI)
  • MRI of the head and neck can demonstrate any pathology along the full length of trigeminal nerve
  • Can also demonstrate demyelinating disease
• Positron emission tomography (PET)

Laboratory Investigations

• Routine bloods
  • FBC, ESR, CRP
• Bloods for specific diseases
  • Aninuclear antibodies, Antibodies to extractable nuclear antigens (Ro/SSA and La/SSB), HBA1C, ACE assay
• Peripheral blood smear (lymphoma/leukaemia screen)
• Infective screen
  • HIV
  • Lyme serology
  • Syphilis
  • Herpes simplex virus testing

Other

• Cerebrospinal fluid analysis
  • Consider if no history of trauma and all above imaging is negative
  • May reveal presence of malignant cells

Management

• Address the cause

Prognosis and Complications

• Again cause dependent
• Numb chin syndrome caused by malignancy has a poor prognosis - overall mortality around 80% with mean survival of 7 months