What are the differential diagnoses for an insensate larynx?

Differential Diagnosis for Insensate Larynx

Immediate Life-Threatening and High-Morbidity Causes

The most critical diagnosis to exclude first is acute stroke, particularly brainstem or cortical lesions affecting the vagal nuclei or their pathways, because loss of laryngeal sensation leads directly to aspiration pneumonia—a life-threatening complication. 1, 2

• Brainstem stroke affecting the nucleus ambiguus or vagal pathways produces loss of supraglottic and glottic sensation, resulting in silent aspiration and rapid-onset pneumonia. 1, 2
• Post-stroke laryngeal anesthesia is a recognized cause of dysphagia and aspiration, requiring emergent neuroimaging when acute sensory loss is identified. 1
• Head and neck malignancy (laryngeal, lung, thyroid cancer) can infiltrate or compress the superior laryngeal nerve, causing progressive sensory loss; delayed diagnosis results in higher staging and reduced survival. 3, 4

Iatrogenic Nerve Injury

Surgical injury to the internal branch of the superior laryngeal nerve (iSLN) is the most common iatrogenic cause of insensate larynx, occurring during thyroidectomy, anterior cervical spine surgery, or carotid procedures. 3, 5, 6

• Thyroidectomy carries a risk of iSLN injury during ligation of the superior thyroid artery and its branches, particularly when anatomical variants are present; the nerve supplies sensation to the supraglottic larynx, epiglottis, and arytenoid region. 3, 5, 6
• Anterior cervical spine surgery produces vocal fold paralysis in 1.69–24.2% of cases and can injure the iSLN when dissection extends superiorly near the superior thyroid vessels. 3, 4
• Signs of iSLN injury include loss of sensation in the upper larynx, resulting in choking and aspiration of swallowed fluids, not voice changes (which occur with external branch injury). 6
• Prolonged intubation causes laryngeal injury in 2.3–84% of cases depending on duration and can produce both motor and sensory deficits. 3, 4

Neurological and Neurodegenerative Disorders

Progressive neurological diseases affecting the vagus nerve or its central pathways produce gradual loss of laryngeal sensation with high aspiration risk. 3, 1, 4

• Amyotrophic lateral sclerosis (ALS) with bulbar involvement causes combined motor and sensory laryngeal dysfunction, leading to progressive dysphagia and aspiration. 3, 1
• Multiple sclerosis can produce demyelinating lesions affecting vagal sensory pathways in the brainstem, resulting in variable laryngeal anesthesia. 3
• Parkinson's disease primarily causes motor dysfunction but can be associated with reduced laryngeal sensation as part of generalized sensory impairment. 3, 1
• Glossopharyngeal neuralgia or neuropathy may extend to involve the superior laryngeal nerve through anatomical proximity and shared pathways. 3

Peripheral Neuropathy and Systemic Disease

Systemic conditions producing peripheral neuropathy can selectively or diffusely affect laryngeal sensory nerves. 3, 4

• Diabetes mellitus causes peripheral neuropathy that can involve cranial nerves, including the vagus and its laryngeal branches, producing insensate larynx. 3, 4
• Rheumatologic and autoimmune diseases (Sjögren's syndrome, rheumatoid arthritis, sarcoidosis, amyloidosis, granulomatosis with polyangiitis) can infiltrate or inflame laryngeal nerves. 3, 4
• Viral neuropathy affecting the vagus nerve (analogous to Bell's palsy for facial nerve) can produce isolated superior laryngeal nerve dysfunction. 3

Tumor Compression and Infiltration

Mass lesions compressing or infiltrating the vagus nerve or superior laryngeal nerve produce progressive sensory loss. 3, 4, 5

• Thyroid cancer infiltrating the superior laryngeal nerve or vagus produces both motor and sensory deficits; tobacco use increases risk 2- to 3-fold. 3, 4
• Lung cancer (particularly apical Pancoast tumors) can involve the vagus nerve in the superior mediastinum. 3, 4
• Skull base tumors (schwannomas, meningiomas, glomus tumors) affecting the jugular foramen compress the vagus nerve before it gives off laryngeal branches. 3
• Posterior fossa masses (cerebellar tumors, brainstem gliomas) can produce central vagal dysfunction. 1

Anatomical Variants and Congenital Causes

Rare anatomical variants of the superior laryngeal nerve increase vulnerability to injury and may present as congenital sensory deficits. 3, 5

• Aberrant course of the internal superior laryngeal nerve relative to the superior laryngeal artery (64.3% superoposterior, 35.7% inferoposterior) increases surgical injury risk. 5
• Congenital absence or hypoplasia of the internal branch of the superior laryngeal nerve is extremely rare but reported. 5, 2

Critical Diagnostic Approach

Begin with targeted history focusing on onset (acute vs. gradual), recent surgery (especially thyroid or cervical spine), aspiration symptoms (choking on liquids, recurrent pneumonia), neurological symptoms (stroke signs, progressive weakness), and cancer risk factors (smoking, prior malignancy). 3, 1, 4, 6

Physical examination must include:

• Cincinnati Prehospital Stroke Scale (facial droop, arm drift, speech) to exclude acute stroke. 1
• Flexible laryngoscopy to visualize vocal fold motion and assess for masses, inflammation, or structural abnormalities. 3, 4
• Sensory testing of the supraglottic larynx using air pulse stimulation (threshold normally 2.09 ± 0.15 mm Hg) or laryngeal adductor reflex testing to quantify sensory loss. 7, 8
• Complete neurological examination including other cranial nerves, cerebellar signs, and motor function. 1

Diagnostic testing should proceed as follows:

• Emergent brain MRI or CT if acute onset or stroke signs are present. 1
• Laryngeal electromyography (LEMG) to differentiate peripheral nerve injury from central disorders and to assess for denervation or reinnervation patterns. 3
• Imaging of the neck and chest (CT or MRI) to evaluate for masses compressing the vagus or superior laryngeal nerve, particularly in patients with cancer risk factors or progressive symptoms. 3, 4
• Swallowing evaluation (modified barium swallow or fiberoptic endoscopic evaluation of swallowing) to assess aspiration risk, as insensate larynx produces silent aspiration. 2, 7

Common Pitfalls to Avoid

Never attribute laryngeal sensory loss to "normal aging" without excluding stroke, malignancy, or progressive neurological disease. 3, 1, 4

Do not assume that normal vocal fold motion on laryngoscopy excludes superior laryngeal nerve injury—the internal branch is purely sensory and does not affect vocal fold movement; only the external branch (motor to cricothyroid) affects voice quality. 5, 6

Failure to recognize insensate larynx leads to recurrent aspiration pneumonia, which carries high morbidity and mortality, particularly in elderly patients. 2, 7

In post-thyroidectomy patients, do not overlook superior laryngeal nerve injury because surgeons and patients focus on recurrent laryngeal nerve function (voice); iSLN injury produces aspiration without voice change. 3, 5, 6

When insensate larynx coexists with vocal fold paralysis, suspect combined recurrent and superior laryngeal nerve injury (high vagal lesion) or brainstem pathology rather than isolated nerve injuries. 3