25th June 2023

Cranial nerves of the oropharynx and of smell – I, X, XII

CN1 – Olfactory nerve

The olfactory nerve arises from the olfactory epithelium, passing through the cribriform plate of the ethmoid bone and running through the olfactory bulb to terminate in the primary olfactory cortex. It carries afferent impulses for the sense of smell and is commonly affected by trauma and infection.

Problems with olfaction can also be a sentinel sign of neurodegenerative disorders which affect the olfactory bulb. Hyposmia is recognised as an early ‘pre-clinical’ sign of Parkinson’s disease, present in 75% of patients with idiopathic Parkinson’s disease, higher than the prevalence of resting tremor. Other α-synuclein and tau-related disorders, e.g. Alzheimer’s dementia, also have associations with olfactory dysfunction.

Kallmann syndrome

Kallmann syndrome is a rare form of hypogonadotropic hypogonadism. It is associated with anosmia or hyposmia as a result of failure of production or activity of gonadotropin-releasing hormone (GnRH) by the hypothalamus. This defect in GnRH producing neurons is also associated with the loss of olfactory neurons, resulting in the syndrome of anosmia and hypogonadism, failure to achieve puberty and cranio-facial defects.

CN X – Vagus nerve

The vagus nerve is a mixed nerve that extends beyond the cranial and cervical area, travelling via the jugular foramen to provide motor parasympathetic function to the cardiac and respiratory systems, and all visceral organs except the adrenal glands. It also innervates part of the pharynx and is responsible for the gag reflex.

Disorders of the vagus nerve can result in dysphagia as part of bulbar and pseudobulbar palsy, loss of the gag reflex which can contribute to aspiration pneumonia and brain-gut axis disorders causing gastroparesis and gastroesophageal reflux disease. Isolated disorder of the vagal nerve is very rare, and it is more likely to be involved as part of the manifestation of systemic pathology like stroke and multiple sclerosis.

CN XII – Hypoglossal nerve

The hypoglossal nerve arises from the medulla and travels through the hypoglossal canal, innervating the intrinsic and extrinsic muscles of the tongue. Disorders of the nerve cause problems with speech and swallowing.

Damage to the hypoglossal nerve itself (a lower motor neuron lesion) is very rare, mostly due to trauma and will cause an ipsilateral weakness of the tongue muscles, manifesting as deviation of the tongue to the ipsilateral side, due to ipsilateral tongue muscles being unable to balance the contraction of the genioglossus from the contralateral side.

Unilateral upper motor neuron lesions disrupting pathways to the hypoglossal nerve nuclei are more common and will result in contralateral weakness of the tongue, and therefore deviate to the contralateral side, as the contralateral muscles are affected and would be unable to balance the action of the unaffected side. Both unilateral and bilateral damage can occur, with bilateral damage resulting in a complete inability of the tongue to function.

Pseudobulbar palsy and bulbar palsy

Pseudobulbar palsy results from a lesion that is supranuclear to the hypoglossal nerve (i.e. an upper motor neuron lesion). Causes can be traumatic, metabolic, vascular, neoplastic, or autoimmune. In this instance, the corticobulbar tracts are most likely to be involved, and the manifestation is similar to that of a true bulbar palsy with impairment of articulation, mastication, and swallowing, but as the lesion is above the bulbar areas, it is termed ‘pseudo’ bulbar. Features include slow dysarthric speech, stiff and spastic tongue without fasciculations, dysphagia, and brisk jaw jerk. Due to the bilateral innervation of CN XII from the cortex, unilateral corticobulbar tract involvement would result in relatively preserved function.

True bulbar palsy is usually caused by a neurodegenerative disease that affects the bulbar areas, in particular, motor neuron disease with bulbar involvement such as progressive bulbar palsy, bulbar amyotrophic lateral sclerosis, and spinal and bulbar muscular atrophy (Kennedy’s disease – an X-linked recessive, CAG repeat mutation of the androgen receptor causing neurological, muscular, and endocrine loss of function.)

It is possible for other general causes of nerve disorders to have bulbar involvement and cause a bulbar palsy, e.g. a medullary brainstem stroke, or tumour specifically involving the brainstem, or neuroinflammatory conditions that can affect any nerve, e.g. Lyme disease, neurosarcoidosis, the acute bulbar palsy plus variant of Guillain Barré syndrome, but these are much rarer manifestations.