11.7 — The Twelve Cranial Nerves

Twelve pairs of nerves leave the brain directly rather than through the spinal cord. Between them they carry every special sense, all the movement of the face, eyes, jaw, tongue and throat, and — through one enormous outlier — the control of the heart, lungs and gut.

Testing all twelve takes about five minutes and localises a lesion in the brainstem to within a few millimetres, which is why the cranial nerve examination is one of the most information-dense procedures in medicine.

The twelve

The cranial nerves seen from below, emerging in numerical order from front to back. Only the first two come from the cerebrum; the remaining ten emerge from the brainstem, which is why brainstem lesions produce such specific combinations. Image: Wikimedia Commons.

No.	Name	Type	Main job
I	Olfactory	Sensory	Smell
II	Optic	Sensory	Vision
III	Oculomotor	Motor	Most eye movement, pupil, eyelid
IV	Trochlear	Motor	One eye muscle (superior oblique)
V	Trigeminal	Both	Facial sensation, chewing
VI	Abducens	Motor	One eye muscle (lateral rectus)
VII	Facial	Both	Facial expression, taste (front of tongue)
VIII	Vestibulocochlear	Sensory	Hearing and balance
IX	Glossopharyngeal	Both	Swallowing, taste (back of tongue)
X	Vagus	Both	Voice, swallowing, heart, lungs, gut
XI	Accessory	Motor	Shoulder shrug, head turn
XII	Hypoglossal	Motor	Tongue movement

Only I and II arise from the cerebrum. The other ten emerge from the brainstem.

And I and II are not really peripheral nerves at all — they are outgrowths of the brain, wrapped in meninges, made of central nervous tissue. This is why the optic nerve does not regenerate after damage while a cut facial nerve can (Chapters 4.4, 11.1).

Nerve by nerve

I — Olfactory

Smell. Fibres pass through the cribriform plate (Chapter 5.2).

Loss of smell (anosmia). Commonest causes: nasal blockage, viral infection, head injury shearing the fibres at the cribriform plate, and neurodegenerative disease.

Two points worth carrying. Loss of smell is an early feature of both Parkinson's and Alzheimer's disease, sometimes preceding other symptoms by years — though it is far too non-specific to be used as a test.

And COVID-19 brought anosmia to general attention, with a distinctive pattern: loss of smell with a clear nose, which is unusual. Most recovered; a minority developed parosmia, in which familiar smells become distorted and often revolting, as the regenerating receptor neurons reconnect imperfectly.

Most of what you experience as taste is smell, which is why food is bland with a blocked nose (Chapter 11.13).

II — Optic

Vision. Tested by acuity, visual fields, colour vision, and looking into the eye.

The visual field defects localise beautifully, because of where the fibres cross.

Fibres from the nasal half of each retina cross at the optic chiasm; fibres from the temporal half do not.

So:

• Damage before the chiasm (the optic nerve itself) — blindness in that one eye.
• Damage at the chiasm — loss of both outer halves of the visual field, because the crossing fibres from both nasal retinas are cut. This is bitemporal hemianopia, and its classic cause is a pituitary tumour pressing up on the chiasm from below (Chapter 12.2). The person bumps into things on both sides and often does not notice the loss.
• Damage behind the chiasm — loss of the same half of the field in both eyes, on the opposite side to the lesion.

One test, and you know whether the problem is in front of, at, or behind a structure the size of a grape.

Looking into the eye is the only place you can see brain tissue and blood vessels directly (Chapter 4.4). A swollen optic disc means raised pressure inside the skull.

III, IV, VI — Eye movement

Six muscles move each eye. III supplies four of them plus the pupil and the eyelid; IV supplies one; VI supplies one.

A memorable formula: LR6 SO4, all the rest 3. Lateral rectus by VI, superior oblique by IV, everything else by III.

III palsy — the eye is turned down and out (because the two muscles still working pull it that way), the eyelid droops, and the pupil is often dilated.

And the pupil is the critical part. A III palsy with a dilated pupil is a surgical emergency — usually compression, from an aneurysm or from the brain shifting (Chapter 11.4), because the parasympathetic fibres run on the nerve's surface. A III palsy with a spared pupil is usually microvascular, from diabetes or hypertension, and it recovers over weeks. Pupil involved means scan today; pupil spared means observe.

IV palsy — difficulty looking down and in, so the person has trouble on stairs. They characteristically tilt their head away from the affected side, which compensates and is often the first thing noticed.

VI palsy — the eye cannot turn outward, giving double vision worse on looking to that side.

VI has the longest intracranial course of any cranial nerve, so it is stretched by any rise in pressure inside the skull. A VI palsy is therefore a "false localising sign" — it indicates raised pressure without indicating where the problem is.

V — Trigeminal

Three branches — ophthalmic, maxillary and mandibular — supplying sensation to the forehead, cheek and jaw respectively, plus the muscles of chewing.

Trigeminal neuralgia is one of the most severe pains in medicine. Sudden, brief, electric-shock-like stabs in one branch's territory, triggered by light touch, chewing, cold air, or brushing teeth. People stop eating and washing to avoid triggering it.

The commonest cause is a blood vessel pressing on the nerve root where it leaves the brainstem, and the pulsation gradually damages the myelin.

It responds well to carbamazepine — an anticonvulsant, used because the mechanism is abnormal nerve firing rather than tissue damage. Ordinary painkillers do essentially nothing, which is diagnostic in itself. Surgical decompression, moving the vessel off the nerve, is curative in a large proportion.

In someone under 40 with trigeminal neuralgia, multiple sclerosis should be considered, because a plaque at the nerve root entry can cause it.

The corneal reflex — touching the cornea causes both eyes to blink. Sensation by V, blink by VII, so it tests both.

VII — Facial

Chapter 6.3 covers this in full. The essential point: the forehead has bilateral supply, so a stroke spares the forehead and a facial nerve palsy does not. That distinction takes three seconds and separates a benign condition from an emergency.

VII also carries taste from the front two thirds of the tongue and supplies the tear and salivary glands, which is why a facial palsy can affect taste and cause a dry eye.

VIII — Vestibulocochlear

Two nerves in one: hearing and balance.

Testing hearing at the bedside uses two tuning fork tests, and together they separate the two kinds of deafness.

Rinne's test — the fork is held on the mastoid bone and then beside the ear. Normally air conduction is better than bone conduction. If bone is better, sound is not getting through the middle ear — conductive deafness.

Weber's test — the fork is placed on the middle of the forehead. Normally it is heard equally in both ears. In conductive deafness it is louder in the affected ear; in sensorineural deafness it is louder in the good ear.

That conductive result is counter-intuitive and worth understanding. A blocked middle ear cannot let sound out either, so it stops competing with the bone-conducted signal and it also excludes background noise — so the bone-conducted tone sounds louder on that side. You can demonstrate it on yourself: hum, and block one ear with a finger. The hum gets louder on the blocked side.

Acoustic neuroma — a benign tumour of the nerve's sheath — causes gradual one-sided hearing loss with tinnitus. One-sided hearing loss always warrants investigation, because the causes differ entirely from the usual symmetrical age-related loss.

IX and X — Glossopharyngeal and vagus

Tested together, because they act together on the palate and pharynx.

Ask the person to say "aah" and watch the palate. It should rise symmetrically. With a one-sided lesion, the uvula deviates away from the affected side, pulled by the working muscle.

The gag reflex — sensation by IX, movement by X. It is absent in a substantial minority of healthy people, so its absence alone means little, and its usefulness in assessing swallowing safety is limited.

The vagus deserves separate emphasis, because it is unlike every other cranial nerve.

Vagus means wandering, and it earns the name. It supplies the larynx, and then descends through the neck into the chest and abdomen, supplying the heart, the lungs, and the gut as far as about two thirds of the way along the colon.

About 80 percent of its fibres are sensory, running from the organs to the brain — which is not what most people assume. The gut sends far more information up than the brain sends down, and this is the anatomical basis of the gut–brain axis (Chapter 9.6).

Its recurrent laryngeal branch takes the absurd detour of Chapter 3.6, which is why hoarseness can be the first sign of a lung tumour.

Vagal stimulation slows the heart (Chapter 7.4), and vagus nerve stimulation with an implanted device is an established treatment for drug-resistant epilepsy and for treatment-resistant depression — an intervention derived directly from the fact that most of the nerve runs upward.

XI — Accessory

Supplies sternocleidomastoid and trapezius (Chapter 6.3).

Tested by shrugging the shoulders and turning the head against resistance.

Its superficial course in the neck makes it vulnerable in surgery, and injury causes shoulder drop, difficulty lifting the arm above the head, and chronic pain.

XII — Hypoglossal

Moves the tongue.

Ask the person to stick their tongue out. It deviates toward the weak side, because the intact muscle on the other side pushes it across.

A useful memory: "the tongue points to the lesion" — for a lower motor neuron lesion. With an upper motor neuron lesion it deviates away, because the supply to that muscle is from the opposite hemisphere.

Wasting and fasciculations of the tongue — visible flickering — indicate a lower motor neuron problem, and they are one of the classic early signs of motor neurone disease (Chapter 20.6).

Patterns worth recognising

Cavernous sinus syndrome — a venous space beside the pituitary through which III, IV, VI and two branches of V all pass. A problem there — thrombosis, tumour, infection — knocks out several at once, giving a paralysed eye with facial numbness, which no other single lesion produces.

Cerebellopontine angle syndrome — the corner where V, VII and VIII emerge together. An acoustic neuroma there causes hearing loss, then facial numbness, then facial weakness, in that order, as it grows and presses on each in turn.

Jugular foramen syndrome — IX, X and XI pass through together, so a lesion there affects swallowing, voice and shoulder shrug simultaneously.

And bulbar versus pseudobulbar palsy. Both affect speech and swallowing.

Bulbar palsy is a lower motor neuron problem in the medulla — flaccid, wasted, fasciculating tongue, nasal speech, absent gag.

Pseudobulbar palsy is an upper motor neuron problem, from bilateral damage above — spastic, small, immobile tongue, strained speech, brisk gag. And characteristically, emotional lability — sudden uncontrollable laughing or crying disconnected from actual mood, which is distressing and is caused by loss of cortical control over brainstem emotional expression rather than by any change in feeling.

Both occur in motor neurone disease and in stroke, and distinguishing them narrows the location considerably.

What the next page fixes

The cranial nerves cover the head. Chapter 11.8 covers the rest — the peripheral nerves and the dermatome map, which turns out to be a direct record of the segments the embryo was built from, and which is how the level of a nerve problem is identified from a pattern of numbness.