Neurological Disorders Flashcards
Over a period of several days a 35-year-old man developed total paralysis of his body and most of his face. He could not swallow or speak. Horizontal eye movements were impaired but vertical eye movements and eye blinks were maintained. Communication via a code of eye movements showed that he remained mentally alert and that sensation over his whole body remained intact. A brainstem vascular lesion was suspected. (Vignette 1)
Which pathway has been affected?
Corticospinal and corticobulbar tracts projecting to spinal and brainstem motor neurones respectively.
Vignette 1
Why are vertical but not horizontal eye movements maintained?
The lesion must be below oculomotor nucleus (vertical eye movements spared) but above abducens (lateral eye movements lost) and all lower motor nuclei. The preserved “eyeblinks” are not true reflexes as the motor nucleus of the facial nerve is below the level of the lesion. They will be intermittent upward movements of the eyelid made by the oculomotor innervation of the levator palpebrae muscles.
Vignette 1
Where in the brainstem is the lesion?
Basal part of the upper pons (locked-in syndrome)
Vignette 1
Why are sensation and consciousness not affected?
The lesion is confined to the ventral (anterior) part of the brainstem thus sparing the ascending sensory tracts and reticular formation which are more dorsally (posteriorly) located.
Vignette 1
Which artery is most likely to have been involved?
Thrombosis of the basilar artery leads to this isolated lesion. The medulla is still supplied by branches of the vertebral arteries while the internal carotid arteries maintain the circle of Willis for supply of the forebrain and midbrain.
A 46-year-old woman presented with weakness of the muscles of the left side of her face, accompanied by reduced sensation in the same area. On examination she was found to have reduced hearing in the left ear. She thought the deafness had been present for several months and also admitted to occasional bouts of dizziness.
(Vignette 2)
These symptoms are all caused by a single lesion – where?
In or near the left internal acoustic meatus.
Vignette 2
What type of lesion is it?
Acoustic schwannoma (also called acoustic neurinoma/neurofibroma) - a benign tumour which develops slowly on the vestibular part of the VIII cranial nerve.
Vignette 2
Explain the symptoms of the structures involved.
The first signs are usually sensorineural deafness due to pressure on the auditory part of the nerve, accompanied or followed by tinnitus and/or vertigo. As the tumour grows out into the posterior fossa, there may be impaired facial sensation (e.g. demonstrated by loss of the corneal reflex) due to distortion of cranial nerve V, and weakness of the facial muscles due to stretching of nerve VII.
Vignette 2
How can the condition be treated?
Removed surgically but there is usually residual deafness.
Vignette 2
What is likely to happen if the condition is left untreated?
Further growth of the tumour could put pressure on the cerebellum and brainstem, causing unilateral ataxia + signs of raised intracranial pressure.
58-year-old gentleman has presented to the eye casualty department complaining of inability to open his right eye. You lift his lid up and find that the eye is deviated down and out. He complains of double vision when you do this. On H-test he cannot elevate or adduct his right eye. His left eye has normal movements in all directions. His right pupil is also dilated in comparison to the left.
(Vignette 3)
Which cranial nerve may account for this presentation?
Nerve lesion can be delineated by assessing which muscles/structures are affected and which nerve these are supplied by.
The complete ptosis is due to failure of Levator Palpebrae Superioris. When stimulated this muscle causes elevation of the lid. It is innervated by the oculomotor nerve.
‘Down and Out’ eye and failure of adduction/elevation suggest unopposed action of Superior Oblique and Lateral Rectus. Cranial nerves 3, 4 and 6 innervate the extra-ocular muscles. These can be remembered using the mnemonic LR6 and SO4. Oculomotor nerve supplies the remaining EOM and hence it is the affected CN.
The dilated pupil is due to either overaction of the sympathetic nervous system (causes mydriasis via dilator pupillae) or underaction of parasympathetic nervous system (causes miosis via sphincter pupillae). In this case the oculomotor nerve, which carries parasympathetic innervation to the eye, is the most likely explanation given the other clinical findings.
Hence the lesion is in the Right Oculomotor Nerve (cranial nerve III).
Vignette 3
Which pathologies may cause this?
Lesions affecting the oculomotor nerve are usually classified into medical and surgical.
Medical lesions, typically affecting the vasculature to the nerve (and hence the central portion of the nerve), tend to not affect the pupil (‘pupil sparing’) as the parasympathetic fibres running to the eye are in the outer portion of the nerve.
Microvascular disease due to hypertension, diabetes mellitus.
Surgical lesions, usually in the form of a posterior communicating artery aneurysm, tend to affect the nerve. The aneurysm typically compresses the outer portion of the nerve fibres, which is where the parasympathetic nerves run.
In relation to the Circle of Willis: CN3 runs in between the posterior cerebral and the superior cerebellar artery.
Vignette 3
What treatments may be offered for this presentation?
Treatment will depend on the cause of the cranial nerve palsy. In this case, the cause is likely to be a posterior communicating artery aneurysm. Hence a neurosurgical opinion is advised, and assessment of the aneurysm is needed (this may involve clipping).
For patients with microvascular disease, they will need better metabolic control of their condition.
Vignette 4
A 28-year old lady presents with a tonically dilated pupil in the right eye. Upon shining light into the right eye, the pupil is slow to react compared to the left. The left eye reacts when light is shone into the right eye. Addition of pilocarpine drops, however, causes rapid constriction of the right pupil. On further examination, she has an absent knee-jerk reflex and impaired sweating.
What is the site of damage causing the pupil defect?
Damage to parasympathetic ciliary ganglion which is involved in the pupillary-light reflex. Parasympathetic fibres travel with CNIII (oculomotor) to synapse at the ciliary ganglion before innervating the iris and ciliary body.
