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An elderly lady with collapse
Q1: What are the findings in figs 1 an 2 (see p 105)?
Figure 1 shows right sided partial ptosis and a divergent squint due to pupil sparing third nerve (oculomotor) palsy—that is, there is no pupil asymmetry. In fig 2, the patient was asked to raise both arms and it shows weakness of the left arm in comparison with the right arm. Indeed the patient had left hemiparesis. The right arm, however, is not fully raised probably due to a degree of proximal weakness. Figure 2 also shows a feeding pump next to the patient and a feeding tube looped over the back of her chair leading to her percutaneous endoscopic gastrostomy (PEG).
Q2: What is the diagnosis?
Crossed hemiplegia due to a vascular lesion affecting the right mid-brain involving the right third nerve and the corticospinal tract at the level of the cerebral peduncles.
Computed tomography of the brain did not reveal any evidence of bleed, but a low density area in the left occipital lobe. The left hemiplegia gradually improved but dysphagia persisted requiring insertion of a PEG for feeding. Eventually the patient was discharged to a residential home. Six months after the onset, she remains with a residual hemiparesis, partial third nerve palsy, and PEG feeding.
Clinical features of the third, fourth, and sixth cranial nerve palsies
The third nerve supplies all the extraocular muscles of the eye except the lateral rectus muscle which is supplied by the sixth (abducent) nerve, and the superior oblique muscle being innervated by the fourth (trochlear nerve) nerve. The features of complete third nerve palsy include ptosis, divergent squint, diplopia, fixed dilatation of the pupil, and paralysis of accommodation.1 Patients with third nerve palsy are often relatively asymptomatic because of the ptosis, but on raising the eyelid, they would have diplopia in all directions except on lateral gaze to the side of the lesion.
However when the fourth nerve palsy develops, it often leads to a very subtle diplopia that is worse when the head is tilted down and away such as when descending the stairs. This abnormality results because of a failure of extorsion leading to an unopposed pull of the inferior rectus which raises the eye. Consequently patients slightly tilt the head and in children this sometimes may be felt to mimic a torticollis.
The sixth nerve palsy causes medial (convergent) squint with failure of abduction.2 Patients with sixth nerve palsy often complain of more diplopia than in those with third nerve palsy because they have no ptosis to mask the false image. In fact patients with sixth nerve palsy voluntarily close their affected eye, and the diplopia persists in all directions.
The third nerve nucleus and its relations
The third nerve nucleus in the mid-brain has two components: the third nerve motor nucleus and the Edinger-Westphal.2 The third nerve has such a long vertical extent that damage at various points can often lead to incomplete lesions. The third nerve motor nucleus is situated in the lower mid-brain and gives rise to the motor fibres supplying the extraocular muscles as mentioned earlier.2 The Edinger-Westphal nucleus is situated adjacent to the third nerve motor nucleus, and receives fibres from the periaqueductal grey matter involved in the consensual light reflex as well as information from the adjacent third nerve nucleus mass which is activated when the medial rectus muscle is activated in accommodation. The Edinger-Westphal nucleus gives rise to the preganglionic parasympathetic fibres.
The third nerve emerges from the mid-brain between the cerebral peduncles where the corticospinal tracts are carried, and at this point it lies between the posterior cerebral artery and superior cerebellar artery and then it runs parallel to the posterior communicating artery until it reaches the cavernous sinus. After entering the orbit through the superior orbital fissure, the third nerve divides into an upper and lower branch. The upper branch supplies the levator palpebrae superioris and superior rectus muscles. The lower branch supplies three muscles: the medial rectus, the inferior rectus, and the inferior oblique muscle.1
Box 1 : Learning points
The third nerve nucleus has two components in the mid-brain: the Edinger-Westphal nucleus and the motor oculomotor nucleus.
The Edinger-Westphal nucleus is situated in the upper mid-brain and gives rise to the preganglionic parasympathetic neurones responsible for the pupilloconstrictor fibres.
The motor third nucleus is situated in the lower mid-brain and gives rise to the motor fibres supplying the extraocular muscles.
The pupilloconstrictor fibres and those innervating the levator palpebrae superioris lie superficially in the trunk of the nerve.
The nerve to the inferior oblique muscle conveys the preganglionic parasympathetic fibres to the ciliary ganglion from which the post-ganglionic parasympathetic fibres arise to supply the ciliary muscle and the muscles of the iris (pupilloconstrictor fibres). After the ciliary ganglion, the parasympathetic fibres subdivide into 8–10 ciliary nerves supplying the pupilloconstrictor muscles.
The pupilloconstrictor fibres and those innervating the levator palpebrae lie in a superficial and dorsal position on the nerve relaying in the ciliary ganglion which is in the posterior orbit. Because of this anatomical characteristic, a fixed dilated pupil is often the first sign of third nerve (oculomotor) compression and ptosis the second, before the external ophthalmoplegia develops.3
The particular anatomical features of the third nuclei, and the nerve trunk described earlier, allows selective damage to the motor nucleus or the fibres in the nerve trunk to present as pupil sparing third nerve palsy, with or without features of long tract signs. For example, the corticospinal fibres descend into the mid-brain where they rotate into the medial part of the of the cerebral peduncles with the fibres supplying the leg lying laterally and those supplying the arm medially. The corticospinal fibres are then spread out by numerous transverse pontine fibres coming together in the lower third of the pons as a preliminary to their decussation in the medullary pyramid. As they decussate, the arm fibres lie medially and cross the mid line above the leg fibres assuming a medial position in the corticospinal tract on the opposite side of the cord.
The sudden occurrence of the pupil sparing third nerve palsy on the right side and hemiplegia on the left side indicate a mid-brain vascular lesion for the clinical manifestations of the patient. Pupil sparing third nerve palsy could occur both intra-axially (within the mid-brain) or extra-axially, that is, in the nerve trunk. Pupil sparing third palsy is a recognised complication in some patients with diabetes mellitus, and in ischaemic damage of the nerve.4 Selective intra-axial lesions of the third nerve with or without pupillary involvement have been observed.5 In the latter report, lesions of the mid-brain sparing the upper part where the Edinger-Westphal nucleus is placed, the pupils were not affected whereas lesions involving both upper and lower parts of the mid-brain were associated with pupillary dilatation. Earlier reports have also demonstrated vascular causes of intra-axial pupil sparing third palsy due to mid-brain haemorrhage and infarction.6,7 The classical description of the Weber's syndrome includes ipsilateral third nerve palsy with mydriasis and crossed hemiplegia.8 However, the patient described here is similar to classical Weber's syndrome but the pupils were spared.
Box 2 : Pupil sparing third palsy may occur in:
Ischaemic damage to the nerve.
Ischaemic mid-brain lesions involving the lower mid-brain.
Crossed hemiplegia due to a vascular lesion affecting the right mid-brain involving the right third nerve and the corticospinal tract.
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