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334 Chapter 7: Nervous system

Clinical features Clinical features
Proximal muscle weakness, particularly of the hips, later Diminished sense of smell and taste (closely linked to
affecting the shoulders. Ptosis may occur. The ocular and smell) although this may be found in elderly patients
bulbar muscles are typically spared. Unlike myasthenia without olfactory nerve lesions. Test ability of each nos-
gravis, weakness tends to be worst in the morning and tril to detect several common smells.
improve with exercise. Reflexes are reduced, but nor-
malise with exercise.
The optic nerve

Investigations Anatomy
Nerveconduction studies show an incremental re- The optic nerve carries information from the retina via

sponse when a motor nerve is repetitively stimulated, the optic chiasm, the lateral geniculate bodies and optic
in direct contrast to the findings in myasthenia gravis radiation to the occipital lobe where the visual cortex is
(where there is a decremental response). situated.
Chest X-ray and CT screening for an underlying tu-

mour. Function
Specific serum antibodies can be detected. Vision

Management Clinical features
Treatment of the underlying tumour can lead to These depend on the location of the lesion (see Fig. 7.4).
improvement. Plasmapheresis and intravenous im- Field loss:
munoglobulin may be used, and drugs which increase Eye lesions include diabetic retinal vascular disease,
acetylcholine release from presynaptic terminals appear glaucoma, retinitis pigmentosa.
to have symptomatic benefit. Optic nerve lesions include multiple sclerosis, com-
pression, syphilis, ischaemia, B 12 deficiency.
Optic chiasm lesions commonly caused by pituitary

Disorders of cranial and neoplasm, rarely secondary neoplasm.
peripheral nerves Optic tract lesions alone are rare but commonly occur
as part of a middle cerebral artery stroke.
Optic cortex lesions (macula spared) caused by pos-
Cranial nerves
terior cerebral artery infarction. Widespread bilateral
occipital damage, e.g. posterior circulation infarction
Olfactory nerve (I) lesion
causescorticalblindness(Anton’sSyndrome)inwhich
Anatomy the patient lacks insight and denies blindness.
The olfactory receptors lie in the olfactory epithelium Tunnel vision occurs in other conditions, e.g. in glau-
in the upper part of the superior turbinate and the nasal coma or late retinitis pigmentosa.
septum. The axons form bundles which pass through the Diseasesaffectingtheopticnerveandtherestoftheoptic
cribiform plate (ethmoid bone) to the olfactory bulb. pathways may also affect visual acuity.
The bundles are wrapped in meninges. The olfactory
bulb neurones project through the olfactory tract to the Abnormalities of the optic disc
frontal cerebral hemispheres, the medial temporal lobe
and the basal ganglia. Definition
The optic disc is where the retinal fibres meet to form
Function the optic nerve. Diseases affecting the optic nerve may
Smell cause the disc to look abnormal:
1 Swollen, i.e. less cupped – papilloedema, optic neuritis
Specific causes (sometimes called papillitis)
Trauma, frontal lobe tumour, meningitis. 2 Paleduetolossofaxonsandvascularity–opticatrophy