Question 116#

A man is recovering after having an operation to remove a meningioma in his left temporal lobe.

What sort of visual field defect is he at risk of having following the procedure?

A. Right inferior homonymous quadrantanopia

Correct Answer B: 

Visual field defects:

• Left homonymous hemianopia means visual field defect to the left, i.e. lesion of right optic tract
• Homonymous quadrantanopias: PITS (Parietal-Inferior, Temporal-Superior)
• Incongruous defects = optic tract lesion; congruous defects= optic radiation lesion or occipital cortex

A congruous defect simply means complete or symmetrical visual field loss and conversely an incongruous defect is incomplete or asymmetric.

Homonymous hemianopia: 

• Incongruous defects: lesion of optic tract
• Congruous defects: lesion of optic radiation or occipital cortex
• Macula sparing: lesion of occipital cortex

Homonymous quadrantanopias*: 

• Superior: lesion of temporal lobe
• Inferior: lesion of parietal lobe
• Mnemonic = PITS (Parietal-Inferior, Temporal-Superior)

Bitemporal hemianopia: 

• Lesion of optic chiasm
• Upper quadrant defect > lower quadrant defect = inferior chiasmal compression, commonly a pituitary tumour
• Lower quadrant defect > upper quadrant defect = superior chiasmal compression, commonly a craniopharyngioma

*this is very much the 'exam answer'. Actual studies suggest that the majority of quadrantanopias are caused by occipital lobe lesions. Please see the following link for more details: http://www.ncbi.nlm.nih.gov/pubmed/9109741

Question 117#

A 63-year-old man is prescribed selegiline for Parkinson's disease. What is the mechanism of action?

A. Dopamine receptor antagonist

Correct Answer C: 

Parkinson's disease: management:
Currently accepted practice in the management of patients with Parkinson's disease (PD) is to delay treatment until the onset of disabling symptoms and then to introduce a dopamine receptor agonist. If the patient is elderly, levodopa is sometimes used as an initial treatment.

Dopamine receptor agonists:

• e.g. Bromocriptine, ropinirole, cabergoline, apomorphine
• Ergot-derived dopamine receptor agonists (bromocriptine, cabergoline, pergolide*) have been associated with pulmonary, retroperitoneal and cardiac fibrosis. The Committee on Safety of Medicines advice that an echocardiogram, ESR, creatinine and chest x-ray should be obtained prior to treatment and patients should be closely monitored
• Patients should be warned about the potential for dopamine receptor agonists to cause impulse control disorders and excessive daytime somnolence
• Nasal congestion and postural hypotension are also seen in some patients

Levodopa: 

• Usually combined with a decarboxylase inhibitor (e.g. Carbidopa or benserazide) to prevent peripheral metabolism of levodopa to dopamine
• Reduced effectiveness with time (usually by 2 years)
• Unwanted effects: dyskinesia (involuntary writhing movements), 'on-off' effect, dry mouth, anorexia, palpitations, postural hypotension, psychosis
• No use in neuroleptic induced parkinsonism 

MAO-B (Monoamine Oxidase-B) inhibitors:

• e.g. Selegiline
• Inhibits the breakdown of dopamine secreted by the dopaminergic neurons

Amantadine: 

• Mechanism is not fully understood, probably increases dopamine release and inhibits its uptake at dopaminergic synapses 

COMT (Catechol-O-Methyl Transferase) inhibitors:

• e.g. Entacapone
• COMT is an enzyme involved in the breakdown of dopamine, and hence may be used as an adjunct to levodopa therapy
• Used in established PD 

Antimuscarinics: 

• Block cholinergic receptors
• Now used more to treat drug-induced parkinsonism rather than idiopathic Parkinson's disease
• Help tremor and rigidity
• e.g. Procyclidine, benzotropine, trihexyphenidyl (benzhexol)

*Pergolide was withdrawn from the US market in March 2007 due to concern regarding increased incidence of valvular dysfunction.

Question 118#

Which one of the following is most associated with downbeat nystagmus?

A. Arnold-Chiari malformation

Correct Answer A: 

Nystagmus:

Upbeat nystagmus:

• Cerebellar vermis lesions

Downbeat nystagmus - foramen magnum lesions: 

• Arnold-Chiari malformation

Question 119#

Which one of the following is least associated with ptosis?

A. Horner's syndrome

Correct Answer E: 

Ptosis:

Ptosis may be unilateral or bilateral.

Causes of bilateral ptosis:

• Myotonic dystrophy
• Myasthenia gravis*
• Syphilis
• Congenital 

Causes of unilateral ptosis, as above plus:

• Third nerve palsy
• Horner's

*Ptosis is much less common in Lambert-Eaton syndrome than myasthenia gravis.

Question 120#

A 31-year-old female with progressive leg weakness has nerve conduction studies for suspected Guillain-Barre syndrome.

Which one of the following findings would be most consistent with this diagnosis?

A. Reduced conduction velocity

Correct Answer A: Slowing of the nerve conduction velocity usually indicates there is damage to the myelin sheath, as in Guillain- Barre syndrome.

Nerve conduction studies:
Nerve conduction studies (NCS) are useful in determining between axonal and demyelinating pathology.

Axonal:

• Normal conduction velocity
• Reduced amplitude 

Demyelinating: 

• Reduced conduction velocity
• Normal amplitude