LESIONS

A Guide to Localisation in Neurology

APPROACH: WHERE IS THE LESION, WHAT IS THE LESION?

Formulation - summing everything up

This should be a concise statement (or several) which summarises the problem and considers the likely cause. Some examples are shown:

‘Sudden-onset dysphasia and right hemiplegia in a patient with atrial fibrillation. The likely cause is a left hemispheric stroke - ischaemic or haemorrhagic.’

‘Subacute lower limb sensory loss with bilateral dorsal column signs and a sensory level at T8, most likely due to demyelination affecting the posterior thoracic spinal cord.’

‘Several months of tingling in the right hand, worse at night, with median nerve territory sensory loss and weakness, and positive provocative testing - suggestive of carpal tunnel syndrome.’

Good neurology always does this - whereas bad neurology skips this stage and people simply start ordering tests. The problem there is they haven’t stated what they think the problem is, and the tests are not usually organised with any particular hypothesis in mind. This is not only inefficient but opens up the risk of all kinds of incidental, misleading findings which not only cloud the picture but will also distress the patient.

The reason people skip the formulation is generally because it is mentally taxing to do it - we are trying to compress information down to something focused and targeted. Sometimes this is difficult to do, particularly when the history - despite our best efforts - remains difficult to interpret, with red herrings or unreliable answers. The exam may also have features that don’t entirely localise. That might be because of red herrings (e.g. unrelated abnormalities) or a mixed process (e.g. myeloneuropathy) - there doesn’t have to be a single lesion.

We may also have a good idea of the ‘Where?’ but not be able to confidently state the most likely ‘What?’. This is OK - we can offer a differential diagnosis in our formulation statement. In many of the cases in this resource, that is as definitive as we can be in the formulation - but the subsequent investigations are still performed with a series of options in mind, rather than a blank canvas. They are also what we write when we request those investigations - the radiologist or neurophysiologist then has an idea what we are asking, what test sequences to perform, and how to answer our questions.

Whatever we do, we must always offer our best effort at a formulation - in writing - for any problem we assess. This is our anchor - and without it, we are adrift.

The entire clinical diagnostic approach in neurology comes down to using the history and exam in a focused manner to think about where the problem might be in the nervous system (which is a very broad system), and what it might be, then attempting a concise formulation of the problem - and then and only then organising investigations to test specific hypotheses.

My goal with this resource is to help teach this process using worked-through case examples.