LESIONS

A Guide to Localisation in Neurology

Case 2. Clumsy hand

What is the lesion?

Every major category of disease can affect the cerebellum (box) –thinking through causes in this fashion is not very helpful when assessing a patient. The causes vary deeply in their manifestations, including tempo, aspects of cerebellar dysfunction seen (gait, limb, eye, speech...), and other associated features due to additional tissues involved – for example other nervous system structures may be affected in some cerebellar diseases, or even other organs.

When considering differential diagnoses it’s better to think clinically - in terms of:

1. What aspects of cerebellar function are impaired, and which side(s)
2. Tempo
3. Other structures involved (which can help localisation)

This is an acute unilateral cerebellar hemisphere problem. On this basis we can immediately remove chronic and/or bilateral entities such as:

• Genetic ataxias (they’re more diffuse and develop slowly)
• Degenerative disorders (likewise, with the rare exception of prion disease which is rapid)
• Toxic exposures, deliberate or not – always worth considering, but symmetrical, not lateralised
• Metabolic disorders (e.g. Wernicke’s) likewise don’t tend to produce lateralised features

Let’s consider what remains.

Young age should not put us off considering a stroke. Young-onset strokes are typically not due to classic risk factors and reflect other causes. For ischaemia this can include heart diseases, arterial dissection and drug abuse. For haemorrhage, lesions prone to bleeding – e.g. arteriovenous malformation (AVM), cavernous malformation (‘cavernoma’) or tumour.

The story sounds acute – but not hyperacute. The problem is getting worse over several days, rather than being maximal at onset.

This seems too slow for ischaemia, which would produce sudden-onset deficits of maximal intensity from the start (worsening is possible due to oedema or haemorrhage however).

It’s also slow for haemorrhage, which is usually high-pressure from an artery (‘burst pipe’). Haemorrhagic strokes may have maximal deficits at onset, though can evolve as the haematoma expands. This is usually quick (hours) – with rapid neurological decline.

If this patient had a haemorrhage it must be of low volume and slowly expanding, as he is still walking, talking, conscious, and with limited deficits - the posterior fossa is a tight space, packed with vital structures, and does not accommodate blood easily; haemorrhage here can quickly lead to rapid decline, including coma and death.

Most inflammatory processes present acutely, with features evolving over hours and days to a maximal intensity. A lesion could be focal or very diffuse, and sometimes with other features.

Demyelination very commonly affects the cerebellum and its white matter tracts, including the peduncles, and could present like this – with focal unilateral signs.

Other inflammatory causes of ataxia seem less likely:

• Miller-Fisher syndrome (MFS) is an acute inflammatory disorder related to Guillain-Barré syndrome. The ataxia is profound and bilateral, and other signs include opthalmoplegia and areflexia – this doesn’t fit here.
• Bickerstaff’s brainstem encephalitis is similar but also causes brainstem signs, autonomic dysfunction and encephalopathy – not present.
• Anti-GAD antibodies (best-known in type 1 diabetes) can affect the brain. Anti-GAD ataxia exists but is not usually acute and is usually bilateral, affecting gait and limbs.

Infections can affect the cerebellum - in some cases presentations are acute while others develop slowly.

Various infections can produce a rhombencephalitis (inflammation of the cerebellum and brainstem) – but features are not usually unilateral, and rhombencephalitis will often feature other signs arising from brainstem pathology (e.g. ophthalmoplegia), also encephalopathy and fever.

The unilateral nature of the ataxia could reflect a space-occupying lesion, such as a bacterial abscess, although fever would be likely. In certain endemic settings, tuberculoma or neurocysticercosis could be plausible, although rapid presentation is unusual – the lesions tend to grow more slowly.

Paraneoplastic neurological syndromes are remote complications of cancer (i.e. not direct metastatic invasion). Many have associated antibodies though these aren’t necessarily pathogenic but are markers of the disease (helpful for diagnosis). Several cause ataxia, typically diffuse and symmetrical, including truncal. All are quickly disabling.

Fortunately, none sound plausible here. They are worth mentioning however:

• Paraneoplastic cerebellar degeneration presents with an isolated ataxic disorder, including gait dysfunction and truncal ataxia.
• Opsoclonus-myoclonus-ataxia (OMA) features vertigo as well as ataxia, myoclonic jerks on actions and chaotic, involuntary eye movements (opsoclonus).
• Paraneoplastic brainstem encephalitis can cause ataxia as well as opthalmoplegia, deafness, other brainstem dysfunction and sleep-wake disorders if the thalamus/hypothalamus are involved.
• Sensory neuronopathy (dorsal root ganglionopathy) causes profound and disabling ataxia and can look similar to cerebellar disease - the issue is not cerebellar but sensory ataxia due to peripheral nerve damage, so areflexia and loss of joint position sense are seen.

Many tumours can affect the cerebellum, including:

• Primary tumours of the cerebellum
• Tumours arising from nearby structures which compress the cerebellum
• Infiltrative metastases

Depending on the lesion type and its aggressiveness (‘grade’), these might present quickly (days) or slowly (months). Latent tumours can also present abruptly, e.g. by haemorrhage or obstruction of the cerebrospinalfluid channels.

It’s certainly possible this patient has a tumour in the cerebellar hemisphere. The acute, evolving presentation perhaps makes it less likely. If this is a tumour it may i) be of higher grade or ii) have developed a haemorrhage.