Overlap with Motor Neurone Disease (MND)

Motor Neurone Disease (MND) and Frontotemporal Dementia (FTD) can overlap in their presenting symptoms, which makes it difficult to distinguish the two diseases from each other in the early stages. The exact degree of overlap is, however, not yet known.

Below we outline in more detail the cognitive and behavioural deficits in Motor Neurone Disease and the overlap with FTD.

 

 

Cognition and Behaviour in Motor Neurone Disease

What are the features of Motor Neurone Disease?

Motor neurone disease (MND) includes a group of neurological conditions characterised by progressive loss of motor neurones, which have an essential role in voluntary movement. There are 2 types of motor neurones present in the brain and spinal cord:

- Upper motor neurones: in the motor cortex (brain)

- Lower motor neurones: in the brainstem and anterior horn of spinal cord.

The most frequent form of presentation of MND is Amyotrophic Lateral Sclerosis (ALS), also known as Lou Gehrig’s disease. This disease affects the upper and lower motor neurone. It is more frequent in males than females (2:1) and the mean age of onset is the sixth decade. MND may also present with isolated speech or swallowing difficulty known as bulbar palsy. Clinically, MND has a wide range of presentation, including:

  • Cramps or twitches of muscles (fasciculations).
  • Progressive weakness of limb and/or bulbar muscles.
  • Muscular wasting.
  • Spasticity (stiffness or tightness).
  • Slurred speech.
  • Difficulties with swallowing (bulbar palsy).

What causes Motor Neurone Disease?

Advances in genetic and pathological studies have increased our understanding of MND, although the exact cause of this disease remains unknown yet, except in a minority of familial cases.

In around 10% of the cases MND is a familial disease and a number of genes have been identified, the commonest is mutation of the gene for Cu/Zn superoxide dismutase (SOD1). People with this mutation appear not to have cognitive impairment. Another recently identified mutation involves the gene coding for the TAR-DNA binding protein (TDP-43).  This protein has an essential role in brain cells and accumulates to an abnormal extent in the motor neurones and other regions of the nervous system in both sporadic and familial cases with MND.

What are the Cognition and behavioural changes in MND?

The patterns of cognitive and behavioural changes in MND are variable across cases, and include:

  • Difficulty in decision making and problem solving.
  • Emotional lability: crying easily even without any stimuli.
  • Changes in personality: notably apathy and lack of motivation.
  • Language impairment: characterised by word finding problems and difficulty to initiation of speech.
  • Memory problems.
  • Psychiatric symptoms: depression, and occasionally psychosis (odd beliefs).

Do the behavioural or motor features come first?

Either can occur first. Some patients with Frontotemporal Dementia, around 10-20%, develop frank Motor Neurone Disease. Such patients typically have rapidly evolving dementia and then develop bulbar (speech slurring or swallowing problems) or limb symptoms usually within 2 years of onset. This combination of features makes management very difficult.

The proportion of patients with classic MND who develop Frontotemporal Dementia is less clear but might be around 15%. A higher proportion has more subtle changes in behaviour or cognition.

The importance of recognising cognition and behaviour impairment is their potential impact on daily life and carer burden. Further work in this area could, we hope, identify strategies to improve the quality of life of people with MND and their carers.

What is the prognosis?

The survival in MND is extremely variable, depending of age, form of presentation and medical complications such as respiratory problems. Different studies have reported a survival of approximately 3-5 years in the 50% of the cases, but in some cases is more than 10 years. The most serious and potentially life threatening complications of MND are problems swallowing and respiratory difficulties. Survival in uncomplicated Frontotemporal Dementia is similar but when MND and FTD co-occur is generally poorer than when they occur separately.