Brain diseases include psychiatric and neurological diseases, conditions and injuries (Ministry of Health and Care Services, 2017; Olesen et al., 2011; Wang et al., 2020). Dependency disorders are also included (Ministry of Health and Care Services, 2017; National Institute on Drug Abuse, undated). Brain diseases can either arise or develop during life, or they can be congenital. They are characterised by disorders of brain development, damage to brain structure and/or impaired brain function (Wang et al. 2020).
The term ‘brain diseases’ is hereinafter used to refer to diseases, conditions and injuries that affect the brain and the rest of the nervous system.
Brain diseases can be acute, progressive or chronic. Some people can experience an absence of symptoms, either completely or during certain periods of time, while others have to live with major health challenges (Ministry of Health and Care Services, 2017). Some brain diseases can also occur in the form of seizures, which can make them difficult to predict and manage.
Certain diseases develop throughout the life course due to the impact of lifestyle choices and the environment, in interaction with genetic predisposition (Polderman et al., 2015). ‘Genetic predisposition’ refers to an increase in the probability of developing certain brain diseases based on inherited genes (Smeland et al., 2023). Other diseases may be congenital and present from birth, caused by genetic or developmental factors.
One particular challenge for people with a brain disease is that it can lead to impaired cognitive function and personality changes. This is often challenging for both the person concerned and their relatives (Ministry of Health and Care Services, 2017).
Comorbidity in brain diseases
Individuals who have one or more brain diseases, a combination of other diseases and/or other health challenges, represent a growing global challenge with a major impact on individuals, relatives and society (Skou et al., 2022).
A study by Barnet et al. (2012) showed that most people with chronic diseases have at least two, and often more, other disorders. The number of diseases and the proportion of people with comorbidity increase significantly with age (ibid.). It is well-known that comorbidity in brain disease is common. Comorbid conditions increase the disease burden and can make it challenging to provide effective treatment (Hesdorffer, 2016).
Drug disorders often occur in conjunction with mental disorders (Long et al., 2017; Torvik et al., 2018). Patients who are treated for a substance abuse disorder often have one or more concomitant mental disorders (Beard et al., 2016). Drug problems can develop as a consequence of mental health problems, but the reverse can also be true (Chassin et al., 2013).
According to Demarin & Morovic (2021), the degree of comorbidity between mental and neurological diseases is high and patients are often under-assessed or misdiagnosed. Helsebiblioteket (2020) refers to the World Health Organization, which claims that premature death and disease as a result of brain diseases can be reduced by devoting more attention to comorbidity.
Common biology and risk factors between mental and neurological disorders
There are numerous types of brain disease. Some have a clear cause in the brain and this is the type that is most often recognised. However, a broader view recognises that there are other ways of developing a brain disease, other than just those with a clear cause in the brain (Jefferson, 2022).
One particular challenge is the management of comorbidity between mental and neurological disorders. This is because the treatment of these disease groups is primarily organised into different disciplines (neurology and psychiatry), which are today characterised by both separate specialisation and separate professional environments. Ibanez & Zimmer (2023) highlight the close link between psychiatry and neurology in terms of brain health. The two categories have common genetic and environmental factors, as well as common lifestyle factors, which can contribute to psychiatric and neurological diseases. A broader interdisciplinary and integrated approach can help understand, prevent and treat brain diseases and develop sound methods for research, prevention and intervention (ibid.).
A recent major genetic study conducted at Oslo University Hospital (Smeland et al., 2023) showed that there is no sharp genetic distinction between neurological and mental disorders, as was previously thought. However, the study also shows that neurological and mental disorders largely share genetic risk and biology linked to the brain. The findings are consistent with the high level of comorbidity between the conditions and suggest that many individuals will have an elevated genetic vulnerability to both mental and neurological disorders. The study thus supports recent research (Ibanez & Zimmer, 2023) which suggests that mental and neurological disorders are more closely associated than is indicated by the traditional delimitation between disease groups.
Although there are many different neurological and psychiatric diagnoses, they have a number of key commonalities. An understanding of the links between psychiatry and neurology could lead to better treatment for diseases affecting the brain (Institute of Medicine, 2001).
When the Directorate of Health prepared the status report on brain health in 2017 (Directorate of Health, 2017), there was some disagreement regarding the definition of the term ‘brain health’ and whether mental disorders should be included. However, the Ministry of Health and Care Services decided that neuropsychiatry and mental disorders with presumed central biological causes should be included in the brain health strategy for 2018-2024. Although there still seem to be differing opinions on this issue, the Directorate of Health opted to follow the decision by the Ministry of Health and Care Services when formulating the draft updated brain health strategy.