Convention

Perhaps a reasonable place to start would be to define neurological illnesses as those treated by neurologists and psychiatric illnesses as those treated by psychiatrists. You might take the Martian to a neurology ward and declare that all patients here have neurological illnesses. Or explain that the people seen by community psychiatric nurses are those with psychiatric illness. Of course, this is circular reasoning and does not stand up to much scrutiny. The Martian would surely not be satisfied by this explanation; he has no prior knowledge of the history, development or social implications of psychiatry v. neurological disease. Therefore, to him, the division is not self-evident.

Neuropathology

You might, therefore, go a little deeper and start to think about what characterises each type of illness. The Martian recognises the notion of pathology and can relate this to other aspects of disease – for example, pulmonary fibrosis causing restrictive lung disease, which we categorise as a respiratory disease. By comparison, you might argue that neuropathological lesions cause neurological disease, whereas psychiatric illness is more to do with abnormal function of the brain. This seems sensible and certainly holds for well-characterised neurological disease such as multiple sclerosis. Demyelination of neuronal axons is the neuropathology, which results in a patient’s symptoms. However, it becomes less tenable as the underlying disease processes are less well characterised. Epilepsy was once regarded as a psychiatric disease. As its neuropathology was better understood, we now regard it as a neurological illness. ^{Reference Reynolds and Trimble1} Similarly, we now know of pathological processes in diseases such as schizophrenia. These processes, while not localised lesions, are evident when comparing brain imaging of people with schizophrenia to healthy controls and are present before the illness manifests clinically. ^{Reference Fusar-Poli, Radua, McGuire and Borgwardt2}

The final nail in the coffin of classifying brain diseases by their pathology is the case of conversion disorder, or neurologically unexplained symptoms. By definition, this disorder cannot be explained by underlying neuropathology. ^{Reference Kanaan, Armstrong, Barnes and Wessely3} It is, presumably, a result of psychological and social factors and is more a ‘functional’ disorder of the central nervous system. And yet it is not treated by psychiatrists but by neurologists. Attempting to categorise brain disease by pathology is clearly troublesome so perhaps we should focus on dividing illnesses based on their symptoms.

Symptomatology

You could try explaining to the Martian that there are ‘neurological symptoms’, weakness, tingling and seizures, for example. By contrast, ‘psychiatric symptoms’ would generally be regarded as dysfunction of higher functions of the nervous system, disturbance in mood, delusions, hallucinations and so forth. Of course, there is some overlap but neurological symptoms would generally signify neurological disease which would interest a neurologist more than a psychiatrist. This holds true for conversion disorder which, although not involving lesions of the nervous system, certainly presents with symptoms more familiar to neurologists.

Unfortunately, it would take little time for our Martian friend to pick holes in this argument. He could point to anti-N-methyl-d-aspartate receptor encephalitis, which can be clinically indistinguishable from the first episode of schizophrenia, though it has a well-defined ‘neurological’ pathophysiology. ^{Reference Barry, Hardiman, Healy, Keogan, Moroney and Molnar4} This disease was only recently discovered, making it difficult to estimate how many patients presenting with psychiatric symptoms actually have neurological diseases. Similarly, psychiatric symptoms are common in traditionally neurological disorders: hallucinations in Parkinson’s disease ^{Reference Williams and Lees5} and depression in multiple sclerosis ^{Reference Feinstein6} are just two examples.