Breathlessness can be many things. For example, it can be the shortness of breath after exercise – short-lived and laced with endorphins – or it can be the frightening gasping for breath experienced by patients with a range of diseases from cardiac failure and cancer to respiratory disease. From a physiological point of view, these may look quite similar, but they are not really the same thing. Breathlessness is the sensation produced by sensory input evaluated within the context of psychological and environmental factors. In short, the quality of breathlessness depends on why, who and where it is experienced – it’s situational and subjective. This means it cannot be approached, from a medical point of view, with a one size fits all solution. It’s important to understand its nuances.
So, which parts of the brain are involved in processing breathlessness? The answer is quite simple: we don’t know yet. To date, studies are few and far between, with the majority being on breathlessness in healthy controls which may or may not translate to various patient groups. While we don’t know anything for certain, particularly for patients, we do however have a few likely suspects.
The usual suspects. The first is the insular cortex. This is a region of the brain that has been identified in the majority of neuroimaging studies on breathlessness. It plays a role in the conscious awareness of body state and is involved in the perception of other unpleasant sensations, such as pain. The role of the insula is two-fold, with the posterior (towards the back) part processing the physical aspects and the anterior (towards the front) part processing the affective (relating to moods and feelings) aspects of the sensation 1. This anterior-posterior division may also be present in breathlessness, as the anterior insula appears to be more associated with the unpleasantness of breathlessness.
The next on the list of likely suspects are somatosensory and motor regions (e.g. 2), which are also activated in a number of breathlessness studies. This is not surprising, given the added work of breathing harder than normal when you’re breathless. In addition to cortical somatosensory/motor regions, we also see activation in the brainstem (particularly in the brainstem respiratory centres) and in the cerebellum (a region associated with motor function). Some of these areas may be mostly involved with breathing itself and not with breathlessness specifically, as they appear frequently in studies looking at simple breathing responses without any breathlessness.
Then there are regions that sometimes appear and sometimes don’t: the prefrontal cortex, the periaqueductal grey (PAG), the amygdala and the anterior cingulate cortex (ACC). In the case of the PAG, which is a structure in the midbrain involved in pain processing and fight-or-flight responses, this may simply be because it is small and hard to image. Recent studies using high resolution imaging have found that the PAG is involved with unpleasant respiratory sensations, and that parts of the PAG have different roles in processing respiratory threat (see Figure 1 below) 3. Looking further into its sub-divisions, the lateral PAG is downregulated during restricted breathing, and the ventrolateral PAG is upregulated. This is interesting as the lateral PAG has been associated with active coping strategies for stressful situations that can be escaped, and the ventrolateral PAG with passive coping strategies for stressful situations that cannot be escaped. In short, it seems as if different parts of the PAG are involved in different aspects of breathlessness.
Fig 1. 7T FMRI of respiratory threat in the PAG. Faull et al. 2016.
The prefrontal cortex has been implicated in the processing of breathlessness cues (i.e. a non-physical stimulus) in COPD patients (medial prefrontal cortex, mPFC), as has the ACC (see Figure 2 below) 4. The latter has also been identified in many studies on breathlessness in healthy volunteers, and the former is an area of the prefrontal cortex involved in emotion processing, and particularly associated with responses to fear and threat. In the above study, both patients and controls showed activation in the insula (labelled ‘Conjunction’). Opioids, which can cause make breathlessness less unpleasant, dampens brain responses to breath holding in the prefrontal and anterior cingulate cortices as well as in the insula 5. So, it’s a good guess that the prefrontal cortex and ACC are involved in the modulation of breathlessness.
Fig 2. COPD patients and control. Response to breathlessness cues. From Herigstad et al. 2015.
Similarly, the amygdala, which is frequently identified in a whole range of studies relating to emotion, threat appraisal and fight-or-flight responses, is also activated in some studies of breathlessness. The amygdala is strongly connected to the anterior insula, and breathlessness studies that have seen activation in the amygdala also show large activation in the insula.
There are also another few regions that are occasionally found and we don’t quite know how they fit in. These include the before-mentioned cerebellum, the dorsolateral prefrontal cortex and the precuneus.
The cerebellum is probably mostly involved in the motoric response to breathlessness, as it is a centre for coordination of motor function, but as the cerebellum has also been associated with cognitive function, we can’t yet determine its role in breathlessness. The dorsolateral prefrontal cortex may be part of the cognitive evaluation of breathlessness, as this is a structure that is associated with attention and working memory. The precuneus could be involved in both sensory and cognitive aspects of breathlessness, depending on which part of the precuneus is active. It is a poorly mapped structure, but in general it has a sensorimotor anterior region which links to sensory/motor areas of the brain and the insula, and a cognitive central region which links to prefrontal regions, including the dorsolateral prefrontal cortex. It also links to the thalamus, which is a subcortical structure that relays a vast range of information between the brainstem and the higher brain areas.
Linking it all together. We don’t know how this all ties together. However, one could speculate ways in which breathlessness is processed in the brain:
Brainstem breathing control. We know that respiratory centres in the brainstem receive peripheral input (input from the rest of the body) and adjust the breathing pattern in response to this. The brainstem is crucial for breathing – without it, no breathing occurs – and it almost certainly plays a part in the actual breathing response to breathlessness. The cerebellum, which is probably involved in the motor response to breathlessness, connects to the higher brain areas through the brainstem. So far so good.
PAG as a gateway. Peripheral input is also believed to be received in the PAG, which could act as a gateway of sorts. The lateral PAG relays signal directly to primary motor/sensory cortices and the (posterior?) insula, and this path may be processing breathlessness intensity. The ventrolateral PAG relays signal directly to the prefrontal cortex, (anterior?) insula and also to motor regions of the brain, and this path may be processing the threat and/or possible responses to the breathlessness. The PAG probably also connects with the thalamus, which in turn acts as a hub (see below).
Thalamus as a hub. Finally, signal could also be relayed via the PAG (or directly) to the thalamus, which has been identified in a range of breathlessness studies and is a common hub for cortical communication. From the thalamus, signal is transmitted to a range of cortical areas (including prefrontal regions and the amygdala via the medial/frontal thalamus, and somatosensory and motor regions via the ventroposterior thalamus). Breathlessness is likely modulated by several of these cortical areas and how they interact.
The complexity of the interactions may be best explained by an example:
Anxiety relating to threat is modulated by activation in both the medial prefrontal cortex (mPFC) and the amygdala. The mPFC possibly influences threat by dampening activation in the amygdala. So far, so good. The prefrontal cortex receives signal from the PAG, and may also receive input from the ACC. Both the ACC and PAG are in turn linked to the anterior insula. The anterior insula also receives input from the thalamus, which relays signal to the amygdala. The thalamus also receives input from the PAG. So now we have a network of interconnected regions which may all influence each other and work to fine-tune the anxiety response. Furthermore, the anterior insula may also receive and integrate information on the physical sensation from the posterior insula, which is linked to the ventroposterior thalamus. This part of the thalamus is connected with somatosensory and motor cortices. These are also influenced by the PAG. The thalamus and somatosensory/motor cortices also relay signal to the precuneus, which in turn could connect to the dorsolateral prefrontal cortex, thus incorporating cognitive processing. And so on. It can get a bit complex.
A suggested network is presented in the figure below.
Fig 3. Possible breathlessness network. Purple=connected cortical regions, black= signal to/from periphery, blue=signal from PAG. ‘Hubs’ are shown in green. PFC = prefrontal cortex (encompasses both medial (emotional/threat processing) and dorsolateral (cognitive processing) PFC)
In short, various studies have shown that a range of brain regions are activated during breathlessness, but we don’t yet know exactly how they are involved in processing the sensation. We may still only guess at the full picture, based on breathlessness work as well as studies on other unpleasant sensations (pain, mostly), threat or emotional processing. Much of the same processing mechanisms are likely to be found in these similar sensations. While the above figure outlines some possible networks for the processing of breathlessness, based on our current understanding of breathlessness and related conditions, (much) more information is needed.
1. Oertel, B., Preibisch, C., et al. Clin Pharmacol Ther 2008; 83:577–588.
2. Hayen, A., Herigstad, M., et al. NeuroImage 2012; 66: 479-488.
3. Faull, O., Jenkinson, M., et al. eLife 2016; 5: e12047
4. Herigstad, M., Hayen, A., et al. Chest 2015; 148(4): 953-61
5. Harvey, A., Pattinson, K., et al. J Magn Reson Imaging 2008; 28:1337–1344.