Introduction

Dehydration is known to be an active process by which the body loses water, particularly when there is a body water deficiency (hypohydration). The causes of dehydration vary from diuretics, diarrhoea, sweat, cold and a lack of water intake. Although there is a known link between dehydration and hypohydration, and headache it is not completely understood. According to the International Classification of Headache Disorders 3rd edition (ICHD-3) a headache is not specifically related to dehydration. This phenomenon would be deemed as a secondary headache and categorised as a headache attributed to disorders of homeostasis. 

There is a suggestion that a water deficiency in the body may cause Monro-Kelli doctrine and provoking a headache. This means restoring intravascular volume could also abort a headache. Contrastingly, the underlying mechanism for dehydration related headaches is variable and has multiple actors that may influence headaches. These include on the individual and associated health conditions e.g primary headaches as well as autonomic disorders.¹

The physiology of sweating and dehydration

Maintaining an adequate amount of total body water volume involves a complex series of homeostatic mechanisms. This is a network that regulates water conservation, oral intake and excretion. Body water spreads across the extracellular and intracellular compartments, causing a mediated movement of water by osmotic gradients. There is an ionic composition difference between the intracellular and extracellular parts but are balanced by electrochemical equilibrium across the cell membrane. This results in water moving across the membranes balancing the osmotic forces.      

Dehydration is part of a heterogeneous group of conditions that possess different clinical and biochemical features. Hypertonic dehydration, when water loss exceeds salt loss, causes an osmotic exert of water from the intracellular compartment (resulting in dehydration and brain shrinkage). This leads to increased plasma osmolarity (around 2%), which is above the threshold of 285 mOsm/kg. Although there may be variations between individuals, generally the loss of intracellular fluid stimulates a hypothalamic osmoreceptor response. This initiates thirst and secretes antidiuretic hormone antidiuretic hormone (ADH) from the pituitary gland. On the other hand, an increase in oral fluids would increase renal retention of water and normalising, extracellular osmolality. This ultimately creates intracellular hydration within the body. A rapid osmotic distribution of intracellular fluid to the extracellular part would protect intravascular volume (such as fluid loss within the cell).     

The rapid osmotic redistribution of intracellular fluid to the extracellular compartment also means that, unless severe water deficits exist, intravascular volume will be relatively protected (i.e. fluid loss is primarily from within the cell).² 

Fluid intake and its role in dehydration

Drinking water is critical for human survival. It comprises 60% of adult body mass, meaning it is the largest constituent in the human body. Water plays an important role that are needed daily such as:

• Many chemical reactions occur, e.g, the distribution of molecules to cells (oxygen to metabolic substrates)
• Removal of waste products of metabolism, e.g carbon dioxide from cells as well as urine
• Thermoregulation by redistributing heat and sweating
• Homeostatic mechanisms allow total body water to be regulated and distributed across intracellular and extracellular compartments²       

The definition of dehydration in medical dictionaries is simply defined as an excess loss of body water. However other comprehensive definitions are based on differing physiological effects on the extracellular compartment. The dehydration council prefers a more clinical based terminology relating to water loss and salt loss dehydration (this will highlight the two etiologies of water deficiency).¹ 

Contrastingly, European guidelines now prefer to define dehydration as low intake dehydration instead of water loss referred to as the cause of insufficient  drinking. Lacey et al have discussed other studies that have referred to dehydration in terms of compartment specific, such as extracellular (salt loss) and intracellular (low intake) dehydration. Whereas some have debated that the term dehydration should only be referred to as (osmotic dependent) intracellular dehydration. Extracellular fluid losses that cause intravascular volume to contract have been described as volume depletion. The varying definitions and categorisations have caused confusion and the lack of detail has interfered with everyday convenience. Due to varying definitions of dehydration and its subtypes has led to concerns and decreased clarity and consistency.¹     

Dehydration and poor outcome 

Dehydration poses a significant factor in the development of acute kidney injury (AKI), with a mortality rate of around 10%. A large multinational cross sectional study was conducted with a sample of 4000 patients. It was found dehydration to be the most common cause of AKI (46% of cases) in low and low to middle income countries. And the second most common cause in high income countries (38% of cases) with the first common cause attributing to hypertension (40%). Causes of dehydration generally in the wider population are attributed to:

• Advanced age
• Comorbidities
• The usage of multiple medications
• Limited access to drinking water
• Excessive hot climate     

Hypertension and diabetes particularly in the UK are mainly the comorbidities that have an association with renal impairment. Dehydration can be seen superimposed in both conditions (either by the result of the disease itself or by diuretics).   

Research by Lacey et al have discussed hospitalised patients who showed markers of water depletion in the body (e.g increased levels of plasma osmolarity) are associated with a greater risk and severity of AKI.²

How dehydration causes headaches

An inadequate fluid intake will cause hypertonicity and ultimately lead to ‘brain dehydration’. This will cause a traction on pain sensitive meninges and vascular structures. However, the exact phenomenon of how dehydration can cause a headache is yet to be investigated.¹       

A research study that was conducted during the COVID-19 pandemic showed dehydration to be associated with increased frequency and severity of head pain. Results seem to be viable due to a large sample size (172 individuals) and dehydration was determined by measuring serum osmolality.¹  

The presentation of headache disorders are variable to every individual and the effect of dehydration. Preventing dehydration would be the ideal treatment, however creative strategies may be needed for a more suitable lifestyle. For example fasting during Ramdhan or following a strict weight loss programme may provoke dehydration related headache attacks. Dehydration can be objectively calculated by measuring osmolarity (using the Khajuria–Krahn formula). This is an effective screening tool recommended to incorporate in routine care for those who may be at risk.² Further studies are needed to investigate the relation between dehydration and headaches to gain a more objective viewpoint of dehydration.¹    

Summary 

Dehydration represents a major challenge to clinicians and holds a significant risk to the wider public. It is significantly prevalent in the healthcare setting as well as in the community, with associations of increased morbidity and mortality rates. It has a complicated pathophysiology which may result in varied clinical presentations making assessments difficult. Dehydration has an ambiguous definition and diagnosis further complicating the condition. However, measuring how hydrated the body is can be represented by measuring plasma osmolarity. This is a highly valuable objective marker of hypertonic dehydration and needs to be utilised more in clinical practices.²

It is important to note, the elderly and comorbid are mostly at risk of developing dehydration. Most healthful individuals consume fluids sufficiently socially and in response to thirst. Lacey et al has suggested encouraging drinking in excess holds to accreditation from a scientific viewpoint. Dehydration is not wholly recognised, poorly managed and holds low prevention methods. Evidence relating to dehydration is limited but holds detrimental associations to health. Further trials are needed to assess the impact on clinical outcomes regimens with the aim to reduce and treat dehydration.²     

FAQs

How can dehydration from sweating lead to a headache?

Brain shrinkage

• The tissue in the brain can temporarily shrink, by pulling away from the skull. This places pressure on nerves. 

Reduced blood flow

• Volume of blood can become reduced in the body, which can effect blood flow to the brain.  

Electrolyte imbalance 

• The balance of electrolytes in the body can be disrupted. This can disrupt normal functioning of the nervous system.