The aim of this study was to determine sex specific co-morbidity of migraine and its subtypes migraine without aura (MO) and migraine with aura (MA) with a number of common somatic diseases.

Large-scale population-based studies are required in order to provide reliable evidence for migraine co-morbidity. This study evaluated the association between a variety of medical disorders and migraine in males and females as well as patients with MA and MO.

In 2002, questionnaires were sent to 46,418 people residing in Denmark and born between 1931 and 1982, as a representation of the whole Danish population. This very large sample size has the statistical power not only to detect associations linked to migraine in general but also to reveal differences with regard to co-morbidity between males and females and patients with MA and MO.

The study found that 21 conditions were co-morbid with migraine, 23 with MA and 12 with MO. Comorbid diseases included asthma, epilepsy and stroke as well as new conditions: kidney stone, psoriasis, rheumatoid arthritis and fibromyalgia.

From this study, it was determined that migraine occurs in 20-30% of several medical conditions. It should be diagnosed and treated along with the primary disease.

MO and MA determination was based on answers to the following questions: Have you ever had migraine? Have you ever had visual disturbances that lasted from 5-60 minutes and were succeeded by a headache?

The medical diseases were divided into 7 categories: cardiovascular diseases, autoimmune disorders, musculoskeletal disorders, thyroid diseases, atopic diseases, audio-vestibular diseases and other diseases.

There were more women than men in the migraine group whereas no significant difference was found in the group without migraine.

Several hypotheses could be put forward from the data collected in this study:

1. That migraine patients over-report co-morbidities in general.
2. Patients with chronic disorders have a general susceptibility due to genetic or environmental factors meaning that once one disease is manifested, more may follow.
3. It is possible that one gene is responsible for, or affecting, more than one phenotypic characteristic which could explain some of the associations between migraine and the other diseases.
4. Migraine patients may have other types of headache or pain conditions that are co-morbid with some of the somatic medical diseases tested. It would not be possible to differentiate if the disease was strictly co morbid to migraine.
5. There may be an information bias related to the fact that, if someone has another medical disorder that has caused consultations and treatments, the likelihood will increase that the diagnosis of migraine has been made. This may be considered a variant of admission-rate bias.
6. Having a medical condition may be stressful and worrying which may increase the likelihood of having migraine.
7. Medicine given for other disorders may provoke migraine. Such effects require further investigation

Several conditions were more co-morbid with MA than MO. This difference between the two types of migraine was mainly seen in females. These findings support the view that migraine co-morbidity should be evaluated separately in MA and MO. Furthermore, they support the notion that the two types of migraines most likely have different path physiology.