A structural irregularity in the skull may cause ovoid structures, or cerebellar tonsils, at the bottom of the brain to protrude or push into the spinal canal. This may cause headaches that a person may mistake for migraine.

Migraine is a debilitating headache that doctors characterize by throbbing, pulsating pain on one side of the head. Many triggers can lead to a migraine episode, including stress, light, hormonal changes, and noise.

Sometimes, migraine can occur as an underlying symptom of another condition. However, while headaches are common in people with low-lying cerebellar tonsils, no evidence links them to migraine.

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The cerebellum is a structure that sits at the back of the brain. It appears below the temporal and occipital lobes and above the brainstem. It sits above the opening of the skull, called the foramen magnum.

Ovoid structures called cerebellar tonsils sit on the underside and middle part of the cerebellum. Although rare, the cerebellar tonsils can move down or protrude below the foramen magnum and into the spinal canal.

Experts call these low-lying cerebellar tonsils Chiari malformation type 1 (CM-1). Other terms for this condition are:

  • cerebellar tonsil ectopia
  • tonsillar herniation
  • herniated cerebellar tonsils

Doctors consider CM a congenital condition that occurs during fetal development. However, they sometimes diagnose acquired forms of this condition. CM-1 is the most common and only involves the cerebellar tonsils extending into the canal. This is in contrast to different types of CM, which involve the presence of other structural irregularities.

Anybody can have low-lying cerebellar tonsils or CM. However, it is generally rare in both adults and children. Health experts estimate that they affect slightly fewer than one person in 1,000.

No studies show a direct link between low-lying cerebellar tonsils and migraine. While the pressure due to CM-1 may trigger migraine, experts have not been able to establish this as the sole cause of migraine.

However, an older 2008 study detected a three-fold increase in migraine in people with CM-1, suggesting it could be a contributing factor to chronic migraine.

Low-lying cerebellar tonsils are often present at birth, meaning it is congenital, and do not cause symptoms until adolescence or adulthood. The degree of descent also determines the presence and severity of symptoms.

One of the symptoms a person may experience is headaches that radiate into the neck and shoulder area. They typically last under 5 minutes and often result from sneezing, coughing, exertion, or straining.

Because the cerebellar ectopia compresses structures in the spinal canal and disrupts the usual flow of cerebrospinal fluid (CSF) to and from the brain, it may lead to a range of symptoms, including:

Complications may also occur as the tonsils block the regular flow of CSF. These complications include hydrocephalus — an accumulation of CSF in the brain, and syringomyelia — a formation of fluid-filled cysts called syrinx due to CSF accumulating in the spinal cord.

The exact cause of CM or low-lying cerebellar tonsils is unknown. However, it is often present at birth and may be due to the lack of brainstem and upper cervical spine development during fetal development.

It may also occur when the space where the cerebellum, known as the posterior fossa, is too small or misshaped. Brain growth causes pressure and crowding, which in turn pushes the cerebellar tonsils into the spinal canal.

In some rare instances, people acquire it from any condition that takes up space within the posterior fossa, including:

  • arachnoid cysts
  • abscesses
  • tumors
  • hematomas
  • hydrocephalus
  • pseudotumor cerebri, or intracranial hypertension
  • basilar invagination
  • diffuse axonal injury
  • prolonged use of lumboperitoneal shunt
  • CSF leakage or over-drainage by lumbar puncture
  • tethered cord syndrome

It can also occur as a part of syndromes, such as Goldenhar syndrome, achondroplasia, and Ehlers-Danlos syndrome. Genetic factors may also play a role. Research is still ongoing to determine genetic mutations that influence its development.

To diagnose the condition, a doctor will take and review the person’s medical history and symptoms and perform a thorough clinical evaluation and neurologic exam.

They will also order imaging tests to determine the cause of symptoms, diagnose the condition, and determine its extent. These include:

  • X-rays: This common imaging technique can show skull abnormalities, spine irregularities, irregular curvature and motions of the spine.
  • MRIs: An MRI can reveal loss of neural tissue and CSF accumulation in the brain or spinal cord. It can also show whether the cerebellar tonsils extend through the spinal canal.
  • Cine MRIs: Doctors and healthcare professionals use this procedure to diagnose irregularities in CSF flow.
  • CT head scan: A CT scan can help identify bony anomalies at the base of the skull. It can also reveal brain damage, tumors, bone deformities, blood vessel problems, and other conditions that may lead to CM-1.

Doctors typically diagnose a person with CM-1 if the cerebellar tonsils extend or descend at least 5 millimeters from the foramen magnum.

Doctors will direct the treatment of low-lying cerebellar tonsils to the condition’s underlying cause. It also depends on the severity of the person’s symptoms. They may recommend monitoring and regular examinations in people with CM-1 who show no symptoms. However, if someone has symptoms or develops a syrinx, they may require treatment. Treatments include the below:

Prescription muscle relaxants and pain relievers

For those with mild neck pain and headaches, doctors may prescribe specific drugs. However, they may recommend surgery for people with significant symptoms or more severe conditions.

Surgical procedures

Surgical procedures for low-lying cerebellar tonsils include:

  • Posterior fossa decompression: This procedure involves removing part of the skull’s base to relieve pressure in the brain.
  • Laminectomy: This involves removing the back of one or several spinal vertebrae to relieve pressure on the spinal cord and restore CSF flow.
  • Duraplasty: In this procedure, surgeons will expand the dura — the brain’s outer protective membrane — which involves cutting and closing the dura with another soft tissue outside the brain.
  • Electrocautery: This procedure involves shrinking the cerebellar tonsils through the application of heat from an electric current.

Many potential triggers can lead to migraine. These include:

  • hormonal changes
  • emotional changes, such as stress and anxiety
  • physical triggers, such as hunger, lack of sleep, or low blood sugar
  • food triggers, such as caffeine, alcohol, or specific foods such as chocolates
  • environmental stimuli, such as smells, noise, and bright lights
  • medicines, such as hormone replacement therapy and contraceptives
  • genetic factors

Read more on migraine triggers.

Low-lying cerebellar tonsils, or CM-1 malformations, are the mildest forms of CMs. It is rarely life threatening but can progress and lead to complications.

Doctors and healthcare professionals do not typically suggest treatment for people with no symptoms. For those with symptoms and who require treatment, outcomes are generally good, and individuals with this condition can get on with their everyday lives.

However, in other types of CM, where other structural irregularities are present, the outlook may vary and depend on the severity of the condition.

Low-lying cerebellar tonsils, or CMs, are rare. While headaches often accompany this condition, people may notice other symptoms, including trouble with breathing and swallowing, tingling sensations in the arms and legs, coordination issues, and balance difficulties.

Some studies suggest that low-lying cerebellar tonsils link to migraine, and some cases show they may coexist. However, there is currently no evidence that directly links the two.

If a person is experiencing migraine without any other symptoms, it is unlikely a result of low-lying cerebellar tonsils. In either case, individuals should consider seeking medical advice so a doctor can investigate and offer an appropriate diagnosis.