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CMI is part of a class of malformation diseases involving the base of the skull and the rhombencephalon. The most used definition is a descent of the cerebellar tonsils into the cervical canal due to a volumetric disproportion between the posterior skull base and the neural content.

Descent of the cerebellar tonsils as diagnosis

Classically, a diagnosis of CMI is proposed if tonsils are at least 5 mm below the foramen magnum. However, this conception has been variously questioned following the evidence of clinical CMI symptoms with only mild tonsils downward dislocation.

The real incidence is not clear but it is believed to affect approximately 1 out of 1500 patients. In almost 1/3 of patients CMI is associated to syringomyelia (see below).  

MacRae’s Line

There are different craniometric lines used to measure the descent of cerebellar tonsils. The most used is the MacRae’s line (joins the basion to the posterior margin of the foramen magnum on a sagittal midline MRI or CT scan).  



MacRae’s Line

Other types of CMs

CM type II

Is characterized by small posterior cranial fossa with downward dislocation of cerebellum, tonsils and brainstem. It is typically associated to mielomenigocele. Other associated malformations are: hydrocephalus, anegesia of the corpus callosum, scoliosis, syringomyelia. 

CM type III

Is characterized by occipital encephalocele.

CM type IV

Typically associated to agenesia of the cerebellum.



There have been proposed many theories to explain CMI, both congenital and acquired. 

The most accredited is that of underdeveloped posterior fossa which eventually pushes the tonsils outside the foramen magnum. This is seen not only when the posterior fossa is “small” but also in case of craniostenosis (where the whole skull’s volume is reduced) or in other conditions which thicken the bone of the skull (osteopetrosis, Paget disease). 

Other mechanism

Increased intracranial pressure (in case of intracranial masses, hydrocephalus, haematoma) can force the tonsils from above. 

  • Intracranial hypotension provoke a pressure gradient between the intracranial and intraspinal compartment;
  • Consequently, tonsils tend to slide down;
  • Rarely, a tethered cord pulls the brainstem and tonsils from below. 

Many other malformation of the cranio-cervical junction are frequently associated to CMI (basilar invagination, platybasia, atlas assimilation, achondroplasia). 

Genetic disorders of the collagen such as Ehlers-Danlos can be also associated to CMI. 

The majority of symptoms associated to CMI relate to the block of cerebrospinal fluid (CSF) flow at the level of the foramen magnum. This affects the intracranial compliance, the ability of the body to manage a volume increase in the intrathecal space without causing a pressure increase and explains many of the symptoms related to CMI. 

Association between CMI and syringomyelia

Syringomyelia is defined as a cavitation into the spinal cord which leads to debilitating myelopathy. The pathophysiology connecting CMI and syringomyelia resides in the obstruction of the cerebrospinal fluid flow at the level of the foramen magnum caused by the low-lying tonsils generating a pulsatile wave onto the spinal cord which favors the penetration of CSF into the cord through the perivascular spaces of Virchow. In this light, operating on CMI can lead to resolve syringomyelia in many patients (see below). 


Symptoms and signs

The most important complaint from almost all the CMI patients is a headache. This is sited in the occipito-nuchal region and is usually triggered by a cough, straining, running and prolonged or extreme neck flexion. 

Differential diagnosis: cough headache (typically this tends to last few seconds and is not associated to other neurological impairment).  

Frequent associated complaints

  • Visual disturbances (diplopia, scotomas, retro-ocular pain, photophobia, visual filed alterations);
  • Oto-neurological symptoms (vertigo, tinnitus, sensation of pressure into the ears, hearing dysfunction);
  • Numbness or tingling in hands or arms can be frequently referred;
  • Fatigue;
  • Concentration and sleeping problems. 

In more severe forms of brainstem compression, patients can show swallowing difficulties, short breath, nystagmus,  the absence of gag reflex, facial hypoestesia, hemiatrophy of the tongue. 

Relationship with syringomyelia

When associated to syringomyelia, symptoms and signs can vary based on the extension, position and severity of the syringomyelia. In fact syringomyelia can either occupy the exact center of the spinal cord or conversely be eccentrically located. In the former case, syringomyelia spares the dorsal column/medial lemniscus, explaining why proprioception, vibration ad pressure sensation are intact whereas pain and temperature sensation is lost. 

The most typical complaint is dysesthesic pain to limbs or trunk; upper extremity weakness and atrophy and long-tract signs.


Radiological diagnosis

MRI is the gold standard to diagnose CMI. Typically, tonsils protrude through foramen magnum and appear triangular. Other signs, such as the absence of the cisterna magna and the disappearance of the CSF spaces around the brainstem, can also be useful.

Radiological diagnosis

It is mandatory to scan the whole cranio-spinal compartment in order to detect syringomyelia and other associated pathologies (intracranial hypertension, hydrocephalus, occipito-cervical malformations, tethered cord, scoliosis). It is worth noting that low-lying tonsils is just a radiological diagnosis and do not define the CMI clinical syndrome. The number of patients diagnosed with low-lying tonsils has increased thanks to the wide availability of MRI scanners. Other modalities such as phase-contrast MRI can support the demonstration of a altered flow of CSF at the level of the foramen magnum. CT scan and flexion-extension radiograms can help in defining skull-base deformities and instability. 

Physical examination and patient’s history are crucial to formulate diagnosis and eventually propose treatment. Moreover, it has been showed that the degree of tonsillar descent does not correlate to severity of symptoms nor to response to treatments.




Management strategies

Asymptomatic patients do not need surgical treatment. Mild complaints could be treated conservatively with pain killers and physical therapy. 

In case of scarce response to medical treatment or when severe neurological impairment appears, surgery should be proposed.

Surgical technique

The most used technique is the posterior fossa decompression with duroplasty (removal on the dura and enlargement with an autologous or heterologous substitute). Coagulation of tonsils, plugging of the obex, arachnoid dissection and other more invasive technique have been progressively abandoned. 

The necessity to create a duroplasty is still under debate. What has emerged from many surgical series is that opening the dura yields higher risks of CSF leaks, infection and pseudomeningocele. On the other hand, leaving the dura intact is reported to increase the risk of symptoms recurrence due to formation of scars and fibrosis. 

CMI and syringomyelia

In case of CMI and syringomyelia, posterior fossa decompression is still the procedure of choice with the goal of eliminating the mechanism of formation of the syringomyelia. There is no consensus about the indication to surgery in asymptomatic patients with CMI and syringomyelia. Many authors advise surgery in these patients to prevent spinal cord damage.  

Syrinx expansion

In case of syrinx expansion despite good posterior fossa decompression, it can be considered a direct approach to syrinx through the positioning of a shunt.  



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Giannantonio Spena, MD

Neurosurgeon Consultant
University of Brescia
"Spedali Civili" Hospital Brescia (Italy)
Scientific Team - UpSurgeOn

Federico Nicolosi, MD

University of Milan
"Spedali Civili" Hospital Brescia (Italy)
Scientific Team - UpSurgeOn