Chiari malformation

Most people meet this diagnosis not in a neurologist's room but in a scan report: an MRI was done for headaches or after a knock on the head, and the report contains a line about descent of the cerebellar tonsils. The phrase is alarming, and by the time an appointment comes round the patient has usually read a great deal about skull surgery. In truth this is an anatomical feature that most people live with all their lives without noticing anything. The doctor is interested in something else: whether the feature is getting in the way — pressing on the brainstem and holding up the flow of cerebrospinal fluid. The condition used to be called Arnold-Chiari malformation; today it is usually just Chiari malformation, and this page is about type I, by far the commonest.

What happens at the base of the skull

The cerebellum sits in the posterior fossa, a cramped compartment above the nape of the neck. Below it the skull ends in a large opening through which the brainstem continues into the spinal cord. If that compartment formed too small for its contents, the lower edges of the cerebellum — the tonsils — are pushed into the opening and end up inside the spinal canal. The scan measures how far: a descent of roughly five millimetres or more is described as Chiari malformation type I.

The figure on its own means little. Trouble starts when the displaced tissue acts as a plug. Cerebrospinal fluid normally moves freely between skull and spine; here it meets a narrow point, most of all in the moments when pressure inside the head rises sharply — coughing, laughing, straining. That is where the whole picture comes from.

The other types are a different matter: type II is found in newborns with spina bifida, while types III and IV are rare and obvious from the first days of life.

The headache that arrives with a cough

The most recognisable symptom resembles neither a migraine nor the ache of tiredness. It sits at the back of the head and the top of the neck, sometimes reaching behind the eyes, and above all it is set off by effort. A cough, a sneeze, a laugh, lifting something heavy, bending to tie a shoelace, straining on the toilet — and the back of the head takes what feels like a blow. It does not last long, usually under five minutes, and settles on its own. If the head aches all day and bears no relation to exertion, the cause is probably not the Chiari, even when the descent is there on the scan.

Alongside the pain there may be:

• dizziness, unsteadiness, a sense of the floor shifting;
• numbness, pins and needles or coldness in the hands and feet;
• weakness in the arms and clumsy fingers — buttons defeat you, things slip from your grip;
• choking on food and drink, hoarseness, a changed voice;
• ringing in the ears, reduced hearing, a blocked feeling;
• a shaking image, double vision, trouble focusing;
• snoring with pauses in breathing at night and waking unrefreshed.

In babies the picture is different and easily misread: the infant feeds poorly, chokes, brings milk back, breathes with an audible whistle, arches the neck, cries weakly. In infants this is watched especially closely, because breathing and swallowing depend on precisely the part of the brainstem that may be under pressure.

The cavity inside the spinal cord

The most valuable thing to know about Chiari concerns the back rather than the head. When fluid passes badly through the large opening, it can collect inside the spinal cord itself and stretch it from within into a long cavity. This is called syringomyelia, and it is this, not the descent of the tonsils, that leaves lasting damage.

The cavity presses on the fibres from inside, and the first to suffer are those carrying pain and temperature. The result is a combination that sounds odd at first: touch is felt perfectly well, heat and sharpness are not. People burn themselves on a hot pan and find out from the blister, cut a finger and notice the cut in the evening. The loss forms a band across the shoulders, arms and upper back, like a cape laid over them. Later come wasting of the hand muscles, weakness in the arms, pain in the neck and shoulders and, in some people, difficulty holding urine.

In children syringomyelia sometimes announces itself with a single sign: a curve of the spine. That is why scoliosis appearing early, progressing quickly or bending to an unusual side is a matter not only for the orthopaedic team but for a scan.

Where it comes from and how it is confirmed

In most cases the fossa simply formed that way before birth and no exact cause can be named. Sometimes several people in a family are affected, but the risk of passing it to a child is small, and a child born with the same feature may never have symptoms.

A separate group is acquired: the tonsils drop because something else pushes or pulls them down. This happens with fluid building up in the brain, with a tumour in the posterior fossa, with a tethered spinal cord in children, and with a leak of cerebrospinal fluid after surgery or a lumbar puncture. The distinction matters, because then it is the cause that needs treating, not the descent. Another frequent companion is joint hypermobility with fragile connective tissue, which adds instability where the skull meets the neck.

Only magnetic resonance imaging confirms the diagnosis. Besides the head, the whole spine is always examined — otherwise a cavity in the cord is missed. A separate sequence assesses whether fluid stalls at the narrow point, which tracks the symptoms far better than the millimetres of descent. If there is choking or night-time pauses in breathing, a sleep study is added.

When it cannot wait

Seek help immediately, without waiting for an appointment, if:

• a baby has pauses in breathing, a noisy whistling breath in, a blue colour around the mouth, or chokes at every feed;
• weakness in the arms or legs is worsening quickly, or walking has changed;
• control of the bladder or bowel has been lost;
• swallowing has become so difficult that food and drink go the wrong way;
• the worst headache ever comes on within seconds;
• symptoms flare sharply after an injury to the neck or head;
• fainting occurs on straining or coughing.

What happens next

If the descent was an incidental finding and there are no symptoms, the usual answer is to do nothing: keep an eye on things and repeat the scan only if symptoms appear. That is the commonest outcome and it deserves to be taken calmly.

For occasional cough-related pain, simple painkillers help, along with the habit of not holding the breath during effort: lift while breathing out, avoid straining on the toilet, treat constipation and a persistent cough. Where there is numbness and burning pain, medicines from the group used for nerve pain are added. No drug can lift the tonsils back into place, and promises of that kind are worth avoiding.

Surgery is offered when there are genuine signs of compression or a cavity in the cord. Usually this means decompression of the posterior fossa: under general anaesthetic, through an incision at the back of the head, a small piece of bone is removed from the base of the skull, often together with the arch of the first neck vertebra, and if needed the covering of the brain is enlarged with a patch. The aim is to free the path of the fluid. What to expect first of all is that deterioration stops: cough-related pain improves in most people, whereas lost sensation and wasted muscle often do not come back, which is why a growing cavity is not something to sit on. The operation carries risks of its own, from fluid leaks and inflammation of the membranes to rare serious complications, and that conversation with the surgeon should be a thorough one.

When the cause lies elsewhere, treatment starts there: fluid collecting in the brain is diverted, a tethered cord is released, an unstable neck may need stabilising. After any operation follow-up runs for years, because only imaging shows whether the cavity has collapsed.

Online consultation

A remote appointment suits the commonest predicament well: the scan report contains the word malformation and nobody has explained what to do about it. The doctor will ask whether the pain is tied to coughing and straining, how long it lasts, whether there is numbness, choking or night-time pauses in breathing, and will set that against the wording of the report. Out of it comes an answer on whether a neurologist is needed soon, whether the whole spine should be imaged, and whether the finding has any bearing on the symptoms at all — very often it has none. The signs on the urgent list are not assessed remotely: they call for in-person care straight away.

This material is for information only and does not replace medical advice.