biology

Superior canal dehiscence syndrome

Superior canal dehiscence syndrome is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Superior canal dehiscence syndrome rather than just read about it. In short: The semicircular canal dehiscence (SCD) is a category of rare neurotological diseases/disorders affecting the inner ears, comprising the superior, lateral and posterior semi-circular ducts. These SCDs induce SCD syndromes (SCDSs), which define specific sets of hearing and balance symptoms.

Key takeaways

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Reference excerpt

The semicircular canal dehiscence (SCD) is a category of rare neurotological diseases/disorders affecting the inner ears, comprising the superior, lateral and posterior semi-circular ducts. These SCDs induce SCD syndromes (SCDSs), which define specific sets of hearing and balance symptoms. This entry mainly deals with the superior SCDS. The superior semicircular canal dehiscence syndrome (SSCDS) is a set of hearing and balance symptoms induced by a rare disease/disorder of the inner ear's superior semicircular canal/duct. The symptoms are caused by a thinning or complete absence of the part of the temporal bone overlying the superior semicircular canal of the vestibular system. There is evidence that this rare defect, or susceptibility, is congenital. There are also numerous cases of symptoms arising after physical trauma to the head. It was first described in 1998 by Lloyd B. Minor of Johns Hopkins University in Baltimore.

Symptoms The superior canal dehiscence can affect both hearing and balance to different extents in different people. Symptoms of the SCDS include:

Autophony – person's own speech or other self-generated noises (e.g. heartbeat, eye movements, creaking joints, chewing) are heard unusually loudly in the affected ear Dizziness/vertigo – chronic disequilibrium caused by the dysfunction of the superior semicircular canal Tullio phenomenon – sound-induced vertigo, disequilibrium or dizziness, nystagmus and oscillopsia Pulse-synchronous oscillopsia Hyperacusis – the over-sensitivity to sound Low-frequency conductive hearing loss A feeling of fullness in the affected ear Brain fog Fatigue Headache/migraine Tinnitus – high pitched ringing in the ear

Symptoms in detail SCDS-related autophony differs greatly in quality and range from the more common form which results from an open, or patulous Eustachian tube through which sufferers of this disorder hear the sound of their own voice and breathing. In contrast, patients with SCDS-related autophony report hearing their own voice as a disturbingly loud and distorted "kazoo-like" sound deep inside the head as if relayed through "a cracked loudspeaker." Additionally they may hear the creaking and cracking of joints, the sound of their footsteps when walking or running, their heartbeat and the sound of chewing and other digestive noises. Some sufferers of this condition experience such a high level of conductive hyperacusis that a tuning fork placed on the ankle will be heard in the affected ear. The bizarre phenomenon of being able to hear the sound of the eyeballs moving in their sockets (e.g. when reading in a quiet room) "like sandpaper on wood" is one of the more distinctive features of this condition and is almost exclusively associated with SCDS. Tullio phenomenon, another of the more identifiable symptoms leading to a positive SCD diagnosis is sound-induced loss of balance. Patients showing this symptom may experience a loss of equilibrium, a feeling of motion sickness or even actual nausea, triggered by normal everyday sounds. Although this is often associated with loud noises, volume is not necessarily a factor. Patients describe a wide range of sounds that affect balance: the 'rattle' of a plastic bag; a cashier tossing coins into the register; a telephone ringing; a knock at the door; music; the sound of children playing and even the patient's own voice are typical examples of sounds that can cause a loss of balance when this condition is present, although there are countless others. The presence of Tullio may also mean that involuntary eye movements (nystagmus), sometimes rotational, are set off by sound, giving the sufferer the impression that the world is tipping, clockwise or anticlockwise, depending on the site of the dehiscence. Some patients report this tilt as being as much as 15°. For such persons, a visit to the concert hall or to a noisy playground may seem like being at the epicenter of an earthquake. A change of pressure within the middle ear (for example when flying or nose-blowing) may equally set off a bout of disequilibrium or nystagmus. Low-frequency conductive hearing loss is present in many patients with SCDS and is explained by the dehiscence acting as a "third window." Vibrations entering the ear canal and middle ear are then abnormally diverted through the superior semicircular canal and up into the intracranial space where they become absorbed instead of being registered as sound in the hearing center, the cochlea. Due to the difference in resistance between the normal round window and the pathological dehiscence window this hearing loss is more serious in the lower frequencies and may initially be mistaken for otosclerosis. In some patients there is true enhancement of low frequency hearing via bone conducted sound. A clinical sign of this phenomenon is the ability of the patient to hear (not feel) a tuning fork placed upon the ankle bone. Pulsatile tinnitus is yet another of the typical symptoms of SCDS and is caused by the gap in the dehiscent bone allowing the normal pulse-related pressure changes within the cranial cavity to enter the inner ear abnormally. These pressure changes affect the sound of the tinnitus which will be perceived as containing a pulse-synchronized "wave" or "blip" which patients describe as a "swooshing" sound or as being like the chirrup of a cricket or grasshopper. Brain fog and fatigue are both common SCDS symptoms and are caused by the brain having to spend an unusual amount of its energy on the simple act of keeping the body in a state of equilibrium when it is constantly receiving confusing signals from the dysfunctional semicircular canal. Headache and migraine are also often mentioned by patients showing other symptoms of SCDS due to the body overcompensating for poor hearing in the affected ear by tensing up nearby parts of the face, head, and neck and using them as almost a secondary eardrum.

Causes According to current research, in approximately 2.5% of the general population the bones of the head develop to only 60–70% of their normal thickness in the months following birth. This genetic predisposition may explain why the section of temporal bone separating the superior semicircular canal from the cranial cavity, normally 0.8 mm thick, shows a thickness of only 0.5 mm, making it more fragile and susceptible to damage through physical head trauma or from slow erosion. An explanation for this erosion of the bone has not yet been found.

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Illustrations

Worked examples

Example 1 — a first encounter with Superior canal dehiscence syndrome

Start with the simplest possible case. Write down what Superior canal dehiscence syndrome claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Superior canal dehiscence syndrome before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Superior canal dehiscence syndrome ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Superior canal dehiscence syndrome

In research: Superior canal dehiscence syndrome appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Superior canal dehiscence syndrome in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Superior canal dehiscence syndrome is common in secondary-school and first-year university syllabi. It links to neighbouring topics Diseases of the ear and mastoid process, Osteopathies, Rare diseases, so understanding it makes those chapters shorter.
In everyday life: Look for Superior canal dehiscence syndrome outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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Frequently asked questions

What is Superior canal dehiscence syndrome in simple terms?

Why does Superior canal dehiscence syndrome matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

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Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

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