Microtia is a congenital condition whereby a baby is born with an incomplete outer ear. The illustration above, by Tetiana Lazunova, depicts a normal ear. (For an overview of how hearing works, please see the tab on "Deaf and Hard of Hearing" or other useful Resources here.)
All of that flesh-coloured part of the ear on the left side of the illustration (above) is affected by microtia. Depending on the degree of severity, that may mean that the person has a small outer ear or it could mean that a person has hardly any outer ear at all.
Microtia is “graded” as Grade I (presence of an outer ear and ear canal although both are small) to Grade IV (absence of an outer ear and no ear canal).
The diagrams above are of ears taken from Stanford University's Otolaryngology Department. (When you have microtia, you learn to spell "otolaryngology.") The diagrams show the different grades (or types) of microtia.
Grade III and Grade IV microtia are almost always accompanied by aural atresia, the absence of an ear canal and may have incomplete or missing middle ear anatomy. For example, the Tympanic membrane (eardrum) may be missing and the three tiny bones that conduct sound vibrations - the malleus, incus and stapes (the smallest bone in the body) - may be incomplete or missing as well.
This affects hearing. Therefore, for most people with microtia, there is a physical part of it (not having an outer ear), and there is also a hearing impairment part of it.
The most common type of microtia is Grade III in which a person has a small piece of cartilage above the ear lobe (that looks like a folded ear) and no ear canal.
For many people with microtia, the inner ear, composed of the cochlea (which processes sound) and the semicircular canals (which helps us to balance), are functional. In addition, the cochlear nerve and vestibular nerve (vestibulocochlear or auditory nerve), which transmit hearing and balance signals, respectively, to the brain, are functional. That is why bone anchored hearing devices (BAHAs), which bypass the middle ear and transmit signals directly to the auditory nerve, can help people with microtia to hear better.
As an isolated condition, microtia does not affect cognitive abilities nor any other developmental issues.
If you do an Internet search on "microtia," you'll see dozens of websites, mostly from the medical community, about what the condition is. What you'll notice is that researchers and doctors don't really know much about what causes it. Some ethnicities have slightly greater prevalence, and there is slightly greater prevalence amongst boys (than girls), so there seems to be a genetic component, but mainly it seems to be random. For the overwhelming majority of instances of microtia, there is no "association" with any risk factor. (See "Who Has Microtia?")
Sources: Department of Defense, Hearing Centers of Excellence; WebMD, Understanding Hearing Loss; The Mayo Clinic, Hearing Loss; World Health Organization: Deafness and Hearing Loss; Johns Hopkins Medicine, Hearing Loss; American Diabetes Association; The Hearing Review: "ABI Trial Helps Children Born without Hearing Nerve," Disabilities Opportunities Internetworking and Technology (DO-IT)