The diagnosis of A-T is usually suspected by the combination of neurologic clinical features (ataxia, abnormal control of eye movement, and postural instability) with telangiectasia and sometimes increased infections, and confirmed by specific laboratory abnormalities (elevated alpha-fetoprotein levels, increased chromosomal breakage or cell death of white blood cells after exposure to X-rays, absence of ATM protein in white blood cells, or mutations in each of the person’s ATM genes).

A variety of laboratory abnormalities occur in most people with A-T, allowing for a tentative diagnosis to be made in the presence of typical clinical features. Not all abnormalities are seen in all patients. These abnormalities include:

- Elevated and slowly increasing alpha-fetoprotein levels in serum after 2 years of age

- Immunodeficiency with low levels of immunoglobulins (especially IgA, IgG subclasses, and IgE) and low number of lymphocytes in the blood

- Chromosomal instability (broken pieces of chromosomes)

- Increased sensitivity of cells to x-ray exposure (cells die or develop even more breaks and other damage to chromosomes)

- Cerebellar atrophy on MRI scan

The diagnosis can be confirmed in the laboratory by finding an absence or deficiency of the ATM protein in cultured blood cells, an absence or deficiency of ATM function (kinase assay), or mutations in both copies of the cell’s ATM gene. These more specialized tests are not always needed, but are particularly helpful if a child’s symptoms are atypical.

Oral intake may be aided by teaching persons with A-T how to drink, chew and swallow more safely. The propriety of treatments for swallowing problems should be determined following evaluation by an expert in the field of speech-language pathology. Dieticians may help treat nutrition problems by recommending dietary modifications, including high calorie foods or food supplements.

A feeding (gastrostomy) tube is recommended when any of the following occur:

- A child cannot eat enough to grow or a person of any age cannot eat enough to maintain weight;

- Aspiration is problematic;

- Mealtimes are stressful or too long, interfering with other activities.

Feeding tubes can decrease the risk of aspiration by enabling persons to avoid liquids or foods that are difficult to swallow and provide adequate calories without the stress and time commitment of prolonged meals. Gastrostomy tubes do not prevent people from eating by mouth. Once a tube is in place, the general goal should be to maintain weight at the 10-25th percentile.

Type 1 (AOA1) usually has an onset of symptoms during childhood. It is an autosomal recessive cerebellar ataxia (ARCA) associated with hypoalbuminemia and hypercholesterolemia. Mutations in the gene APTX, which encodes for aprataxin, have been identified to be responsible for AOA1. Elevated creatine kinase is occasionally present, in addition to a sensorimotor axonal neuropathy, as shown by nerve conduction velocity studies. In addition, MRI studies have shown cerebellar atrophy, mild brainstem atrophy, and, in advanced cases, cortical atrophy

Ataxia with oculomotor apraxia type 2 (AOA2) has its onset during adolescence. It is characterized by cerebellar atrophy and peripheral neuropathy. Sufferers of Type 2 have high amounts of another protein called alpha-fetoprotein (AFP), and may also have high amounts of the protein creatine phosphokinase (CPK). Mutations in the SETX gene are the cause of the disease. AOA2 shows cerebellar atrophy, loss of Purkinje cells, and demyelination. In particular, there is a failure of the cerebrocerebellar circuit in AOA2 that has been shown to be responsible for the weaker coordination of complex cognitive functions such as working memory, executive functions, speech, and sequence learning. Although there is no sign of mental retardation or severe dementia, even after long disease duration, research on families with possible AOA2 have shown mild cognitive impairment as indexed by the Mini-Mental State Examination (MMSE) and the Mattis Dementia Rating Scale. These impairments appear to be mostly due to a deficit in initiation and concept subtests.