Transient hypogammaglobulinaemia of infancy

A 14-month-old boy is brought to your GP clinic by his mother with recurrent upper respiratory tract infections. His mother reports that he attends full time day care and he is ‘constantly sick.’ You note that one of your colleagues has seen him twice for acute otitis media this year.

 What happens to serum immunoglobulin levels during gestation and in early life?

IgG levels of maternal origin in the fetal circulation increase over the course of pregnancy. At the end of gestation, fetal IgG levels exceed those in the maternal circulation, which indicates that the placenta develops an active transport mechanism for IgG molecules (1).

Evolution of maternal and serum IgG levels during pregnancy.

Maternal immunoglobulin G (IgG) is transported across the placenta, mainly in the third trimester. Because of this, babies born prematurely have less IgG.

After birth, the maternally derived IgG levels fall to a physiological nadir of 400 mg/dL at 3–6 months, at the same time, the infant’s production of IgG is not fully developed. Preterm babies have a more pronounced nadir. At the age of 1 year, IgA levels are approximately 20% of adult levels, IgG approximately 60% of adult levels and IgM approximately 75% of adult levels (2).

Immunoglobulin levels by age (from 3)

What are the clinical features of transient hypogammaglobulinaemia of infancy (THI)?

The usual clinical features include:

• Upper respiratory tract infections in about two-thirds of patients, most commonly ear and sinus infections
• Lower respiratory tract infections in approximately one-third of patients
• Allergic manifestations in 30% of patients, namely asthma, eczema, food allergy
• Gastrointestinal difficulties in 10% of patients e.g. chronic diarrhoea, persistent vomiting, food allergy and/or intolerance

Typically, children have a combination of the above features.

Other less common or rare clinical features include:

• Urinary tract infections
• Opportunistic infections caused by fungi (e.g. Pneumocystis jiroveci pneumonia or chronic candida), staphylococcus
• Severe invasive infections such as sepsis and meningitis
• Severe varicella zoster infection

How is transient hypogammaglobulinaemia of infancy defined and diagnosed?

There are differing definitions of THI in the literature but the following criteria is widely accepted:

• Serum IgG level must be less than 2SD below the mean for age, with or without depression of IgA or IgM on two separate occasions.
• Immunoglobulins and specific antibody formation must normalise usually during childhood
• Other primary immunodeficiency must be excluded.

THI can only truly be diagnosed retrospectively when age-adjusted immunoglobulin levels and specific antibody formation (function) have become normal, usually by age 4 years. There are no specific markers or a genetic signature.

Note, in a normal population, 2.5% of individuals will fall below that value, and clinicians may encounter otherwise well children with low immunoglobulin levels and normal antibody responses to vaccine antigens with no increased susceptibility to infection.

Diagnosis of putative THI is based on clinical picture and investigations. Most infants with THI have a normal physical examination with none of the classic findings present in other primary immunodeficiency diseases, although the tonsils and lymph nodes may be small.

THI is seen in two cohorts of patients:

• Firstly, asymptomatic patients whose relatives have an immunodeficiency and are tested and found to have low IgG. These patients are not excessively prone to infection and would not otherwise have come to medical attention without the index case.
• Secondly, those who present with clinically significant THI with frequent infections and/or poor antibody responses to one or more vaccine antigens.

How common is transient hypogammaglobulinaemia of infancy?

The estimated incidence of THI is between 0.061 and 1.1 per 1000 live births (5,6). it is likely to be significantly underdiagnosed.

There is a male predominance (2:1), and approximately 60% of patients are diagnosed by age 1 and the remaining 40% thereafter. Male predominance may be a genuine finding or may suggest that the normal values we use for evaluation should reflect not only age but also sex.

What investigations should be ordered?

 Immunoglobulins (IgG, IgA, IgM)

• By definition, the IgG level is <2SD below the mean for age. Typically the IgG level is less than 400 mg/dl but more than one-half of children with THI have IgG levels as low as 200 mg/dl.
• Levels less than 100 mg/dl and/or pan-hypogammaglobulinemia (very low IgM and IgA as well as IgG) may suggest a permanent immunodeficiency.

Pneumococcal antibody serology, tetanus serology (adapted from 7)

• Most children with THI produce normal antibodies to vaccines including tetanus toxoid, diphtheria, conjugated haemophilus influenzae, hepatitis A and B vaccines as well as to measles, mumps and rubella vaccines.
• Most infants will receive Prevenar 13 (conjugated pneumococcal vaccine) given on the immunisation schedule at 2m of age in Australia and will respond adequately.
• Tetanus antigen given at the same age in Infanrix hexa is a potent vaccine and lack of response suggests a more serious defect and should prompt a more comprehensive evaluation.
• If protective antibodies are not found, i.e. if the child lacks protective antibodies against a prior vaccine, a booster shot can be given and the antibody response checked four to six weeks later. If the antibody response is poor, repeat titres may be done in six months.
• A lack of response to tetanus antigen or poor antibody responses, or frequent or severe infections, should prompt additional studies and referral/consultation with an immunologist.

Full blood count

• Haematological abnormalities in THI are uncommon but a complete blood count with differential may suggest a primary immunodeficiency

B and T lymphocyte subsets

• Indicated if there is lymphopaenia, persistent or severe fungal or opportunistic infections, failure to thrive, chronic diarrhoea, or severe skin disease.

Imaging

• Chest or sinus X-ray may be helpful in the child with chronic respiratory tract infection. In THI, the chest x-ray is usually normal, and the thymus is typically normal in size.

What is the differential diagnosis of transient hypogammaglobulinaemia of infancy?

THI should be distinguished from other permanent forms of hypogammaglobulinemia such as CVID or X-linked agammaglobulinaemia (XLA).

• CVID: Typically these patients are unable to produce protective levels of specific antibodies with variable immunoglobulin concentrations, with most having some impairment in cellular immunity.
• XLA: Typically, XLA can be easily differentiated from THI. XLA has a severe deficiency of all Ig isotypes, an inability to produce antibodies, an absence of normal peripheral lymphoid tissues, an absence or a very low number of circulating B lymphocytes, and severe pyogenic infections starting in the first to second year of life.

What is the pathogenesis of transient hypogammaglobulinaemia of infancy?

The pathogenesis is unknown but proposed causes for THI include: (1) suppressive maternal antibodies (IgG) which cross the placenta and suppress foetal immunoglobulin production; (2) genetic variation in certain families with a propensity to immunodeficiency; 3) abnormal T-lymphocytes that fail to stimulate antibody production by B-cells; 4) unbalanced cytokine production; (5) abnormal or immature B-cells, not unlike patients CVID (7).

What is the treatment of transient hypogammaglobulinemia of infancy?

For asymptomatic patients, counselling is the main treatment required. Immunoglobulins should be repeated approximately every 6 months to ensure they normalise.

For children with clinically significant THI, the main objective is the prevention and treatment of infection. Judicious reduction of exposure to infection, and prompt and appropriate treatment of respiratory infections is warranted.

Antimicrobial prophylaxis during the respiratory infection season may be prescribed if recurrent respiratory and ear infections occur. Bactrim (5 mg/kg of the trimethoprim component) or amoxicillin (40 mg/kg) are suggested options.

Immunoglobulin replacement is reserved for rare instances of recurrent, severe infections only.

Immunisations should be given as per the schedule.

What is the prognosis of transient hypogammaglobulinemia of infancy?

Most children with THI develop normalise IgG levels by age 4 but many normalise before this and some children’s IgG levels do not normalise until adolescence.

Most suffer from increased episodes of otitis media and upper and lower respiratory infections.