A 9-month-old with failure to thrive and a chronic cough

The most likely diagnosis is cystic fibrosis (CF).

How the findings fit:

• Respiratory: chronic cough since 4 months, recurrent 'chest infections', one admission with bronchiolitis, and a chest X-ray showing hyperinflation and peribronchial thickening. In CF, thick secretions impair mucociliary clearance, leading to chronic infection and inflammation. The early onset and recurrence are typical.
• Gastrointestinal: 3–4 loose, greasy stools per day and a protuberant abdomen suggest pancreatic exocrine insufficiency with steatorrhoea. Fat malabsorption contributes to poor weight gain.
• Growth: weight and length below the 0.4th centile with a relatively preserved head circumference (9th centile) suggests nutritional rather than primary brain growth failure - consistent with malabsorption and increased energy expenditure from chronic lung disease.
• Development: she is not sitting at 9 months, which is delayed; this can reflect chronic illness and undernutrition rather than a primary neurodevelopmental disorder, but it must be assessed.
• Consanguinity: first-cousin parents raise the probability of an autosomal recessive condition such as CF.

Differential to keep in mind:

• Primary ciliary dyskinesia - chronic cough and recurrent infections, but usually no steatorrhoea; situs inversus in about half.
• Immunodeficiency - recurrent infections, but the greasy stools and malabsorption are less typical.
• Coeliac disease - malabsorption and poor growth, but the chronic respiratory symptoms are not explained.
• Post-infectious bronchiolitis obliterans - chronic cough and hyperinflation, but not the malabsorption.
• Tuberculosis - chronic cough and poor growth; consider if there is exposure.

CF explains all three domains (respiratory, GI, growth) with a single diagnosis.

Do not wait 6 weeks to act. The child is failing to thrive and has chronic respiratory symptoms; treatment should start on clinical suspicion while confirmation proceeds.

Confirmatory pathway:

1. Sweat chloride test - the gold standard. Two separate tests on two separate days are needed. A chloride of greater than 60 mmol/L is diagnostic; 30–59 is intermediate; less than 30 is normal (in infants, values can be lower). The local lab cannot do it, so arrange referral to a centre that can - do not simply wait for a posted result.
2. Genetic testing - CFTR gene sequencing. This is increasingly used as a first-line or parallel test and can be done on a blood or saliva sample. It is especially useful given consanguinity, where a homozygous mutation may be found.
3. Newborn screening - check whether she was screened (immunoreactive trypsinogen). A normal newborn screen does not exclude CF (some mutations are missed), but a positive one would support the diagnosis.
4. Faecal elastase - to document pancreatic exocrine insufficiency; a low value supports the diagnosis and guides enzyme replacement.
5. Stool culture and respiratory culture - to identify pathogens (Pseudomonas, Staphylococcus aureus, Haemophilus).
6. Chest X-ray and, if available, chest CT - to document structural lung disease.

What to start now:

• Pancreatic enzyme replacement therapy with meals (lipase units per gram of fat, or a standard starting dose), plus fat-soluble vitamins (A, D, E, K) and salt supplementation in hot weather.
• Nutritional support - high-calorie diet, dietitian referral, and monitoring of weight and length.
• Airway clearance - physiotherapy and consider nebulised hypertonic saline or dornase alfa once the diagnosis is confirmed.
• Immunisations - ensure up to date, including influenza and pneumococcal.
• Referral to a specialist CF centre - this is essential; management should be by a multidisciplinary team.
• Genetic counselling for the family, given consanguinity and the 25% recurrence risk.

Safety-net: return if respiratory distress, fever, or poor feeding.

Answering 'will she grow out of it?':

• CF is a life-long genetic condition; it does not go away. But it is no longer the diagnosis it was a generation ago. With modern care, many children reach adulthood with good lung function and normal growth.
• Be honest but hopeful: "This is a serious condition that needs lifelong care, but with the right treatment many people with CF live full and active lives."

What the future holds:

• Lung health depends on early and consistent airway clearance, infection control, and prompt treatment of exacerbations.
• Growth depends on pancreatic enzyme replacement, adequate calories, and fat-soluble vitamins. With good nutrition, many children catch up.
• CFTR modulator therapies are now available for many (but not all) mutations and can substantially improve lung function and quality of life; eligibility depends on the specific mutations, which is another reason to pursue genetic testing.
• Complications can include diabetes, liver disease, and infertility (most males with CF are infertile due to congenital bilateral absence of the vas deferens); these should be discussed sensitively and at an age-appropriate time.

Role of the multidisciplinary team:

• Paediatric pulmonologist - lung function, airway clearance, and exacerbation management.
• Dietitian - calorie targets, enzyme dosing, and vitamin supplementation.
• Physiotherapist - airway clearance techniques taught to the family.
• CF nurse specialist - coordination, education, and sick-day advice.
• Pharmacist - nebulised therapies and drug interactions.
• Psychologist/social worker - support for the family, school liaison, and financial help.
• Genetic counsellor - 25% recurrence risk for future pregnancies, and cascade testing.

Practical points for the family:

• Keep a symptom diary and know the signs of an exacerbation (increased cough, sputum, breathlessness, poor appetite).
• Avoid smoking in the home and avoid unnecessary contact with others with CF (cross-infection risk).
• Ensure vaccinations are up to date.
• Give written information and a clear contact number for the CF team.