Difference Between Stridor and Stertor ( Stridor in children )
Stridor is an auditory manifestation of a disordered respiratory function. In simple terms it could be termed as noisy breathing. It should be borne in mind that not all noisy breathing is stridor. Voluntary or involuntary vocalisations, moist sounds such as bubbling of secretions in the larynx or pharynx are to be excluded.
There is one more noisy breathing known as stertor, derived from the latin word stertere meaning to snore. This is caused by obstruction of airway above the level of the larynx. It is low pitched snoring or snuffly sound produced by vibrations of tissue of the naso pharynx, pharynx or soft palate. Stridor on the other hand is caused due to air flow changes within the larynx, trachea or bronchi. Stridor is infact more often musical in character. The frequency of sound produced also ranges from lwo pitched to high pitched frequencies.Aerodynamic explanation of stridor and stertor:
Stridor and stertor are both due to turbulence of the air flow within a partially obstructed respiratory tract. In order to understand its mechanism we will have to go back to the physics of airflow. According to Bedrnoulli's theorem if air passes through a tube which has constrictions, the velocity of gas increases at the level of the narrowing, and in order to preserve the law of conservation of energy, the local gas pressure falls. The same principle can be applied to air flowing through the respiratory tract, which is flaccid and compressible in a child.
There is a natural increase in the pressure gradiant at the sites of constriction, resulting in a collapse of the airway and temporary cessation of airflow. Now Pascal's principle come into play, which states that in fluid / gas at rest the pressure changes in one part is transmitted without loss to every portion of the fluid or gas and to the walls of the container. In addition to this the resilence of the cartilagenous support of the airway in children cause the airway to spring open thus resuming the air flow. Thus the fluttering vibrations caused are amplified by the various resonators giving rise to audible sounds which are known as either a stridor or a stertor.Stertor is always inspiratory in its timing and has a low frequency, where as a stridor can occur both during inspiration or expiration with varying frequencies. Caution should be exercised while applying these criteria to differentiate stridor from stertor because there can always be a overlap between these conditions.1. During inspiration, the relatively supple, poorly supported structures of the infantile supraglottis tend to be drawn into the glottic aperture due to the pressure differential that exist between the pharynx and trachea during inspiration, on the contrary expiration forces the prolapsed tissues out of the laryngeal inlet. The noise caused by this process is limited to the inspiratory phase and is often low pitched in character. Stridor of this type occur in laryngomalacia.
2. In smaller bronchi and bronchioles, during expiration there is accentuation of bronchial muscle contraction combined with high velocity gas flow cause collapse of the airway causing stridor during expiratory phase, it could be heard as a wheeze. This is commonly caused by retained secretions or foreign bodies.
3. The relatively rigid walls of rima glottis and trachea prevent collapse and hence stridor arising from these areas is due to turbulance of airflow alone. If the obstruction is severe to and fro stridor can occur (biphasic).The associated signs / symptoms associated with stridor are:
1. Dyspnoea: Occurs in all patients with airway obstruction. It is also associated with other signs like flaring of the nostrils, use of accessory muscles of respiration, and cyanosis may also be present in extreme cases. During obstruction an extraordinarily high negative pressure causing indrawing of soft tissues in the subcostal, intercostal, suprasternal and substernal areas. Severe indrawing of the suprasternal area may cause a phenomenon known as the tracheal tug, where the trachea appears to be pulled down into the chest with each inspiration. In infants & neonates who have a soft compliant chest wall the indrawing of the subcostal space may be a normal phenomenon. Similarly these signs of distress may not be seen in infants even during chronic severe airway obstruction.
The respiratory obstruction which produces stridor & stertor can lead to hypoxia, hypercapnia, pulmonary oedema, cor pulmonale, vomiting, aspiration & pneumonia.
2. Cough : Is usually harsh and barking in nature. This symptom is commonly associated with subglottic inflammation or tracheal compression.
3. Hoarseness : Suggests laryngeal inflammation, trauma, tumors or vocal cord mobility problems. In a child with stretor there will not be any hoarseness of voice but the cry is usually muffled.
4. Deglutition : The process of deglutition & respiration share the common pathway i.e. the oropharynx. Hence disorders of swallowing may affect breathing and the vice versa is also possible.
Figure showing the differences between stridor and stertorThese patients have narrowing of oropharynx, nasopharynx and nasal cavities. The may also additionally manifest with macroglossia. The various anamolies associated with respiratory difficulties are:History :
As with anyother disease a good history is a must for accurate diagnosis of the etilogy of infantile stridor. History should concentrate on the duration of the respiratory distress, Positional variations if any, associated problems with feeding, voice changes, and other congential abnormalities. In addition h/o trauma or ingestion of foreign body is a must.
Causes of stridor are anatomically classified:
1. Supralaryngeal causes:
a. Nose - choanal atresia
Obstruction due to infection / truama / tubes
b. Cranio facial anamolies:
- Pierre Robin syndrome
- Treacher collin syndrome
- Apert's syndrome
- Cruzon's syndrome
- Mobieus syndrome
-----c. Macroglossia :
- Beckwith Wiedemann syndrome
- Down's syndrome
e. Laryngomalacia : Is caused by an excessiely elastic cartilagenous support to the airway seen in infants. This commonly affects the glottic and supra glottic airway of infants. This excessively soft and elastic cartilage causes inspiratory collapse of the arytenoid, aryepiglottic folds and epiglottis during inspiration. The omega shaped epiglottis seen often in the infants adds to the problem. This causes occlusion of the laryngeal inlet. These patients have inspiratory stridor which becomes better on prone position or when the child is calm. Stridor is worsened if the child is restless or excited.
The cry of the child is usually normal. The child may also have aspiration and feeding difficulties. It is commonly seen during the first few months of life.
Direct laryngoscopy shows indrawing and falling forwards of the arytenoid and the aryepiglottic folds. The epiglottis may be infolded.
This condition may be managed conservatively, as the cartilage in infants tend to become stiffer as the child grows. In difficult cases the patient may be subjected to tracheostomy to secure the airway and to prevent aspiration, and feeding gastrostomy to maintain the nutritional status of the child. Epiglottoplasty may be considered in resistant cases.
2. Glottic causes:
a. Vocal cord palsy : Is one of the commonest cause of airway obstruction. In 80% of patients it is unilateral.
Etiology: Could be caused due to injury to vagus nerve at the level of Nucleus ambiguus - it is often bilateral.
Injury to the left recurrent laryngeal nerve due to cardio vascular causes and thoracic causes.
It could be caused due to increased intracranial pressure - i.e. Meningomyelocoele with Arnold Chiari malformation.
Clinical features: Inspiratory stridor at birth
Weak, hoarse cry or aphonia.
If unilateral the patient feel better when placed on the side of the lesion.
Direct laryngoscopy under local anaesthesia.
Reduction of elevagted intracranial tension.
In bilateral palsy tracheostomy is indicated.
b. Tumors :
d. Webs: are caused due to failure of recanalisation of the larynx . It can range between a complete occlusion by mucosa and submucous tissue or partial occlusion by a thin membranous web. It can occur in supraglottis, glottis and subglottis area. Commonly it is seen in the glottic area. It occurs in one in 10,000 live births.
Stridor is inspiratory in nature and is present from birth. The degree of airway obstruction depends on the extent of the web. The cry is weak or absent because of fixity of the cord. Symptoms are not positional in nature.
Tracheostomy may be life saving.
Perforation of the web.
Dilatation of the web.
3. Subglottic causes:
Stenosis - congenital / acquired : is the most common cause of neonatal airway obstruction. It may either be congenital or acquired. Subglottic area is the narrowest portion of neonatal airway. Even a 1mm mucosal oedema in this area is sufficient to reduced the circumference by 1/3 compromising the airway. Congenital stenosis is more common in male children. Acquired subglottic stenosis is due to prolonged intubation, insertion of a large endotracheal tube etc. These patients present with inspiratory and expiratory stridor (to & fro stridor).
This condition is diagnosed by performing an endoscopy.
Tracheostomy may be life saving.
Anterior cricoid split
Resection of the stenotic segment with reconstruction
Tumors like hemangioma & cystic hygroma.
Subglottic hemangioma is may be asymptomatic at birth. As it grows it may produce symptoms at a later date. These children manifest with stridor, change in voice is possible if the tumor involves the under surface of the vocal cord. These children become symptomatic by 6 months. Female child is more commonly affected than males. Subcutaneous hemangiomas may also be seen in 50% of these children.
Xray soft tissue lateral view may show an eccentric swelling in the subglottic region. Endoscopy is diagnostic.
Tracheostomy is indicated to tide over acute crisis
Steroids may be prescribed to reduce subglottic oedema.
4. Tracheal causes:
Tracheomalacia : More or less similar to laryngomalacia. The stridor produced is expiratory in nature.
Atresia : Often fatal at birth. It is also associated with other congenital abnormalities.
5. Extrinsic causes:
Vascular rings : Compression of trachea and oesophagus due to abnormalities in the development of great vessels. It could be due to right sided aortic arch, double aortic arch, anomalous right subclavian artery, or aberrent left pulmonary artery.
Dyspnoea is present at birth, becomes worse on neck extension. Stridor is expiratory in nature.
Diagnosis is by:
Contrast CT scans
Figure showing the various craniofacial anamolies