Anatomy and Physiology of the Larynx (Voice Box)
The vocal fold can be divided into five anatomical layers and three mechanical layers.
The five anatomical layers of the vocal cord are:
1. Nonkeratinising stratified squamous epithelium
2. Superficial layer of the lamina propria (Reinke’s space) – this consists of connective tissue made up of a loose fibrous substance
3. Intermediate layer of the lamina propria (elastic fibres)
4. Deep layer of the lamina propria (collagenous fibres) – this layer and the intermediate layer of the lamina propria forms the vocal ligament
5. Vocalis muscle
Mechanically the fold can be divided into three layers:
The cover (squamous epithelium and Reinke’s space)
The transitional zone (the vocal ligament)
The body (vocalis muscle)
The layered structure is important for the function of voice production.
The vocal cord acts as a multi-layered vibrator with the cover vibrating as a ‘mucosal wave’ on the underlying transition layer and body. The vocal cords vibrate at 100-300 Hz during conversation. A pathological process that disturbs the vibration will give rise to dysphonia (abnormality of the voice).
The voice is controlled by:
The lungs that produces an air stream
The larynx which can be seen as a ‘voice generator’
The pharynx and oral cavity which act as a ‘voice resonator’
These are all under the control of the central and peripheral nervous system. Abnormalities of the respiratory system or the structure and/or function of the larynx can cause hoarseness.
Hoarseness may be caused by either:
An abnormality of the structure of the vocal cords which can be generalised e.g. laryngitis, or localised e.g. a cyst.
An abnormality of the movement of the vocal cords e.g. recurrent laryngeal nerve palsy or arthritis affecting the laryngeal joints.
Malignancy should always be excluded as the cause of hoarseness.
We will discuss the structure and function of the larynx in order to demonstrate how pathological processes can cause hoarseness.
In terms of voice, the equivalent to hoarseness will be an abnormal cry in children before the development of speech. Hoarseness describes a rough quality to the voice. Dysphonia encompasses the spectrum of voice disorders.
The quality and severity of voice disturbance varies markedly depending on the pathology. For example, it may be so severe that it makes it difficult for the patient to make themselves understood or may be a subtle abnormality affecting a professional singer’s singing voice in a certain register.
The cause of hoarseness is often multi-factorial and the classification of hoarseness is not necessarily straightforward. For example, disorders of the movement of vocal cords can often lead to structural abnormalities, functional factors can be difficult to define and the place of some factors e.g. acid reflux, is controversial.
Respiratory pathology can exacerbate hoarseness by altering the control of the air stream presented to the larynx.