INVESTIGATION OF LARYNGEAL DISEASE 153 patient has a picture taken while breathing quietly, during phonation, while doing a Valsalva manoeuvre to outline the ventricles, and while doing forced inspiration. A laryngogram has the advantage over the tomogram that it can be screened and the laryngeal movements better assessed. SPECIAL INVESTIGATIONS. These tests are applied to voice problems rather than to cases with organic disease processes. 1. Tape Recordings. These are essential to record the progress of any case having speech therapy. 2. Stroboscopy. This produces an optical illusion whereby an object moving rapidly appears to move very slowly. The illusion is obtained by viewing the larynx by indirect laryngoscopy using a stroboscope as the light source and synchronizing the illumination with the frequency of the phonated 'ee\ This arrests or slows down the rapid movement of the vibrating vocal cords and allows of their inspection in any desired position by changing the phase of the light. 3. Pitch and Frequency Measurements. These measurements should be performed on all patients with voice problems. The ideal speaking pitch should be one third of the way up the patient's range. To measure this the patient should sing his lowest note and then sing a scale so that his highest frequency can be measured. The pitch frequency of his speaking voice should then be measured and the speech therapist will then be able to adjust this to his ideal conversational pitch one-third of the way up the range. 4. Respiratory Function Tests. These tests are occasionally useful because altered breathing patterns can cause vocal abnormalities. 5. Fibreoptic Laryngoscopy. This is a new addition to the investigative armoury resulting from the technological advances in fibreoptic instrumenta- tion. It is possible to pass a small fibreoptic bundle through the nose and into the larynx to examine certain areas very closely and also to avoid the need for direct laryngoscopy in some cases with an overhanging epiglottis. It may also be combined with Stroboscopy for laryngeal examination. DIRECT LARYNGOSCOPY. A fuller description of the technique of this will be given in Chapter 41. Suffice it to say at this point that it is the most important laryngeal investigation. It permits a close examination of the laryngeal and pharyngeal structures, it allows the extent of tumours to be assessed and biopsied, and it is a route for minor laryngeal surgery such as vocal cord stripping or removal of polyps and other benign tumours. In recent years use has been made of the Zeiss operating microscope with a 400-mm lens for microlaryngoscopy. With this innovation many new instru- ments have been developed to allow for fine microscopic work to be performed on the laryngeal structures with a high degree of accuracy and thoroughness. A stroboscopic light source may also be used with the microscope as an additional help in micro-examination of the larynx.