"Secondary signs of lateralization" in apogeotropic lateral canalolithiasis

The diagnosis of lateral canalolithiasis is based on the typical finding of the horizontal paroxysmal positional nystagmus induced by the Pagnini-Mc Clure manoeuvre. This technique usually identifies also the affected side, namely, the side where the paroxysmal nystagmus is more intense in geotropic forms and the side where the paroxysmal nystagmus is less intense in apogeotropic forms. However, this method is not always applicable since, especially in apogeotropic forms, the intensity of the nystagmus is not so distinctly different between the two sides. Further useful signs to identify the affected side have been described in the Literature: Pseudo-spontaneous nystagmus in the sitting position; Leaning and Bowing nystagmus; Null-point in the vertical plane; Sitting to supine positioning nystagmus; Null-point in the horizontal plane. They are fully explained at pathophysiological level by the mechanism of canalolithiasis, as they are caused by otoliths moving in the ampullopetal direction in the non-ampullary segment (geotropic canalolithiasis) of the lateral canal or in ampullofugal direction in the ampullary segment (apogeotropic canalolithiasis) of the lateral canal. In other words, the movement of otoliths determines excitatory or inhibitory endolymphatic flow that generates specific nystagmic eye-movements. Authors analyse the characteristics of these signs, that they define as "Secondary signs of lateralization", in 64 cases of apogeotropic lateral canalolithiasis. A decisive contribution of the "Secondary signs of lateralization" to diagnose the pathological side has been verified in 13 out of 64 cases, whereas, in all other cases, Pagnini-Mc Clure diagnostic manoeuvre proved successful in correctly identifying the affected side by itself. Among the "Secondary signs of lateralization", the Pseudo-spontaneous nystagmus in the sitting position and the Sitting to the supine positioning nystagmus were particularly useful, due to both their frequency and their easy detection, as they do not require additional manoeuvres besides those usually performed during health examination of a patient affected by lateral canalolithiasis. In conclusion, the Authors propose a decision-making algorithm to diagnose and treat lateral canalolithiasis based on the attempt to obtain as much information as possible for a correct diagnosis, with the least trouble and inconvenience for patients.