SD is formally classified as a movement disorder, one of the focal dystonias, and is also known as laryngeal dystonia. Supporting evidence that SD is a neurological disorder includes:
- SD may co-occur with other neurological movement disorders such as blepharospasm (excessive eye blinking and involuntary forced eye closure), tardive dyskinesia (involuntary and repetitious movement of muscles of the face, tongue, body, arms and legs), oromandibular dystonia (involuntary movements of the jaw muscles, lips and tongue), torticollis (involuntary movements of the neck muscles), or tremor (rhythmic, quivering muscle movements).
- Spasmodic dysphonia runs in some families and is thought to be inherited. Research has identified a possible gene on chromosome 9 that may contribute to the spasmodic dysphonia that is common to certain families.
Histological examination of the nerve to the vocal cords in patients with SD demonstrates that the percentage of abnormally thin nerve fibers was higher than in normal controls
Functional MRI signal is reduced in sensorimotor cortices associated with movement of the affected body part in laryngeal dystonia, supporting a dystonic basis for this voice disorder