“Hyperacusis” and Origins of Lowered Sound Tolerance

SIR: Nields et al.¹ rightly highlighted the severe psychological sequelae of lowered sound tolerance in Lyme disease (as in many other conditions) and claimed a central origin for “hyperacusis.” There is a serious problem with intolerance definitions and nomenclature. Itard, who coined hypercousie, said it had a great number of varieties. Many loudness phenomena occur in damaged ears, although it remains to be seen if these deficits are totally independent:

1. Hyperacusis, best considered as increased sensitivity to quiet sounds. I defined it² as mean pure tone hearing between 250 and 4,000 Hz better than zero dB Hearing Level (HL). This is very similar to the definition of hyperacute hearing¹} as “ability to distinguish sounds below zero dB HL.”

2. Audiosensitivity, defined as aversion to the output of television sets, radios, or sound systems at volume levels tolerated by normal listeners.²

3. Intolerance to other loud noises. Unlike strict audiosensitivity, in this condition stapedial reflexes were found to be normal.²

4. Recruitment, when loud noises in impaired ears sound as loud in the deaf ear as in a normal ear.³ It does not physiologically explain increased loudness and should not be used for clinical intolerances.

5. Loudness discomfort level, where patients indicate the intensity at which sounds start to feel uncomfortably loud. This is a standard audiometric test, though I have not found it an accurate indicator of audiosensitivity, intolerance, or recruitment,^2,3 stapedial reflex measures being far better. Confusingly, Nields et al.¹ defined sound tolerance at or below 100 dB HL as hyperacusis.

6. Tullio phenomenon, well described by Nields and Kveton,⁴ an idiosyncratic vestibular response to loud noise, probably due to labyrinthine fistulas.

7. Audiogenic seizures from loud noises in susceptible animals,⁵ a standard assay for anticonvulsants like carbamazepine.

Nields et al.¹ state that hypersensitivity to sound can originate anywhere from ear to cortex. Agreed, but does it? The best-attested cause of audiosensitivity is Meniere's disease, as in C. Darwin and Van Gogh.⁶ Fortunately, excellent clinical descriptions^1,7,8 provide abundant clinical evidence for endolymphatic hydrops⁹ in Lyme disease: blurry vision, tinnitus, transient unilateral deafness, sensation of head pressure, extreme sound sensitivity, agoraphobia, gastrointestinal problems, dizziness, imbalance, nausea, vomiting, depersonalization, motion sickness, paranoia. Such recognizable clinical symptoms are not always confirmed by objective neuro-otological findings. Nevertheless, there is a definite link with classic Meniere's disease in some with Lyme disease¹⁰ (characteristic low tone or fluctuant losses and vertigo attacks, as in early syphilis). There is no plausible encephalopathic cause for deafness and vertigo, even occurring separately.⁶

Another approach is to consider the pathogenesis of musical hallucinations (MHs) such as occur in Lyme disease.⁸ A review⁹ clearly showed a common mechanism for all MHs, with incipient hydrops probably a necessary and sufficient cause. Risk factors included anything likely to reduce labyrinthine pressure, like fevers, dehydration, fistulas, and weight loss (prominent in Lyme disease¹).

A further argument for a peripheral locus for auditory disorders comes from syphilis, which Lyme disease closely mimics. There is a large literature on syphilitic ear disease, including Meniere's syndrome. Audiosensitivity was surprisingly common in composers, many of whom had otosyphilis, including Beethoven, Haydn,⁶ Rossini, Donizetti, Smetana, Delius, and Wolf, who plugged his ears with bread and shot noisy songbirds!

Nields et al.¹ noted a kindling phenomenon where repeated sound stimulation led to reduced tolerance. I have never seen this reported before, nor observed it myself, but have not specifically checked with patients. This emphasizes the continuing value of careful clinical observations. In some rodents acoustic priming kindles audiogenic brainstem seizures, with secondary limbic epilepsy,¹¹ especially in the amygdala. The inferior colliculus is widely believed to play a major but unexplained role in auditory seizure initiation. Brainstem potentials show enhanced excitability along the auditory pathway, but only the peripherally generated Wave Ia had significantly shorter latency than in both control groups.¹² Such cochlear hyperactivation is not surprising given deafness and cochlear pathology in genetically seizure-prone rodents.

Most tinnitus clearly comes from the ear, but recent physiological work has led to the claim, reported in the Times (London), that it has a central origin,¹³ especially in the inferior colliculus. The standard animal model involves salicylates, which lowered activity in cochlear nucleus and inferior colliculus but increased it in auditory cortex.¹⁴ However, salicylate acts on the cochlea and can cause hydrops and otogenic MHs.⁹ This controversy could be rapidly settled by the production of clinical cases of tinnitus due to a brain lesion in patients with normal ears and hearing. Despite a specific appeal,¹³ no such case has appeared. Lyme disease is mimicked by experimental activation of the amygdala by procaine.¹ However, procaine also overactivates the cochlea,¹⁵ producing audiosensitivity, elementary auditory hallucinations (i.e., tinnitus), and seizure activity, as when the labyrinth is overstimulated by loud noise¹² or movements.⁵

In line with the maxim that the better and more original today's idea, the further back one has to go to find out who thought of it first, a peripheral locus of audiosensitivity had a sound 18th-century basis. E. Darwin was an acute observer, and there were plenty of febrile patients.¹⁶ “Inflammation of the brain is attended with intolerance of light and sound; which shews that the extremities of the nerves of those senses are at the same time inflamed.” After extensive dissections of man and beast, Comparetti¹⁷ stated that tinnitus and hypersensitive fluid were characterized by labyrinthine fluid deficiency (“Sed, humore deficiente, annon potius debilior sensus, quam exquisitior?”) Even when this occurred, as it so often did, in hysteria, hypochondria, and mental illness, he still said it was of labyrinthine origin. Itard in 1821 also noted that symptomatic audiosensitivity, as occurs in otitis, also occurred in hysteria and hypochondria.²